diff --git a/audio/common/all-versions/default/service/service.cpp b/audio/common/all-versions/default/service/service.cpp
index 8a7b2ea..2a6571b 100644
--- a/audio/common/all-versions/default/service/service.cpp
+++ b/audio/common/all-versions/default/service/service.cpp
@@ -36,6 +36,15 @@
 using namespace android::hardware;
 using android::OK;
 
+/** Try to register the provided factories in the provided order.
+ *  If any registers successfully, do not register any other and return true.
+ *  If all fail, return false.
+ */
+template <class... Factories>
+bool registerPassthroughServiceImplementations() {
+    return ((registerPassthroughServiceImplementation<Factories>() != OK) && ...);
+}
+
 int main(int /* argc */, char* /* argv */ []) {
     ::android::ProcessState::initWithDriver("/dev/vndbinder");
     // start a threadpool for vndbinder interactions
@@ -50,30 +59,30 @@
     }
     configureRpcThreadpool(16, true /*callerWillJoin*/);
 
-    bool fail = registerPassthroughServiceImplementation<audio::V5_0::IDevicesFactory>() != OK &&
-                registerPassthroughServiceImplementation<audio::V4_0::IDevicesFactory>() != OK &&
-                registerPassthroughServiceImplementation<audio::V2_0::IDevicesFactory>() != OK;
-    LOG_ALWAYS_FATAL_IF(fail, "Could not register audio core API 2, 4 nor 5");
+    LOG_ALWAYS_FATAL_IF((registerPassthroughServiceImplementations<audio::V5_0::IDevicesFactory,
+                                                                   audio::V4_0::IDevicesFactory,
+                                                                   audio::V2_0::IDevicesFactory>()),
+                        "Could not register audio core API");
 
-    fail = registerPassthroughServiceImplementation<audio::effect::V5_0::IEffectsFactory>() != OK &&
-           registerPassthroughServiceImplementation<audio::effect::V4_0::IEffectsFactory>() != OK &&
-           registerPassthroughServiceImplementation<audio::effect::V2_0::IEffectsFactory>() != OK,
-    LOG_ALWAYS_FATAL_IF(fail, "Could not register audio effect API 2, 4 nor 5");
+    LOG_ALWAYS_FATAL_IF(
+            (registerPassthroughServiceImplementations<audio::effect::V5_0::IEffectsFactory,
+                                                       audio::effect::V4_0::IEffectsFactory,
+                                                       audio::effect::V2_0::IEffectsFactory>()),
+            "Could not register audio effect API");
 
-    fail = registerPassthroughServiceImplementation<soundtrigger::V2_2::ISoundTriggerHw>() != OK &&
-           registerPassthroughServiceImplementation<soundtrigger::V2_1::ISoundTriggerHw>() != OK &&
-           registerPassthroughServiceImplementation<soundtrigger::V2_0::ISoundTriggerHw>() != OK,
-    ALOGW_IF(fail, "Could not register soundtrigger API 2.0, 2.1 nor 2.2");
+    ALOGW_IF((registerPassthroughServiceImplementations<soundtrigger::V2_2::ISoundTriggerHw,
+                                                        soundtrigger::V2_1::ISoundTriggerHw,
+                                                        soundtrigger::V2_0::ISoundTriggerHw>()),
+             "Could not register soundtrigger API");
 
-    fail = registerPassthroughServiceImplementation<
-                   bluetooth::audio::V2_0::IBluetoothAudioProvidersFactory>() != OK;
-    ALOGW_IF(fail, "Could not register Bluetooth Audio API 2.0");
+    ALOGW_IF(registerPassthroughServiceImplementations<
+                     bluetooth::audio::V2_0::IBluetoothAudioProvidersFactory>(),
+             "Could not register Bluetooth audio API");
 
     // remove the old HIDL when Bluetooth Audio Hal V2 has offloading supported
-    fail =
-        registerPassthroughServiceImplementation<bluetooth::a2dp::V1_0::IBluetoothAudioOffload>() !=
-        OK;
-    ALOGW_IF(fail, "Could not register Bluetooth audio offload 1.0");
+    ALOGW_IF(registerPassthroughServiceImplementations<
+                     bluetooth::a2dp::V1_0::IBluetoothAudioOffload>(),
+             "Could not register Bluetooth audio offload API");
 
     joinRpcThreadpool();
 }
diff --git a/audio/core/all-versions/default/Android.bp b/audio/core/all-versions/default/Android.bp
index a1af3c4..97b4553 100644
--- a/audio/core/all-versions/default/Android.bp
+++ b/audio/core/all-versions/default/Android.bp
@@ -26,6 +26,7 @@
         "libhidlbase",
         "libhidltransport",
         "liblog",
+        "libmedia_helper",
         "libutils",
         "android.hardware.audio.common-util",
     ],
@@ -37,10 +38,6 @@
         "libhardware_headers",
         "libmedia_headers",
     ],
-
-    whole_static_libs: [
-        "libmedia_helper",
-    ],
 }
 
 cc_library_shared {
diff --git a/audio/effect/5.0/xml/Android.bp b/audio/effect/5.0/xml/Android.bp
index 967135c..eb2bcee 100644
--- a/audio/effect/5.0/xml/Android.bp
+++ b/audio/effect/5.0/xml/Android.bp
@@ -1,6 +1,6 @@
 
 xsd_config {
-    name: "audio_effects_conf",
+    name: "audio_effects_conf_V5_0",
     srcs: ["audio_effects_conf.xsd"],
     package_name: "audio.effects.V5_0",
 }
diff --git a/drm/1.1/vts/functional/drm_hal_clearkey_test.cpp b/drm/1.1/vts/functional/drm_hal_clearkey_test.cpp
index 7dedd7f..6be30d3 100644
--- a/drm/1.1/vts/functional/drm_hal_clearkey_test.cpp
+++ b/drm/1.1/vts/functional/drm_hal_clearkey_test.cpp
@@ -24,7 +24,7 @@
 #include <android/hardware/drm/1.0/types.h>
 #include <android/hardware/drm/1.1/types.h>
 #include <android/hidl/allocator/1.0/IAllocator.h>
-#include <android/hidl/manager/1.0/IServiceManager.h>
+#include <android/hidl/manager/1.2/IServiceManager.h>
 #include <gtest/gtest.h>
 #include <hidl/HidlSupport.h>
 #include <hidl/ServiceManagement.h>
@@ -129,9 +129,9 @@
         ALOGD("DrmHalClearkeyTest: Running test %s.%s", test_info->test_case_name(),
                 test_info->name());
 
-        auto manager = android::hardware::defaultServiceManager();
+        auto manager = android::hardware::defaultServiceManager1_2();
         ASSERT_NE(nullptr, manager.get());
-        manager->listByInterface(IDrmFactory::descriptor,
+        manager->listManifestByInterface(IDrmFactory::descriptor,
                 [&](const hidl_vec<hidl_string> &registered) {
                     for (const auto &instance : registered) {
                         sp<IDrmFactory> drmFactory =
@@ -144,7 +144,7 @@
                 }
             );
 
-        manager->listByInterface(ICryptoFactory::descriptor,
+        manager->listManifestByInterface(ICryptoFactory::descriptor,
                 [&](const hidl_vec<hidl_string> &registered) {
                     for (const auto &instance : registered) {
                         sp<ICryptoFactory> cryptoFactory =
diff --git a/gnss/1.1/vts/functional/gnss_hal_test_cases.cpp b/gnss/1.1/vts/functional/gnss_hal_test_cases.cpp
index 4288d0d..ee236ba 100644
--- a/gnss/1.1/vts/functional/gnss_hal_test_cases.cpp
+++ b/gnss/1.1/vts/functional/gnss_hal_test_cases.cpp
@@ -24,6 +24,9 @@
 
 using android::hardware::hidl_vec;
 
+using IGnssMeasurement_1_0 = android::hardware::gnss::V1_0::IGnssMeasurement;
+using IGnssMeasurement_1_1 = android::hardware::gnss::V1_1::IGnssMeasurement;
+
 using android::hardware::gnss::V1_0::GnssConstellationType;
 using android::hardware::gnss::V1_0::GnssLocation;
 using android::hardware::gnss::V1_0::IGnssDebug;
@@ -43,11 +46,15 @@
  * Gets the GnssMeasurementExtension and verify that it returns an actual extension.
  */
 TEST_F(GnssHalTest, TestGnssMeasurementCallback) {
-    auto gnssMeasurement = gnss_hal_->getExtensionGnssMeasurement_1_1();
-    ASSERT_TRUE(gnssMeasurement.isOk());
+    auto gnssMeasurement_1_1 = gnss_hal_->getExtensionGnssMeasurement_1_1();
+    ASSERT_TRUE(gnssMeasurement_1_1.isOk());
+    auto gnssMeasurement_1_0 = gnss_hal_->getExtensionGnssMeasurement();
+    ASSERT_TRUE(gnssMeasurement_1_0.isOk());
     if (last_capabilities_ & IGnssCallback::Capabilities::MEASUREMENTS) {
-        sp<IGnssMeasurement> iGnssMeas = gnssMeasurement;
-        EXPECT_NE(iGnssMeas, nullptr);
+        sp<IGnssMeasurement_1_1> iGnssMeas_1_1 = gnssMeasurement_1_1;
+        sp<IGnssMeasurement_1_0> iGnssMeas_1_0 = gnssMeasurement_1_0;
+        // At least one interface must be non-null.
+        ASSERT_TRUE(iGnssMeas_1_1 != nullptr || iGnssMeas_1_0 != nullptr);
     }
 }
 
@@ -59,6 +66,11 @@
  * each received location.
  */
 TEST_F(GnssHalTest, GetLocationLowPower) {
+    if (!IsGnssHalVersion_1_1()) {
+        ALOGI("Test GetLocationLowPower skipped. GNSS HAL version is greater than 1.1.");
+        return;
+    }
+
     const int kMinIntervalMsec = 5000;
     const int kLocationTimeoutSubsequentSec = (kMinIntervalMsec / 1000) * 2;
     const int kNoLocationPeriodSec = (kMinIntervalMsec / 1000) / 2;
diff --git a/gnss/2.0/vts/functional/gnss_hal_test_cases.cpp b/gnss/2.0/vts/functional/gnss_hal_test_cases.cpp
index 6e0887f..39736cc 100644
--- a/gnss/2.0/vts/functional/gnss_hal_test_cases.cpp
+++ b/gnss/2.0/vts/functional/gnss_hal_test_cases.cpp
@@ -416,6 +416,70 @@
 }
 
 /*
+ * GetLocationLowPower:
+ * Turns on location, waits for at least 5 locations allowing max of LOCATION_TIMEOUT_SUBSEQUENT_SEC
+ * between one location and the next. Also ensure that MIN_INTERVAL_MSEC is respected by waiting
+ * NO_LOCATION_PERIOD_SEC and verfiy that no location is received. Also perform validity checks on
+ * each received location.
+ */
+TEST_F(GnssHalTest, GetLocationLowPower) {
+    if (!(gnss_cb_->last_capabilities_ & IGnssCallback::Capabilities::LOW_POWER_MODE)) {
+        ALOGI("Test GetLocationLowPower skipped. LOW_POWER_MODE capability not supported.");
+        return;
+    }
+
+    const int kMinIntervalMsec = 5000;
+    const int kLocationTimeoutSubsequentSec = (kMinIntervalMsec / 1000) * 2;
+    const int kNoLocationPeriodSec = (kMinIntervalMsec / 1000) / 2;
+    const int kLocationsToCheck = 5;
+    const bool kLowPowerMode = true;
+
+    // Warmup period - VTS doesn't have AGPS access via GnssLocationProvider
+    gnss_cb_->location_cbq_.reset();
+    StartAndCheckLocations(kLocationsToCheck);
+    StopAndClearLocations();
+    gnss_cb_->location_cbq_.reset();
+
+    // Start of Low Power Mode test
+    SetPositionMode(kMinIntervalMsec, kLowPowerMode);
+
+    // Don't expect true - as without AGPS access
+    if (!StartAndCheckFirstLocation()) {
+        ALOGW("GetLocationLowPower test - no first low power location received.");
+    }
+
+    for (int i = 1; i < kLocationsToCheck; i++) {
+        // Verify that kMinIntervalMsec is respected by waiting kNoLocationPeriodSec and
+        // ensure that no location is received yet
+
+        gnss_cb_->location_cbq_.retrieve(gnss_cb_->last_location_, kNoLocationPeriodSec);
+        const int locationCalledCount = gnss_cb_->location_cbq_.calledCount();
+
+        // Tolerate (ignore) one extra location right after the first one
+        // to handle startup edge case scheduling limitations in some implementations
+        if ((i == 1) && (locationCalledCount == 2)) {
+            CheckLocation(gnss_cb_->last_location_, true);
+            continue;  // restart the quiet wait period after this too-fast location
+        }
+        EXPECT_LE(locationCalledCount, i);
+        if (locationCalledCount != i) {
+            ALOGW("GetLocationLowPower test - not enough locations received. %d vs. %d expected ",
+                  locationCalledCount, i);
+        }
+
+        if (!gnss_cb_->location_cbq_.retrieve(
+                    gnss_cb_->last_location_,
+                    kLocationTimeoutSubsequentSec - kNoLocationPeriodSec)) {
+            ALOGW("GetLocationLowPower test - timeout awaiting location %d", i);
+        } else {
+            CheckLocation(gnss_cb_->last_location_, true);
+        }
+    }
+
+    StopAndClearLocations();
+}
+
+/*
  * MapConstellationType:
  * Given a GnssConstellationType_2_0 type constellation, maps to its equivalent
  * GnssConstellationType_1_0 type constellation. For constellations that do not have
@@ -537,6 +601,8 @@
  */
 TEST_F(GnssHalTest, BlacklistIndividualSatellites) {
     if (!(gnss_cb_->last_capabilities_ & IGnssCallback::Capabilities::SATELLITE_BLACKLIST)) {
+        ALOGI("Test BlacklistIndividualSatellites skipped. SATELLITE_BLACKLIST capability"
+              " not supported.");
         return;
     }
 
@@ -680,6 +746,7 @@
  */
 TEST_F(GnssHalTest, BlacklistConstellation) {
     if (!(gnss_cb_->last_capabilities_ & IGnssCallback::Capabilities::SATELLITE_BLACKLIST)) {
+        ALOGI("Test BlacklistConstellation skipped. SATELLITE_BLACKLIST capability not supported.");
         return;
     }
 
diff --git a/graphics/composer/2.4/Android.bp b/graphics/composer/2.4/Android.bp
new file mode 100644
index 0000000..0e1bc09
--- /dev/null
+++ b/graphics/composer/2.4/Android.bp
@@ -0,0 +1,23 @@
+// This file is autogenerated by hidl-gen -Landroidbp.
+
+hidl_interface {
+    name: "android.hardware.graphics.composer@2.4",
+    root: "android.hardware",
+    vndk: {
+        enabled: true,
+    },
+    srcs: [
+        "IComposer.hal",
+        "IComposerClient.hal",
+    ],
+    interfaces: [
+        "android.hardware.graphics.common@1.0",
+        "android.hardware.graphics.common@1.1",
+        "android.hardware.graphics.common@1.2",
+        "android.hardware.graphics.composer@2.1",
+        "android.hardware.graphics.composer@2.2",
+        "android.hardware.graphics.composer@2.3",
+        "android.hidl.base@1.0",
+    ],
+    gen_java: false,
+}
diff --git a/graphics/composer/2.4/IComposer.hal b/graphics/composer/2.4/IComposer.hal
new file mode 100644
index 0000000..34801da
--- /dev/null
+++ b/graphics/composer/2.4/IComposer.hal
@@ -0,0 +1,34 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.hardware.graphics.composer@2.4;
+
+import IComposerClient;
+
+import @2.1::Error;
+import @2.3::IComposer;
+
+interface IComposer extends @2.3::IComposer {
+
+    /**
+     * Creates a v2.4 client of the composer. Supersedes @2.3::createClient.
+     *
+     * @return error is NONE upon success. Otherwise,
+     *         NO_RESOURCES when the client could not be created.
+     * @return client is the newly created client.
+     */
+    createClient_2_4() generates (Error error, IComposerClient client);
+};
diff --git a/graphics/composer/2.4/IComposerClient.hal b/graphics/composer/2.4/IComposerClient.hal
new file mode 100644
index 0000000..8fe0976
--- /dev/null
+++ b/graphics/composer/2.4/IComposerClient.hal
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.hardware.graphics.composer@2.4;
+
+import @2.1::Display;
+import @2.1::Error;
+import @2.3::IComposerClient;
+
+interface IComposerClient extends @2.3::IComposerClient {
+
+    /**
+     * Required capabilities which are supported by the display. The
+     * particular set of supported capabilities for a given display may be
+     * retrieved using getDisplayCapabilities.
+     */
+    enum DisplayCapability : uint32_t {
+        /**
+         * Indicates that the display supports protected contents.
+         * When returned, hardware composer must be able to accept client target
+         * with protected buffers.
+         */
+        PROTECTED_CONTENTS = 4,
+    };
+
+    /**
+     * Provides a list of supported capabilities (as described in the
+     * definition of DisplayCapability above). This list must not change after
+     * initialization.
+     *
+     * @return error is NONE upon success. Otherwise,
+     *     BAD_DISPLAY when an invalid display handle was passed in.
+     * @return capabilities is a list of supported capabilities.
+     */
+    getDisplayCapabilities_2_4(Display display)
+              generates (Error error,
+                         vec<DisplayCapability> capabilities);
+};
diff --git a/graphics/composer/2.4/default/Android.bp b/graphics/composer/2.4/default/Android.bp
new file mode 100644
index 0000000..1bf51e0
--- /dev/null
+++ b/graphics/composer/2.4/default/Android.bp
@@ -0,0 +1,47 @@
+//
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+cc_binary {
+    name: "android.hardware.graphics.composer@2.4-service",
+    defaults: ["hidl_defaults"],
+    vendor: true,
+    relative_install_path: "hw",
+    srcs: ["service.cpp"],
+    init_rc: ["android.hardware.graphics.composer@2.4-service.rc"],
+    header_libs: [
+        "android.hardware.graphics.composer@2.4-passthrough",
+    ],
+    shared_libs: [
+        "android.hardware.graphics.composer@2.1",
+        "android.hardware.graphics.composer@2.2",
+        "android.hardware.graphics.composer@2.3",
+        "android.hardware.graphics.composer@2.4",
+        "android.hardware.graphics.mapper@2.0",
+        "android.hardware.graphics.mapper@3.0",
+        "libbase",
+        "libbinder",
+        "libcutils",
+        "libfmq",
+        "libhardware",
+        "libhidlbase",
+        "libhidltransport",
+        "libhwc2on1adapter",
+        "libhwc2onfbadapter",
+        "liblog",
+        "libsync",
+        "libutils",
+    ],
+}
diff --git a/graphics/composer/2.4/default/OWNERS b/graphics/composer/2.4/default/OWNERS
new file mode 100644
index 0000000..cc6d937
--- /dev/null
+++ b/graphics/composer/2.4/default/OWNERS
@@ -0,0 +1,4 @@
+# Graphics team
+lpy@google.com
+stoza@google.com
+vhau@google.com
diff --git a/graphics/composer/2.4/default/android.hardware.graphics.composer@2.4-service.rc b/graphics/composer/2.4/default/android.hardware.graphics.composer@2.4-service.rc
new file mode 100644
index 0000000..a296b0a
--- /dev/null
+++ b/graphics/composer/2.4/default/android.hardware.graphics.composer@2.4-service.rc
@@ -0,0 +1,7 @@
+service vendor.hwcomposer-2-4 /vendor/bin/hw/android.hardware.graphics.composer@2.4-service
+    class hal animation
+    user system
+    group graphics drmrpc
+    capabilities SYS_NICE
+    onrestart restart surfaceflinger
+    writepid /dev/cpuset/system-background/tasks
diff --git a/graphics/composer/2.4/default/service.cpp b/graphics/composer/2.4/default/service.cpp
new file mode 100644
index 0000000..98dac3e
--- /dev/null
+++ b/graphics/composer/2.4/default/service.cpp
@@ -0,0 +1,55 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <sched.h>
+
+#include <android/hardware/graphics/composer/2.4/IComposer.h>
+#include <binder/ProcessState.h>
+#include <composer-passthrough/2.4/HwcLoader.h>
+#include <hidl/HidlTransportSupport.h>
+
+using android::hardware::graphics::composer::V2_4::IComposer;
+using android::hardware::graphics::composer::V2_4::passthrough::HwcLoader;
+
+int main() {
+    // the conventional HAL might start binder services
+    android::ProcessState::initWithDriver("/dev/vndbinder");
+    android::ProcessState::self()->setThreadPoolMaxThreadCount(4);
+    android::ProcessState::self()->startThreadPool();
+
+    // same as SF main thread
+    struct sched_param param = {0};
+    param.sched_priority = 2;
+    if (sched_setscheduler(0, SCHED_FIFO | SCHED_RESET_ON_FORK, &param) != 0) {
+        ALOGE("Couldn't set SCHED_FIFO: %d", errno);
+    }
+
+    android::hardware::configureRpcThreadpool(4, true /* will join */);
+
+    android::sp<IComposer> composer = HwcLoader::load();
+    if (composer == nullptr) {
+        return 1;
+    }
+    if (composer->registerAsService() != android::NO_ERROR) {
+        ALOGE("failed to register service");
+        return 1;
+    }
+
+    android::hardware::joinRpcThreadpool();
+
+    ALOGE("service is terminating");
+    return 1;
+}
diff --git a/graphics/composer/2.4/utils/OWNERS b/graphics/composer/2.4/utils/OWNERS
new file mode 100644
index 0000000..cc6d937
--- /dev/null
+++ b/graphics/composer/2.4/utils/OWNERS
@@ -0,0 +1,4 @@
+# Graphics team
+lpy@google.com
+stoza@google.com
+vhau@google.com
diff --git a/graphics/composer/2.4/utils/command-buffer/Android.bp b/graphics/composer/2.4/utils/command-buffer/Android.bp
new file mode 100644
index 0000000..8acf0e1
--- /dev/null
+++ b/graphics/composer/2.4/utils/command-buffer/Android.bp
@@ -0,0 +1,18 @@
+cc_library_headers {
+    name: "android.hardware.graphics.composer@2.4-command-buffer",
+    defaults: ["hidl_defaults"],
+    vendor_available: true,
+    shared_libs: [
+        "android.hardware.graphics.composer@2.4",
+    ],
+    export_shared_lib_headers: [
+        "android.hardware.graphics.composer@2.4",
+    ],
+    header_libs: [
+        "android.hardware.graphics.composer@2.3-command-buffer",
+    ],
+    export_header_lib_headers: [
+        "android.hardware.graphics.composer@2.3-command-buffer",
+    ],
+    export_include_dirs: ["include"],
+}
diff --git a/graphics/composer/2.4/utils/command-buffer/include/composer-command-buffer/2.4/ComposerCommandBuffer.h b/graphics/composer/2.4/utils/command-buffer/include/composer-command-buffer/2.4/ComposerCommandBuffer.h
new file mode 100644
index 0000000..6b64c16
--- /dev/null
+++ b/graphics/composer/2.4/utils/command-buffer/include/composer-command-buffer/2.4/ComposerCommandBuffer.h
@@ -0,0 +1,58 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifndef LOG_TAG
+#warn "ComposerCommandBuffer.h included without LOG_TAG"
+#endif
+
+#undef LOG_NDEBUG
+#define LOG_NDEBUG 0
+
+#include <android/hardware/graphics/composer/2.4/IComposer.h>
+#include <android/hardware/graphics/composer/2.4/IComposerClient.h>
+#include <composer-command-buffer/2.3/ComposerCommandBuffer.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+
+using android::hardware::MessageQueue;
+using android::hardware::graphics::composer::V2_1::Error;
+using android::hardware::graphics::composer::V2_4::IComposerClient;
+
+// This class helps build a command queue.  Note that all sizes/lengths are in
+// units of uint32_t's.
+class CommandWriterBase : public V2_3::CommandWriterBase {
+  public:
+    CommandWriterBase(uint32_t initialMaxSize) : V2_3::CommandWriterBase(initialMaxSize) {}
+};
+
+// This class helps parse a command queue.  Note that all sizes/lengths are in
+// units of uint32_t's.
+class CommandReaderBase : public V2_3::CommandReaderBase {
+  public:
+    CommandReaderBase() : V2_3::CommandReaderBase(){};
+};
+
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/hal/Android.bp b/graphics/composer/2.4/utils/hal/Android.bp
new file mode 100644
index 0000000..3ee4e19
--- /dev/null
+++ b/graphics/composer/2.4/utils/hal/Android.bp
@@ -0,0 +1,36 @@
+//
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+cc_library_headers {
+    name: "android.hardware.graphics.composer@2.4-hal",
+    defaults: ["hidl_defaults"],
+    vendor_available: true,
+    shared_libs: [
+        "android.hardware.graphics.composer@2.4",
+    ],
+    export_shared_lib_headers: [
+        "android.hardware.graphics.composer@2.4",
+    ],
+    header_libs: [
+        "android.hardware.graphics.composer@2.3-hal",
+        "android.hardware.graphics.composer@2.3-command-buffer",
+    ],
+    export_header_lib_headers: [
+        "android.hardware.graphics.composer@2.3-hal",
+        "android.hardware.graphics.composer@2.3-command-buffer",
+    ],
+    export_include_dirs: ["include"],
+}
diff --git a/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/Composer.h b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/Composer.h
new file mode 100644
index 0000000..129bae6
--- /dev/null
+++ b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/Composer.h
@@ -0,0 +1,89 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifndef LOG_TAG
+#warning "Composer.h included without LOG_TAG"
+#endif
+
+#include <android/hardware/graphics/composer/2.4/IComposer.h>
+#include <composer-hal/2.3/Composer.h>
+#include <composer-hal/2.4/ComposerClient.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace hal {
+
+namespace detail {
+
+// ComposerImpl implements V2_*::IComposer on top of V2_*::ComposerHal
+template <typename Interface, typename Hal>
+class ComposerImpl : public V2_3::hal::detail::ComposerImpl<Interface, Hal> {
+  public:
+    static std::unique_ptr<ComposerImpl> create(std::unique_ptr<Hal> hal) {
+        return std::make_unique<ComposerImpl>(std::move(hal));
+    }
+
+    explicit ComposerImpl(std::unique_ptr<Hal> hal) : BaseType2_3(std::move(hal)) {}
+
+    // IComposer 2.4 interface
+
+    Return<void> createClient_2_4(IComposer::createClient_2_4_cb hidl_cb) override {
+        std::unique_lock<std::mutex> lock(mClientMutex);
+        if (!waitForClientDestroyedLocked(lock)) {
+            hidl_cb(Error::NO_RESOURCES, nullptr);
+            return Void();
+        }
+
+        sp<ComposerClient> client = ComposerClient::create(mHal.get()).release();
+        if (!client) {
+            hidl_cb(Error::NO_RESOURCES, nullptr);
+            return Void();
+        }
+
+        auto clientDestroyed = [this]() { onClientDestroyed(); };
+        client->setOnClientDestroyed(clientDestroyed);
+
+        mClient = client;
+        hidl_cb(Error::NONE, client);
+        return Void();
+    }
+
+  private:
+    using BaseType2_3 = V2_3::hal::detail::ComposerImpl<Interface, Hal>;
+    using BaseType2_1 = V2_1::hal::detail::ComposerImpl<Interface, Hal>;
+
+    using BaseType2_1::mClient;
+    using BaseType2_1::mClientMutex;
+    using BaseType2_1::mHal;
+    using BaseType2_1::onClientDestroyed;
+    using BaseType2_1::waitForClientDestroyedLocked;
+};
+
+}  // namespace detail
+
+using Composer = detail::ComposerImpl<IComposer, ComposerHal>;
+
+}  // namespace hal
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerClient.h b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerClient.h
new file mode 100644
index 0000000..7110c80
--- /dev/null
+++ b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerClient.h
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifndef LOG_TAG
+#warning "ComposerClient.h included without LOG_TAG"
+#endif
+
+#include <android/hardware/graphics/composer/2.4/IComposerClient.h>
+#include <composer-hal/2.4/ComposerHal.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace hal {
+
+namespace detail {
+
+// ComposerClientImpl implements V2_*::IComposerClient on top of V2_*::ComposerHal
+template <typename Interface, typename Hal>
+class ComposerClientImpl : public V2_3::hal::detail::ComposerClientImpl<Interface, Hal> {
+  public:
+    ComposerClientImpl(Hal* hal) : BaseType2_3(hal) {}
+
+    Return<void> getDisplayCapabilities_2_4(
+            Display display, IComposerClient::getDisplayCapabilities_2_4_cb hidl_cb) override {
+        std::vector<IComposerClient::DisplayCapability> capabilities;
+        Error error = mHal->getDisplayCapabilities_2_4(display, &capabilities);
+        hidl_cb(error, capabilities);
+        return Void();
+    }
+
+    static std::unique_ptr<ComposerClientImpl> create(Hal* hal) {
+        auto client = std::make_unique<ComposerClientImpl>(hal);
+        return client->init() ? std::move(client) : nullptr;
+    }
+
+  private:
+    using BaseType2_3 = V2_3::hal::detail::ComposerClientImpl<Interface, Hal>;
+    using BaseType2_1 = V2_1::hal::detail::ComposerClientImpl<Interface, Hal>;
+    using BaseType2_1::mHal;
+};
+
+}  // namespace detail
+
+using ComposerClient = detail::ComposerClientImpl<IComposerClient, ComposerHal>;
+
+}  // namespace hal
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerHal.h b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerHal.h
new file mode 100644
index 0000000..0074808
--- /dev/null
+++ b/graphics/composer/2.4/utils/hal/include/composer-hal/2.4/ComposerHal.h
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <composer-hal/2.3/ComposerHal.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace hal {
+
+using common::V1_1::RenderIntent;
+using common::V1_2::ColorMode;
+using common::V1_2::Dataspace;
+using common::V1_2::Hdr;
+using common::V1_2::PixelFormat;
+using V2_1::Display;
+using V2_1::Error;
+using V2_1::Layer;
+
+class ComposerHal : public V2_3::hal::ComposerHal {
+  public:
+    virtual Error getDisplayCapabilities_2_4(
+            Display display, std::vector<IComposerClient::DisplayCapability>* outCapabilities) = 0;
+};
+
+}  // namespace hal
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/passthrough/Android.bp b/graphics/composer/2.4/utils/passthrough/Android.bp
new file mode 100644
index 0000000..43d9aaa
--- /dev/null
+++ b/graphics/composer/2.4/utils/passthrough/Android.bp
@@ -0,0 +1,30 @@
+//
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+cc_library_headers {
+    name: "android.hardware.graphics.composer@2.4-passthrough",
+    defaults: ["hidl_defaults"],
+    vendor: true,
+    header_libs: [
+        "android.hardware.graphics.composer@2.3-passthrough",
+        "android.hardware.graphics.composer@2.4-hal",
+    ],
+    export_header_lib_headers: [
+        "android.hardware.graphics.composer@2.3-passthrough",
+        "android.hardware.graphics.composer@2.4-hal",
+    ],
+    export_include_dirs: ["include"],
+}
diff --git a/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcHal.h b/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcHal.h
new file mode 100644
index 0000000..65d47d7
--- /dev/null
+++ b/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcHal.h
@@ -0,0 +1,88 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifndef LOG_TAG
+#warning "HwcHal.h included without LOG_TAG"
+#endif
+
+#include <type_traits>
+
+#include <composer-hal/2.4/ComposerHal.h>
+#include <composer-passthrough/2.3/HwcHal.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace passthrough {
+
+namespace detail {
+
+using common::V1_1::RenderIntent;
+using common::V1_2::ColorMode;
+using common::V1_2::Dataspace;
+using common::V1_2::Hdr;
+using common::V1_2::PixelFormat;
+using V2_1::Display;
+using V2_1::Error;
+
+// HwcHalImpl implements V2_*::hal::ComposerHal on top of hwcomposer2
+template <typename Hal>
+class HwcHalImpl : public V2_3::passthrough::detail::HwcHalImpl<Hal> {
+  public:
+    Error getDisplayCapabilities_2_4(
+            Display display,
+            std::vector<IComposerClient::DisplayCapability>* outCapabilities) override {
+        std::vector<V2_3::IComposerClient::DisplayCapability> capabilities;
+        Error error = BaseType2_3::getDisplayCapabilities(display, &capabilities);
+        if (error != Error::NONE) {
+            return error;
+        }
+        outCapabilities->clear();
+        outCapabilities->reserve(capabilities.size());
+        for (auto capability : capabilities) {
+            outCapabilities->push_back(static_cast<IComposerClient::DisplayCapability>(capability));
+        }
+        return Error::NONE;
+    }
+
+  protected:
+    bool initDispatch() override {
+        if (!BaseType2_3::initDispatch()) {
+            return false;
+        }
+        return true;
+    }
+
+  private:
+    using BaseType2_1 = V2_1::passthrough::detail::HwcHalImpl<Hal>;
+    using BaseType2_3 = V2_3::passthrough::detail::HwcHalImpl<Hal>;
+    using BaseType2_1::mDevice;
+};
+
+}  // namespace detail
+
+using HwcHal = detail::HwcHalImpl<hal::ComposerHal>;
+
+}  // namespace passthrough
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcLoader.h b/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcLoader.h
new file mode 100644
index 0000000..a7cc588
--- /dev/null
+++ b/graphics/composer/2.4/utils/passthrough/include/composer-passthrough/2.4/HwcLoader.h
@@ -0,0 +1,79 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#ifndef LOG_TAG
+#warning "HwcLoader.h included without LOG_TAG"
+#endif
+
+#include <composer-hal/2.4/Composer.h>
+#include <composer-hal/2.4/ComposerHal.h>
+#include <composer-passthrough/2.3/HwcLoader.h>
+#include <composer-passthrough/2.4/HwcHal.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace passthrough {
+
+class HwcLoader : public V2_3::passthrough::HwcLoader {
+  public:
+    static IComposer* load() {
+        const hw_module_t* module = loadModule();
+        if (!module) {
+            return nullptr;
+        }
+
+        auto hal = createHalWithAdapter(module);
+        if (!hal) {
+            return nullptr;
+        }
+
+        return createComposer(std::move(hal)).release();
+    }
+
+    // create a ComposerHal instance
+    static std::unique_ptr<hal::ComposerHal> createHal(const hw_module_t* module) {
+        auto hal = std::make_unique<HwcHal>();
+        return hal->initWithModule(module) ? std::move(hal) : nullptr;
+    }
+
+    // create a ComposerHal instance, insert an adapter if necessary
+    static std::unique_ptr<hal::ComposerHal> createHalWithAdapter(const hw_module_t* module) {
+        bool adapted;
+        hwc2_device_t* device = openDeviceWithAdapter(module, &adapted);
+        if (!device) {
+            return nullptr;
+        }
+        auto hal = std::make_unique<HwcHal>();
+        return hal->initWithDevice(std::move(device), !adapted) ? std::move(hal) : nullptr;
+    }
+
+    // create an IComposer instance
+    static std::unique_ptr<IComposer> createComposer(std::unique_ptr<hal::ComposerHal> hal) {
+        return hal::Composer::create(std::move(hal));
+    }
+};
+
+}  // namespace passthrough
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/vts/Android.bp b/graphics/composer/2.4/utils/vts/Android.bp
new file mode 100644
index 0000000..b87a116
--- /dev/null
+++ b/graphics/composer/2.4/utils/vts/Android.bp
@@ -0,0 +1,42 @@
+//
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+cc_library_static {
+    name: "android.hardware.graphics.composer@2.4-vts",
+    defaults: ["hidl_defaults"],
+    srcs: [
+        "ComposerVts.cpp",
+    ],
+    static_libs: [
+        "VtsHalHidlTargetTestBase",
+        "android.hardware.graphics.composer@2.1",
+        "android.hardware.graphics.composer@2.2",
+        "android.hardware.graphics.composer@2.3-vts",
+        "android.hardware.graphics.composer@2.3",
+        "android.hardware.graphics.composer@2.4",
+    ],
+    header_libs: [
+        "android.hardware.graphics.composer@2.1-command-buffer",
+        "android.hardware.graphics.composer@2.2-command-buffer",
+        "android.hardware.graphics.composer@2.3-command-buffer",
+    ],
+    cflags: [
+        "-O0",
+        "-g",
+        "-DLOG_TAG=\"ComposerVts\"",
+    ],
+    export_include_dirs: ["include"],
+}
diff --git a/graphics/composer/2.4/utils/vts/ComposerVts.cpp b/graphics/composer/2.4/utils/vts/ComposerVts.cpp
new file mode 100644
index 0000000..ee4f3a3
--- /dev/null
+++ b/graphics/composer/2.4/utils/vts/ComposerVts.cpp
@@ -0,0 +1,69 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <composer-vts/2.4/ComposerVts.h>
+
+#include <VtsHalHidlTargetTestBase.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace vts {
+
+using V2_1::Error;
+
+Composer::Composer() : Composer(::testing::VtsHalHidlTargetTestBase::getService<IComposer>()) {}
+
+Composer::Composer(const std::string& name)
+    : Composer(::testing::VtsHalHidlTargetTestBase::getService<IComposer>(name)) {}
+
+Composer::Composer(const sp<IComposer>& composer)
+    : V2_3::vts::Composer(composer), mComposer(composer) {}
+
+std::unique_ptr<ComposerClient> Composer::createClient() {
+    std::unique_ptr<ComposerClient> client;
+    mComposer->createClient_2_4([&client](const auto& tmpError, const auto& tmpClient) {
+        ASSERT_EQ(Error::NONE, tmpError) << "failed to create client";
+        client = std::make_unique<ComposerClient>(tmpClient);
+    });
+
+    return client;
+}
+
+sp<IComposerClient> ComposerClient::getRaw() const {
+    return mClient;
+}
+
+Error ComposerClient::getDisplayCapabilities(
+        Display display, std::vector<IComposerClient::DisplayCapability>* outCapabilities) {
+    std::vector<IComposerClient::DisplayCapability> capabilities;
+    Error error = Error::NONE;
+    mClient->getDisplayCapabilities_2_4(display,
+                                        [&](const auto& tmpError, const auto& tmpCapabilities) {
+                                            error = tmpError;
+                                            *outCapabilities = tmpCapabilities;
+                                        });
+    return error;
+}
+
+}  // namespace vts
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/utils/vts/include/composer-vts/2.4/ComposerVts.h b/graphics/composer/2.4/utils/vts/include/composer-vts/2.4/ComposerVts.h
new file mode 100644
index 0000000..0a301c6
--- /dev/null
+++ b/graphics/composer/2.4/utils/vts/include/composer-vts/2.4/ComposerVts.h
@@ -0,0 +1,83 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <memory>
+#include <vector>
+
+#include <VtsHalHidlTargetTestBase.h>
+#include <android/hardware/graphics/composer/2.4/IComposer.h>
+#include <android/hardware/graphics/composer/2.4/IComposerClient.h>
+#include <composer-vts/2.3/ComposerVts.h>
+#include <utils/StrongPointer.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace vts {
+
+using common::V1_1::RenderIntent;
+using common::V1_2::ColorMode;
+using common::V1_2::Dataspace;
+using common::V1_2::Hdr;
+using common::V1_2::PixelFormat;
+using V2_1::Display;
+using V2_1::Error;
+using V2_4::IComposer;
+using V2_4::IComposerClient;
+
+class ComposerClient;
+
+// A wrapper to IComposer.
+class Composer : public V2_3::vts::Composer {
+  public:
+    Composer();
+    explicit Composer(const std::string& name);
+
+    std::unique_ptr<ComposerClient> createClient();
+
+  protected:
+    explicit Composer(const sp<IComposer>& composer);
+
+  private:
+    const sp<IComposer> mComposer;
+};
+
+// A wrapper to IComposerClient.
+class ComposerClient : public V2_3::vts::ComposerClient {
+  public:
+    explicit ComposerClient(const sp<IComposerClient>& client)
+        : V2_3::vts::ComposerClient(client), mClient(client) {}
+
+    sp<IComposerClient> getRaw() const;
+
+    Error getDisplayCapabilities(
+            Display display,
+            std::vector<IComposerClient::DisplayCapability>* outDisplayCapabilities);
+
+  private:
+    const sp<IComposerClient> mClient;
+};
+
+}  // namespace vts
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
diff --git a/graphics/composer/2.4/vts/functional/Android.bp b/graphics/composer/2.4/vts/functional/Android.bp
new file mode 100644
index 0000000..d437f24
--- /dev/null
+++ b/graphics/composer/2.4/vts/functional/Android.bp
@@ -0,0 +1,48 @@
+//
+// Copyright 2019 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+cc_test {
+    name: "VtsHalGraphicsComposerV2_4TargetTest",
+    defaults: ["VtsHalTargetTestDefaults"],
+    srcs: ["VtsHalGraphicsComposerV2_4TargetTest.cpp"],
+
+    // TODO(b/64437680): Assume these libs are always available on the device.
+    shared_libs: [
+        "libfmq",
+        "libhidltransport",
+        "libsync",
+    ],
+    static_libs: [
+        "android.hardware.graphics.allocator@2.0",
+        "android.hardware.graphics.composer@2.1",
+        "android.hardware.graphics.composer@2.1-vts",
+        "android.hardware.graphics.composer@2.2",
+        "android.hardware.graphics.composer@2.2-vts",
+        "android.hardware.graphics.composer@2.3",
+        "android.hardware.graphics.composer@2.3-vts",
+        "android.hardware.graphics.composer@2.4",
+        "android.hardware.graphics.composer@2.4-vts",
+        "android.hardware.graphics.mapper@2.0",
+        "android.hardware.graphics.mapper@2.0-vts",
+        "android.hardware.graphics.mapper@2.1",
+    ],
+    header_libs: [
+        "android.hardware.graphics.composer@2.1-command-buffer",
+        "android.hardware.graphics.composer@2.2-command-buffer",
+        "android.hardware.graphics.composer@2.3-command-buffer",
+        "android.hardware.graphics.composer@2.4-command-buffer",
+    ],
+}
diff --git a/graphics/composer/2.4/vts/functional/OWNERS b/graphics/composer/2.4/vts/functional/OWNERS
new file mode 100644
index 0000000..b3ea6be
--- /dev/null
+++ b/graphics/composer/2.4/vts/functional/OWNERS
@@ -0,0 +1,8 @@
+# Graphics team
+lpy@google.com
+stoza@google.com
+vhau@google.com
+
+# VTS team
+yim@google.com
+zhuoyao@google.com
diff --git a/graphics/composer/2.4/vts/functional/VtsHalGraphicsComposerV2_4TargetTest.cpp b/graphics/composer/2.4/vts/functional/VtsHalGraphicsComposerV2_4TargetTest.cpp
new file mode 100644
index 0000000..0fccc58
--- /dev/null
+++ b/graphics/composer/2.4/vts/functional/VtsHalGraphicsComposerV2_4TargetTest.cpp
@@ -0,0 +1,197 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#define LOG_TAG "graphics_composer_hidl_hal_test@2.4"
+
+#include <algorithm>
+
+#include <VtsHalHidlTargetTestBase.h>
+#include <android-base/logging.h>
+#include <android/hardware/graphics/mapper/2.0/IMapper.h>
+#include <composer-command-buffer/2.4/ComposerCommandBuffer.h>
+#include <composer-vts/2.1/GraphicsComposerCallback.h>
+#include <composer-vts/2.1/TestCommandReader.h>
+#include <composer-vts/2.4/ComposerVts.h>
+#include <mapper-vts/2.0/MapperVts.h>
+
+namespace android {
+namespace hardware {
+namespace graphics {
+namespace composer {
+namespace V2_4 {
+namespace vts {
+namespace {
+
+using common::V1_0::BufferUsage;
+using common::V1_1::RenderIntent;
+using common::V1_2::ColorMode;
+using common::V1_2::Dataspace;
+using common::V1_2::PixelFormat;
+using mapper::V2_0::IMapper;
+using mapper::V2_0::vts::Gralloc;
+
+// Test environment for graphics.composer
+class GraphicsComposerHidlEnvironment : public ::testing::VtsHalHidlTargetTestEnvBase {
+  public:
+    // get the test environment singleton
+    static GraphicsComposerHidlEnvironment* Instance() {
+        static GraphicsComposerHidlEnvironment* instance = new GraphicsComposerHidlEnvironment;
+        return instance;
+    }
+
+    virtual void registerTestServices() override { registerTestService<IComposer>(); }
+
+  private:
+    GraphicsComposerHidlEnvironment() {}
+
+    GTEST_DISALLOW_COPY_AND_ASSIGN_(GraphicsComposerHidlEnvironment);
+};
+
+class GraphicsComposerHidlTest : public ::testing::VtsHalHidlTargetTestBase {
+  protected:
+    void SetUp() override {
+        ASSERT_NO_FATAL_FAILURE(
+                mComposer = std::make_unique<Composer>(
+                        GraphicsComposerHidlEnvironment::Instance()->getServiceName<IComposer>()));
+        ASSERT_NO_FATAL_FAILURE(mComposerClient = mComposer->createClient());
+
+        mComposerCallback = new V2_1::vts::GraphicsComposerCallback;
+        mComposerClient->registerCallback(mComposerCallback);
+
+        // assume the first display is primary and is never removed
+        mPrimaryDisplay = waitForFirstDisplay();
+
+        mInvalidDisplayId = GetInvalidDisplayId();
+
+        // explicitly disable vsync
+        mComposerClient->setVsyncEnabled(mPrimaryDisplay, false);
+        mComposerCallback->setVsyncAllowed(false);
+
+        mWriter = std::make_unique<CommandWriterBase>(1024);
+        mReader = std::make_unique<V2_1::vts::TestCommandReader>();
+    }
+
+    void TearDown() override {
+        ASSERT_EQ(0, mReader->mErrors.size());
+        ASSERT_EQ(0, mReader->mCompositionChanges.size());
+        if (mComposerCallback != nullptr) {
+            EXPECT_EQ(0, mComposerCallback->getInvalidHotplugCount());
+            EXPECT_EQ(0, mComposerCallback->getInvalidRefreshCount());
+            EXPECT_EQ(0, mComposerCallback->getInvalidVsyncCount());
+        }
+    }
+
+    // returns an invalid display id (one that has not been registered to a
+    // display.  Currently assuming that a device will never have close to
+    // std::numeric_limit<uint64_t>::max() displays registered while running tests
+    Display GetInvalidDisplayId() {
+        std::vector<Display> validDisplays = mComposerCallback->getDisplays();
+        uint64_t id = std::numeric_limits<uint64_t>::max();
+        while (id > 0) {
+            if (std::find(validDisplays.begin(), validDisplays.end(), id) == validDisplays.end()) {
+                return id;
+            }
+            id--;
+        }
+
+        return 0;
+    }
+
+    void execute() { mComposerClient->execute(mReader.get(), mWriter.get()); }
+
+    std::unique_ptr<Composer> mComposer;
+    std::unique_ptr<ComposerClient> mComposerClient;
+    sp<V2_1::vts::GraphicsComposerCallback> mComposerCallback;
+    // the first display and is assumed never to be removed
+    Display mPrimaryDisplay;
+    Display mInvalidDisplayId;
+    std::unique_ptr<CommandWriterBase> mWriter;
+    std::unique_ptr<V2_1::vts::TestCommandReader> mReader;
+
+  private:
+    Display waitForFirstDisplay() {
+        while (true) {
+            std::vector<Display> displays = mComposerCallback->getDisplays();
+            if (displays.empty()) {
+                usleep(5 * 1000);
+                continue;
+            }
+
+            return displays[0];
+        }
+    }
+};
+
+// Tests for IComposerClient::Command.
+class GraphicsComposerHidlCommandTest : public GraphicsComposerHidlTest {
+  protected:
+    void SetUp() override {
+        ASSERT_NO_FATAL_FAILURE(GraphicsComposerHidlTest::SetUp());
+
+        ASSERT_NO_FATAL_FAILURE(mGralloc = std::make_unique<Gralloc>());
+
+        mWriter = std::make_unique<CommandWriterBase>(1024);
+        mReader = std::make_unique<V2_1::vts::TestCommandReader>();
+    }
+
+    void TearDown() override {
+        ASSERT_EQ(0, mReader->mErrors.size());
+        ASSERT_NO_FATAL_FAILURE(GraphicsComposerHidlTest::TearDown());
+    }
+
+    const native_handle_t* allocate() {
+        IMapper::BufferDescriptorInfo info{};
+        info.width = 64;
+        info.height = 64;
+        info.layerCount = 1;
+        info.format = static_cast<common::V1_0::PixelFormat>(PixelFormat::RGBA_8888);
+        info.usage =
+                static_cast<uint64_t>(BufferUsage::CPU_WRITE_OFTEN | BufferUsage::CPU_READ_OFTEN);
+
+        return mGralloc->allocate(info);
+    }
+
+    void execute() { mComposerClient->execute(mReader.get(), mWriter.get()); }
+
+    std::unique_ptr<CommandWriterBase> mWriter;
+    std::unique_ptr<V2_1::vts::TestCommandReader> mReader;
+
+  private:
+    std::unique_ptr<Gralloc> mGralloc;
+};
+
+TEST_F(GraphicsComposerHidlTest, getDisplayCapabilitiesBadDisplay) {
+    std::vector<IComposerClient::DisplayCapability> capabilities;
+    const auto error = mComposerClient->getDisplayCapabilities(mInvalidDisplayId, &capabilities);
+    EXPECT_EQ(Error::BAD_DISPLAY, error);
+}
+
+}  // namespace
+}  // namespace vts
+}  // namespace V2_4
+}  // namespace composer
+}  // namespace graphics
+}  // namespace hardware
+}  // namespace android
+
+int main(int argc, char** argv) {
+    using android::hardware::graphics::composer::V2_4::vts::GraphicsComposerHidlEnvironment;
+    ::testing::AddGlobalTestEnvironment(GraphicsComposerHidlEnvironment::Instance());
+    ::testing::InitGoogleTest(&argc, argv);
+    GraphicsComposerHidlEnvironment::Instance()->init(&argc, argv);
+    int status = RUN_ALL_TESTS();
+    return status;
+}
diff --git a/neuralnetworks/1.0/vts/functional/Callbacks.cpp b/neuralnetworks/1.0/vts/functional/Callbacks.cpp
index 2b5723d..37afcf0 100644
--- a/neuralnetworks/1.0/vts/functional/Callbacks.cpp
+++ b/neuralnetworks/1.0/vts/functional/Callbacks.cpp
@@ -14,130 +14,78 @@
  * limitations under the License.
  */
 
+#define LOG_TAG "Callbacks"
+
 #include "1.0/Callbacks.h"
+
 #include <android-base/logging.h>
 
-namespace android {
-namespace hardware {
-namespace neuralnetworks {
-namespace V1_0 {
-namespace implementation {
+namespace android::hardware::neuralnetworks::V1_0::implementation {
 
-CallbackBase::CallbackBase() : mNotified(false) {}
-
-CallbackBase::~CallbackBase() {
-    // Note that we cannot call CallbackBase::join_thread from here:
-    // CallbackBase is intended to be reference counted, and it is possible that
-    // the reference count drops to zero in the bound thread, causing the
-    // bound thread to call this destructor. If a thread tries to join
-    // itself, it throws an exception, producing a message like the
-    // following:
-    //
-    //     terminating with uncaught exception of type std::__1::system_error:
-    //     thread::join failed: Resource deadlock would occur
-}
-
-void CallbackBase::wait() {
-    std::unique_lock<std::mutex> lock(mMutex);
-    mCondition.wait(lock, [this]{return mNotified;});
-    join_thread_locked();
-}
-
-bool CallbackBase::on_finish(std::function<bool(void)> post_work) {
-    std::lock_guard<std::mutex> lock(mMutex);
-    if (mPostWork != nullptr) {
-        LOG(ERROR) << "CallbackBase::on_finish -- a post-work function has already been bound to "
-                   "this callback object";
-        return false;
-    }
-    if (post_work == nullptr) {
-        LOG(ERROR) << "CallbackBase::on_finish -- the new post-work function is invalid";
-        return false;
-    }
-    mPostWork = std::move(post_work);
-    return true;
-}
-
-bool CallbackBase::bind_thread(std::thread&& asyncThread) {
-    std::lock_guard<std::mutex> lock(mMutex);
-    if (mThread.joinable()) {
-        LOG(ERROR) << "CallbackBase::bind_thread -- a thread has already been bound to this "
-                   "callback object";
-        return false;
-    }
-    if (!asyncThread.joinable()) {
-        LOG(ERROR) << "CallbackBase::bind_thread -- the new thread is not joinable";
-        return false;
-    }
-    mThread = std::move(asyncThread);
-    return true;
-}
-
-void CallbackBase::join_thread() {
-    std::lock_guard<std::mutex> lock(mMutex);
-    join_thread_locked();
-}
-
-void CallbackBase::notify() {
-    {
-        std::lock_guard<std::mutex> lock(mMutex);
-        mNotified = true;
-        if (mPostWork != nullptr) {
-            bool success = mPostWork();
-            if (!success) {
-                LOG(ERROR) << "CallbackBase::notify -- post work failed";
-            }
-        }
-    }
-    mCondition.notify_all();
-}
-
-void CallbackBase::join_thread_locked() {
-    if (mThread.joinable()) {
-        mThread.join();
-    }
-}
-
-PreparedModelCallback::PreparedModelCallback() :
-        mErrorStatus(ErrorStatus::GENERAL_FAILURE), mPreparedModel(nullptr) {}
-
-PreparedModelCallback::~PreparedModelCallback() {}
+// PreparedModelCallback methods begin here
 
 Return<void> PreparedModelCallback::notify(ErrorStatus errorStatus,
-                                           const sp<V1_0::IPreparedModel>& preparedModel) {
-    mErrorStatus = errorStatus;
-    mPreparedModel = preparedModel;
-    CallbackBase::notify();
+                                           const sp<IPreparedModel>& preparedModel) {
+    {
+        std::lock_guard<std::mutex> hold(mMutex);
+
+        // quick-return if object has already been notified
+        if (mNotified) {
+            return Void();
+        }
+
+        // store results and mark as notified
+        mErrorStatus = errorStatus;
+        mPreparedModel = preparedModel;
+        mNotified = true;
+    }
+
+    mCondition.notify_all();
     return Void();
 }
 
-ErrorStatus PreparedModelCallback::getStatus() {
+void PreparedModelCallback::wait() const {
+    std::unique_lock<std::mutex> lock(mMutex);
+    mCondition.wait(lock, [this] { return mNotified; });
+}
+
+ErrorStatus PreparedModelCallback::getStatus() const {
     wait();
     return mErrorStatus;
 }
 
-sp<V1_0::IPreparedModel> PreparedModelCallback::getPreparedModel() {
+sp<IPreparedModel> PreparedModelCallback::getPreparedModel() const {
     wait();
     return mPreparedModel;
 }
 
-ExecutionCallback::ExecutionCallback() : mErrorStatus(ErrorStatus::GENERAL_FAILURE) {}
-
-ExecutionCallback::~ExecutionCallback() {}
+// ExecutionCallback methods begin here
 
 Return<void> ExecutionCallback::notify(ErrorStatus errorStatus) {
-    mErrorStatus = errorStatus;
-    CallbackBase::notify();
+    {
+        std::lock_guard<std::mutex> hold(mMutex);
+
+        // quick-return if object has already been notified
+        if (mNotified) {
+            return Void();
+        }
+
+        mErrorStatus = errorStatus;
+        mNotified = true;
+    }
+    mCondition.notify_all();
+
     return Void();
 }
 
-ErrorStatus ExecutionCallback::getStatus() {
+void ExecutionCallback::wait() const {
+    std::unique_lock<std::mutex> lock(mMutex);
+    mCondition.wait(lock, [this] { return mNotified; });
+}
+
+ErrorStatus ExecutionCallback::getStatus() const {
     wait();
     return mErrorStatus;
 }
 
-}  // namespace implementation
-}  // namespace V1_0
-}  // namespace neuralnetworks
-}  // namespace hardware
-}  // namespace android
+}  // namespace android::hardware::neuralnetworks::V1_0::implementation
diff --git a/neuralnetworks/1.0/vts/functional/include/1.0/Callbacks.h b/neuralnetworks/1.0/vts/functional/include/1.0/Callbacks.h
index 36318ea..820bb10 100644
--- a/neuralnetworks/1.0/vts/functional/include/1.0/Callbacks.h
+++ b/neuralnetworks/1.0/vts/functional/include/1.0/Callbacks.h
@@ -17,310 +17,187 @@
 #ifndef ANDROID_HARDWARE_NEURALNETWORKS_V1_0_CALLBACKS_H
 #define ANDROID_HARDWARE_NEURALNETWORKS_V1_0_CALLBACKS_H
 
+#include <android-base/thread_annotations.h>
 #include <android/hardware/neuralnetworks/1.0/IExecutionCallback.h>
 #include <android/hardware/neuralnetworks/1.0/IPreparedModelCallback.h>
 #include <hidl/Status.h>
-#include <chrono>
 #include <condition_variable>
-#include <functional>
 #include <mutex>
-#include <thread>
 
-namespace android {
-namespace hardware {
-namespace neuralnetworks {
-namespace V1_0 {
-namespace implementation {
-
-/**
- * The CallbackBase class is used internally by the NeuralNetworks runtime to
+/*
+ * The Callback classes are used internally by the NeuralNetworks runtime to
  * synchronize between different threads. An asynchronous task is launched
  * paired with a callback object. When a client thread requires the output being
  * generated by the asynchronous task, the client thread can wait for the result
- * and be blocked until it has completed or a timeout condition has been
- * reached. Any wait* may safely be called concurrently, even on the same
- * callback object. When the asynchronous task has finished its workload, it
- * must immediately call "notify". If the asynchronous task has failed to launch,
- * the function that tried to launch the asynchronous task must immediately call
- * "notify". This "notify" call awakens any client threads waiting on the
- * callback object.
+ * and be blocked until it has completed. Any wait may safely be called
+ * concurrently, even on the same callback object. When the asynchronous task
+ * has finished its workload, it must immediately call "notify". If the
+ * asynchronous task has failed to launch, the function that tried to launch the
+ * asynchronous task must immediately call "notify". This "notify" call
+ * awakens any client threads waiting on the callback object.
  *
- * The CallbackBase class implements some of the base synchronization common to
- * both PrepareModelCallback and ExecutionCallback. For consistency, any HIDL
- * callback class must inherit from CallbackBase as well as the HIDL callback
- * interface it implements.
- *
- * This class exists to enable synchronization across HIDL. When synchronization
- * is only required in the same process, consider using std::future, std::mutex,
- * std::condition_variable, or std::experimental::latch instead.
+ * These classes exist to enable synchronization across HIDL. When
+ * synchronization is only required in the same process, consider using
+ * std::future, std::mutex, std::condition_variable, or std::experimental::latch
+ * instead.
  */
-class CallbackBase {
-  public:
-    CallbackBase();
-    ~CallbackBase();
 
-    /**
-     * CallbackBase::wait blocks until notify has been called on the callback
-     * object.
-     */
-    void wait();
-
-    /**
-     * CallbackBase::wait_for blocks until notify has been called on the
-     * callback object or the time duration from the time the wait_for function
-     * was called has expired, whichever comes first.
-     *
-     * @return Status std::cv_status::no_timeout if the callback was notified
-     *                before the time duration expired, std::cv_status::timeout
-     *                otherwise.
-     */
-    template <class Rep, class Period>
-    std::cv_status wait_for(const std::chrono::duration<Rep, Period>& timeout_duration);
-
-    /**
-     * CallbackBase::on_finish binds a function to the callback object. This
-     * bound function will be executed when CallbackBase::notify is called,
-     * before any calls to wait* return. (Note that CallbackBase::wait_for can
-     * return std::cv_status::timeout before CallbackBase::notify is called for
-     * the first time, and hence before the bound function is executed.)
-     *
-     * The bound function must not synchronize with or otherwise access the
-     * callback object it is bound to, as this could cause a deadlock.
-     *
-     * CallbackBase::on_finish can be called at most once on a given callback
-     * object, and the call to CallbackBase::on_finish must finish before
-     * CallbackBase::notify is called.
-     *
-     * @param post_work Function to be invoked the first time
-     *                  CallbackBase::notify is called. Must have a target --
-     *                  i.e., must not compare equal to nullptr. post_work
-     *                  returns true if it successfully completes, false if it
-     *                  fails.
-     * @return bool True if the function was successfully bound, false if
-     *              unsuccessful.
-     *
-     * TODO: Why does the return value of the callback matter?
-     */
-    bool on_finish(std::function<bool(void)> post_work);
-
-    /**
-     * CallbackBase::bind_thread binds a thread to the event for later use by
-     * CallbackBase::join_thread.
-     *
-     * The thread must be passed using std::move.
-     *
-     * Once a thread is bound with CallbackBase::bind_thread, the client code
-     * should ensure that one of the following occurs before the event is
-     * destroyed:
-     * - CallbackBase::join_thread has been called.
-     * - CallbackBase::wait has been called.
-     * - CallbackBase::wait_for has been called and returned other than
-     *   std::cv_status::no_timeout.
-     *
-     * The bound thread shall not call any CallbackBase method with the
-     * exception of CallbackBase::notify, which it must call when the thread has
-     * finished its computation.
-     *
-     * CallbackBase::bind_thread can be called at most once on a given callback
-     * object.
-     *
-     * @param asyncThread Thread to be bound to the callback object. The thread
-     *                    object must represent a thread of execution -- i.e.,
-     *                    asyncThread.joinable() must be true.
-     * @return bool True if successful, false if thread was not properly bound.
-     */
-    bool bind_thread(std::thread&& asyncThread);
-
-    /**
-     * CallbackBase::join_thread ensures that the thread (if any) bound to this
-     * event with CallbackBase::bind_thread has fully finished and cleaned its
-     * resources. It is legal to call this function multiple times, concurrently
-     * or sequentially.
-     */
-    void join_thread();
-
-  protected:
-    /**
-     * CallbackBase::notify enables all prior and future wait* calls on the
-     * callback object to proceed. The call to CallbackBase::notify happens
-     * before any wait* calls on this callback object return (except in the case
-     * of wait_for timing out). The asynchronous call the callback object is
-     * paired with must ensure that any update to state that should be visible
-     * to the caller of wait* happens before the call to CallbackBase::notify.
-     *
-     * CallbackBase::notify must be called exactly once on a given callback
-     * object.
-     */
-    void notify();
-
-  private:
-    // Same as CallbackBase::join_thread but assumes we already hold a lock on
-    // mMutex.
-    void join_thread_locked();
-
-    bool mNotified;
-    std::mutex mMutex;
-    std::condition_variable mCondition;
-    std::function<bool(void)> mPostWork;
-    std::thread mThread;
-};
+namespace android::hardware::neuralnetworks::V1_0::implementation {
 
 /**
  * The PreparedModelCallback class is used to receive the error status of
  * preparing a model as well as the prepared model from a task executing
- * asynchronously with respect to the runtime. If a calling thread calls wait*
+ * asynchronously with respect to the runtime. If a calling thread calls wait
  * or get* on a PreparedModelCallback object and the corresponding asynchronous
  * task has not finished preparing the model, the calling thread will block
- * until the asynchronous task has called notify. For more information on the
- * synchronization behavior, refer to the CallbackBase class.
+ * until the asynchronous task has called notify.
  *
- * This class inherits the basic blocking and signaling calls from
- * CallbackBase, and implements the HIDL notify call from
- * IPreparedModelCallback. This callback object is passed as an argument to
- * IDevice::prepareModel.
+ * If the callback object is notified more than once, only the results of the
+ * first call to notify are used, and the results from subsequent calls are
+ * discarded.
+ *
+ * This callback object is passed as an argument to IDevice::prepareModel*.
  */
-class PreparedModelCallback : public CallbackBase, public IPreparedModelCallback {
+class PreparedModelCallback : public IPreparedModelCallback {
   public:
-    PreparedModelCallback();
-    ~PreparedModelCallback() override;
-
     /**
      * IPreparedModelCallback::notify marks the callback object with the return
      * status of the asynchronous model preparation along with the prepared
-     * model and calls CallbackBase::notify, enabling all prior and future
-     * wait* calls on the PreparedModelCallback object to proceed.
-     * For more information on the synchronization behavior, refer to the
-     * CallbackBase class.
+     * model, and allows all prior and future wait calls on the
+     * PreparedModelCallback object to proceed.
      *
-     * IPreparedModelCallback::notify must be called exactly once on a given
+     * IPreparedModelCallback::notify must be called on a given
      * PreparedModelCallback object.
      *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify are used, and the results from subsequent calls
+     * are discarded.
+     *
      * @param status Error status returned from asynchronously preparing the
-     *               model; will be:
-     *               - NONE if the asynchronous preparation was successful
-     *               - DEVICE_UNAVAILABLE if driver is offline or busy
-     *               - GENERAL_FAILURE if there is an unspecified error
-     *               - INVALID_ARGUMENT if the input model is invalid
+     *     model; will be:
+     *     - NONE if the asynchronous preparation was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - INVALID_ARGUMENT if the input model is invalid
      * @param preparedModel Returned model that has been prepared for execution,
-     *                      nullptr if the model was unable to be prepared.
+     *     nullptr if the model was unable to be prepared.
      */
-    Return<void> notify(ErrorStatus status, const sp<V1_0::IPreparedModel>& preparedModel) override;
+    Return<void> notify(ErrorStatus status, const sp<IPreparedModel>& preparedModel) override;
+
+    /**
+     * PreparedModelCallback::wait blocks until notify has been called on the
+     * callback object.
+     */
+    void wait() const;
 
     /**
      * Retrieves the error status returned from the asynchronous task launched
-     * by IDevice::prepareModel. If IDevice::prepareModel has not finished
+     * by IDevice::prepareModel*. If IDevice::prepareModel* has not finished
      * asynchronously preparing the model, this call will block until the
      * asynchronous task notifies the object.
      *
      * @return status Error status returned from asynchronously preparing the
-     *                model; will be:
-     *                - NONE if the asynchronous preparation was successful
-     *                - DEVICE_UNAVAILABLE if driver is offline or busy
-     *                - GENERAL_FAILURE if there is an unspecified error
-     *                - INVALID_ARGUMENT if the input model is invalid
+     *     model; will be:
+     *     - NONE if the asynchronous preparation was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - INVALID_ARGUMENT if the input model is invalid
      */
-    ErrorStatus getStatus();
+    ErrorStatus getStatus() const;
 
     /**
      * Retrieves the model that has been prepared for execution from the
-     * asynchronous task launched by IDevice::prepareModel. If
-     * IDevice::prepareModel has not finished asynchronously preparing the
+     * asynchronous task launched by IDevice::prepareModel*. If
+     * IDevice::prepareModel* has not finished asynchronously preparing the
      * model, this call will block until the asynchronous task notifies the
      * object.
      *
      * @return preparedModel Returned model that has been prepared for
-     *                       execution, nullptr if the model was unable to be
-     *                       prepared.
+     *     execution, nullptr if the model was unable to be prepared.
      */
-    sp<V1_0::IPreparedModel> getPreparedModel();
+    sp<IPreparedModel> getPreparedModel() const;
 
   private:
-    ErrorStatus mErrorStatus;
-    sp<V1_0::IPreparedModel> mPreparedModel;
+    mutable std::mutex mMutex;
+    mutable std::condition_variable mCondition;
+    bool mNotified GUARDED_BY(mMutex) = false;
+    ErrorStatus mErrorStatus = ErrorStatus::GENERAL_FAILURE;
+    sp<IPreparedModel> mPreparedModel;
 };
 
 /**
- * The ExecutionCallback class is used to receive the error status of the
- * execution from a task executing asynchronously with respect to the runtime.
- * If a calling thread calls wait* or get* on a PreparedModelCallback object and
- * the corresponding asynchronous task has not finished the execution, the
- * calling thread will block until the asynchronous task has called notify.
- * For more information on the synchronization behavior, refer to the
- * CallbackBase class.
+ * The ExecutionCallback class is used to receive the results of the execution
+ * from a task executing asynchronously with respect to the runtime. If a
+ * calling thread calls wait or get* on a ExecutionCallback object and the
+ * corresponding asynchronous task has not finished the execution, the calling
+ * thread will block until the asynchronous task has called notify.
  *
- * This class inherits the basic blocking and signaling calls from
- * CallbackBase, and implements the HIDL notify call from IExecutionCallback.
- * This callback object is passed as an argument to IPreparedModel::execute.
+ * If the callback object is notified more than once, only the results of the
+ * first call to notify are used, and the results from subsequent calls are
+ * discarded.
+ *
+ * This callback object is passed as an argument to IPreparedModel::execute*.
  */
-class ExecutionCallback : public CallbackBase, public IExecutionCallback {
+class ExecutionCallback : public IExecutionCallback {
   public:
-    ExecutionCallback();
-    ~ExecutionCallback() override;
-
     /**
      * IExecutionCallback::notify marks the callback object with the return
-     * status of the asynchronous execution that held this callback and enable
-     * all prior and future wait* calls on the ExecutionCallback object to
-     * proceed. For more information on the synchronization behavior, refer to
-     * the CallbackBase class.
+     * status of the asynchronous execution that held this callback and enables
+     * all prior and future wait calls on the ExecutionCallback object to
+     * proceed.
      *
-     * IExecutionCallback::notify must be called exactly once on a given
-     * ExecutionCallback object.
+     * IExecutionCallback::notify must be called on a given ExecutionCallback
+     * object.
+     *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify are used, and the results from subsequent calls
+     * are discarded.
      *
      * @param status Error status returned from launching the asynchronous task
-     *               (if the launch fails) or from the asynchronous task itself
-     *               (if the launch succeeds). Must be:
-     *               - NONE if the asynchronous execution was successful
-     *               - DEVICE_UNAVAILABLE if driver is offline or busy
-     *               - GENERAL_FAILURE if there is an unspecified error
-     *               - OUTPUT_INSUFFICIENT_SIZE if provided output buffer is
-     *                 not large enough to store the resultant values
-     *               - INVALID_ARGUMENT if the input request is invalid
+     *     (if the launch fails) or from the asynchronous task itself (if the
+     *     launch succeeds). Must be:
+     *     - NONE if the asynchronous execution was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - OUTPUT_INSUFFICIENT_SIZE if provided output buffer is not large
+     *         enough to store the resultant values
+     *     - INVALID_ARGUMENT if the input request is invalid
      */
     Return<void> notify(ErrorStatus status) override;
 
     /**
+     * ExecutionCallback::wait blocks until notify has been called on the
+     * callback object.
+     */
+    void wait() const;
+
+    /**
      * Retrieves the error status returned from the asynchronous task launched
      * by IPreparedModel::execute. If IPreparedModel::execute has not finished
      * asynchronously executing, this call will block until the asynchronous
      * task notifies the object.
      *
      * @return status Error status returned from launching the asynchronous task
-     *                (if the launch fails) or from the asynchronous task itself
-     *                (if the launch succeeds). Must be:
-     *                - NONE if the asynchronous execution was successful
-     *                - DEVICE_UNAVAILABLE if driver is offline or busy
-     *                - GENERAL_FAILURE if the asynchronous task resulted in an
-     *                  unspecified error
-     *                - OUTPUT_INSUFFICIENT_SIZE if at least one output
-     *                  operand buffer is not large enough to store the
-     *                  corresponding output
-     *                - INVALID_ARGUMENT if one of the input arguments to
-     *                  prepareModel is invalid
+     *     (if the launch fails) or from the asynchronous task itself (if the
+     *     launch succeeds). Must be:
+     *     - NONE if the asynchronous execution was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if the asynchronous task resulted in an unspecified
+     *         error
+     *     - OUTPUT_INSUFFICIENT_SIZE if at least one output operand buffer is
+     *         not large enough to store the corresponding output
+     *     - INVALID_ARGUMENT if one of the input arguments to prepareModel is
+     *         invalid
      */
-    ErrorStatus getStatus();
+    ErrorStatus getStatus() const;
 
   private:
+    mutable std::mutex mMutex;
+    mutable std::condition_variable mCondition;
+    bool mNotified GUARDED_BY(mMutex) = false;
     ErrorStatus mErrorStatus = ErrorStatus::GENERAL_FAILURE;
 };
 
-// template function implementation(s) below this point
-
-template <class Rep, class Period>
-std::cv_status CallbackBase::wait_for(const std::chrono::duration<Rep, Period>& timeout_duration) {
-    std::unique_lock<std::mutex> lock(mMutex);
-    std::cv_status status =
-            mCondition.wait_for(lock, timeout_duration, [this] { return mNotified; });
-    if (status != std::cv_status::timeout) {
-        join_thread_locked();
-    }
-    return status;
-}
-
-}  // namespace implementation
-}  // namespace V1_0
-}  // namespace neuralnetworks
-}  // namespace hardware
-}  // namespace android
+}  // namespace android::hardware::neuralnetworks::V1_0::implementation
 
 #endif  // ANDROID_HARDWARE_NEURALNETWORKS_V1_0_CALLBACKS_H
diff --git a/neuralnetworks/1.2/vts/functional/Callbacks.cpp b/neuralnetworks/1.2/vts/functional/Callbacks.cpp
index cfaf91d..a607a08 100644
--- a/neuralnetworks/1.2/vts/functional/Callbacks.cpp
+++ b/neuralnetworks/1.2/vts/functional/Callbacks.cpp
@@ -14,160 +14,128 @@
  * limitations under the License.
  */
 
+#define LOG_TAG "Callbacks"
+
 #include "1.2/Callbacks.h"
+
 #include <android-base/logging.h>
 
-namespace android {
-namespace hardware {
-namespace neuralnetworks {
-namespace V1_2 {
-namespace implementation {
+#include <limits>
 
-CallbackBase::CallbackBase() : mNotified(false) {}
+namespace android::hardware::neuralnetworks::V1_2::implementation {
 
-CallbackBase::~CallbackBase() {
-    // Note that we cannot call CallbackBase::join_thread from here:
-    // CallbackBase is intended to be reference counted, and it is possible that
-    // the reference count drops to zero in the bound thread, causing the
-    // bound thread to call this destructor. If a thread tries to join
-    // itself, it throws an exception, producing a message like the
-    // following:
-    //
-    //     terminating with uncaught exception of type std::__1::system_error:
-    //     thread::join failed: Resource deadlock would occur
-}
+constexpr Timing kNoTiming = {.timeOnDevice = std::numeric_limits<uint64_t>::max(),
+                              .timeInDriver = std::numeric_limits<uint64_t>::max()};
 
-void CallbackBase::wait() {
-    std::unique_lock<std::mutex> lock(mMutex);
-    mCondition.wait(lock, [this] { return mNotified; });
-    join_thread_locked();
-}
-
-bool CallbackBase::on_finish(std::function<bool(void)> post_work) {
-    std::lock_guard<std::mutex> lock(mMutex);
-    if (mPostWork != nullptr) {
-        LOG(ERROR) << "CallbackBase::on_finish -- a post-work function has already been bound to "
-                      "this callback object";
-        return false;
-    }
-    if (post_work == nullptr) {
-        LOG(ERROR) << "CallbackBase::on_finish -- the new post-work function is invalid";
-        return false;
-    }
-    mPostWork = std::move(post_work);
-    return true;
-}
-
-bool CallbackBase::bind_thread(std::thread&& asyncThread) {
-    std::lock_guard<std::mutex> lock(mMutex);
-    if (mThread.joinable()) {
-        LOG(ERROR) << "CallbackBase::bind_thread -- a thread has already been bound to this "
-                      "callback object";
-        return false;
-    }
-    if (!asyncThread.joinable()) {
-        LOG(ERROR) << "CallbackBase::bind_thread -- the new thread is not joinable";
-        return false;
-    }
-    mThread = std::move(asyncThread);
-    return true;
-}
-
-void CallbackBase::join_thread() {
-    std::lock_guard<std::mutex> lock(mMutex);
-    join_thread_locked();
-}
-
-void CallbackBase::notify() {
-    {
-        std::lock_guard<std::mutex> lock(mMutex);
-        mNotified = true;
-        if (mPostWork != nullptr) {
-            bool success = mPostWork();
-            if (!success) {
-                LOG(ERROR) << "CallbackBase::notify -- post work failed";
-            }
-        }
-    }
-    mCondition.notify_all();
-}
-
-void CallbackBase::join_thread_locked() {
-    if (mThread.joinable()) {
-        mThread.join();
-    }
-}
-
-PreparedModelCallback::PreparedModelCallback()
-    : mErrorStatus(ErrorStatus::GENERAL_FAILURE), mPreparedModel(nullptr) {}
-
-PreparedModelCallback::~PreparedModelCallback() {}
+// PreparedModelCallback methods begin here
 
 Return<void> PreparedModelCallback::notify(ErrorStatus errorStatus,
                                            const sp<V1_0::IPreparedModel>& preparedModel) {
-    mErrorStatus = errorStatus;
-    mPreparedModel = preparedModel;
-    CallbackBase::notify();
+    {
+        std::lock_guard<std::mutex> hold(mMutex);
+
+        // quick-return if object has already been notified
+        if (mNotified) {
+            return Void();
+        }
+
+        // store results and mark as notified
+        mErrorStatus = errorStatus;
+        mPreparedModel = preparedModel;
+        mNotified = true;
+    }
+
+    mCondition.notify_all();
     return Void();
 }
 
 Return<void> PreparedModelCallback::notify_1_2(ErrorStatus errorStatus,
                                                const sp<V1_2::IPreparedModel>& preparedModel) {
-    mErrorStatus = errorStatus;
-    mPreparedModel = preparedModel;
-    CallbackBase::notify();
-    return Void();
+    return notify(errorStatus, preparedModel);
 }
 
-ErrorStatus PreparedModelCallback::getStatus() {
+void PreparedModelCallback::wait() const {
+    std::unique_lock<std::mutex> lock(mMutex);
+    mCondition.wait(lock, [this] { return mNotified; });
+}
+
+ErrorStatus PreparedModelCallback::getStatus() const {
     wait();
     return mErrorStatus;
 }
 
-sp<V1_0::IPreparedModel> PreparedModelCallback::getPreparedModel() {
+sp<V1_0::IPreparedModel> PreparedModelCallback::getPreparedModel() const {
     wait();
     return mPreparedModel;
 }
 
-ExecutionCallback::ExecutionCallback() : mErrorStatus(ErrorStatus::GENERAL_FAILURE) {}
-
-ExecutionCallback::~ExecutionCallback() {}
+// ExecutionCallback methods begin here
 
 Return<void> ExecutionCallback::notify(ErrorStatus errorStatus) {
-    mErrorStatus = errorStatus;
-    mOutputShapes = {};
-    mTiming = {.timeOnDevice = UINT64_MAX, .timeInDriver = UINT64_MAX};
-    CallbackBase::notify();
+    notifyInternal(errorStatus, {}, kNoTiming);
     return Void();
 }
 
 Return<void> ExecutionCallback::notify_1_2(ErrorStatus errorStatus,
                                            const hidl_vec<OutputShape>& outputShapes,
                                            const Timing& timing) {
-    mErrorStatus = errorStatus;
-    mOutputShapes = outputShapes;
-    mTiming = timing;
-    CallbackBase::notify();
+    if (errorStatus == ErrorStatus::OUTPUT_INSUFFICIENT_SIZE) {
+        // outputShapes must not be empty if OUTPUT_INSUFFICIENT_SIZE.
+        if (outputShapes.size() == 0) {
+            LOG(ERROR) << "Notified with empty output shape vector when OUTPUT_INSUFFICIENT_SIZE";
+            notifyInternal(ErrorStatus::GENERAL_FAILURE, {}, kNoTiming);
+            return Void();
+        }
+    } else if (errorStatus != ErrorStatus::NONE) {
+        // outputShapes must be empty if errorStatus is neither NONE nor OUTPUT_INSUFFICIENT_SIZE.
+        if (outputShapes.size() != 0) {
+            LOG(ERROR) << "Notified with non-empty output shape vector when error status is "
+                          "neither NONE nor OUTPUT_INSUFFICIENT_SIZE";
+            notifyInternal(ErrorStatus::GENERAL_FAILURE, {}, kNoTiming);
+            return Void();
+        }
+    }
+    notifyInternal(errorStatus, outputShapes, timing);
     return Void();
 }
 
-ErrorStatus ExecutionCallback::getStatus() {
+void ExecutionCallback::wait() const {
+    std::unique_lock<std::mutex> lock(mMutex);
+    mCondition.wait(lock, [this] { return mNotified; });
+}
+
+ErrorStatus ExecutionCallback::getStatus() const {
     wait();
     return mErrorStatus;
 }
 
-const std::vector<OutputShape>& ExecutionCallback::getOutputShapes() {
+const std::vector<OutputShape>& ExecutionCallback::getOutputShapes() const {
     wait();
     return mOutputShapes;
 }
 
-Timing ExecutionCallback::getTiming() {
+Timing ExecutionCallback::getTiming() const {
     wait();
     return mTiming;
 }
 
-}  // namespace implementation
-}  // namespace V1_2
-}  // namespace neuralnetworks
-}  // namespace hardware
-}  // namespace android
+void ExecutionCallback::notifyInternal(ErrorStatus errorStatus,
+                                       const hidl_vec<OutputShape>& outputShapes,
+                                       const Timing& timing) {
+    {
+        std::lock_guard<std::mutex> hold(mMutex);
+
+        // quick-return if object has already been notified
+        if (mNotified) {
+            return;
+        }
+
+        mErrorStatus = errorStatus;
+        mOutputShapes = outputShapes;
+        mTiming = timing;
+        mNotified = true;
+    }
+    mCondition.notify_all();
+}
+
+}  // namespace android::hardware::neuralnetworks::V1_2::implementation
diff --git a/neuralnetworks/1.2/vts/functional/CompilationCachingTests.cpp b/neuralnetworks/1.2/vts/functional/CompilationCachingTests.cpp
index adbf224..ac92a5b 100644
--- a/neuralnetworks/1.2/vts/functional/CompilationCachingTests.cpp
+++ b/neuralnetworks/1.2/vts/functional/CompilationCachingTests.cpp
@@ -26,6 +26,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <random>
+#include <thread>
 
 #include "1.2/Callbacks.h"
 #include "GeneratedTestHarness.h"
diff --git a/neuralnetworks/1.2/vts/functional/include/1.2/Callbacks.h b/neuralnetworks/1.2/vts/functional/include/1.2/Callbacks.h
index 212a887..2992c0c 100644
--- a/neuralnetworks/1.2/vts/functional/include/1.2/Callbacks.h
+++ b/neuralnetworks/1.2/vts/functional/include/1.2/Callbacks.h
@@ -17,299 +17,218 @@
 #ifndef ANDROID_HARDWARE_NEURALNETWORKS_V1_2_CALLBACKS_H
 #define ANDROID_HARDWARE_NEURALNETWORKS_V1_2_CALLBACKS_H
 
+#include <android-base/thread_annotations.h>
+#include <android/hardware/neuralnetworks/1.0/IExecutionCallback.h>
+#include <android/hardware/neuralnetworks/1.0/IPreparedModelCallback.h>
 #include <android/hardware/neuralnetworks/1.2/IExecutionCallback.h>
 #include <android/hardware/neuralnetworks/1.2/IPreparedModelCallback.h>
 #include <hidl/Status.h>
-#include <chrono>
 #include <condition_variable>
-#include <functional>
 #include <mutex>
-#include <thread>
 
-namespace android {
-namespace hardware {
-namespace neuralnetworks {
-namespace V1_2 {
-namespace implementation {
+/*
+ * The Callback classes are used internally by the NeuralNetworks runtime to
+ * synchronize between different threads. An asynchronous task is launched
+ * paired with a callback object. When a client thread requires the output being
+ * generated by the asynchronous task, the client thread can wait for the result
+ * and be blocked until it has completed. Any wait may safely be called
+ * concurrently, even on the same callback object. When the asynchronous task
+ * has finished its workload, it must immediately call "notify*". If the
+ * asynchronous task has failed to launch, the function that tried to launch the
+ * asynchronous task must immediately call "notify*". This "notify*" call
+ * awakens any client threads waiting on the callback object.
+ *
+ * These classes exist to enable synchronization across HIDL. When
+ * synchronization is only required in the same process, consider using
+ * std::future, std::mutex, std::condition_variable, or std::experimental::latch
+ * instead.
+ */
+
+namespace android::hardware::neuralnetworks::V1_2::implementation {
 
 using V1_0::ErrorStatus;
 
 /**
- * The CallbackBase class is used internally by the NeuralNetworks runtime to
- * synchronize between different threads. An asynchronous task is launched
- * paired with a callback object. When a client thread requires the output being
- * generated by the asynchronous task, the client thread can wait for the result
- * and be blocked until it has completed or a timeout condition has been
- * reached. Any wait* may safely be called concurrently, even on the same
- * callback object. When the asynchronous task has finished its workload, it
- * must immediately call "notify". If the asynchronous task has failed to launch,
- * the function that tried to launch the asynchronous task must immediately call
- * "notify". This "notify" call awakens any client threads waiting on the
- * callback object.
- *
- * The CallbackBase class implements some of the base synchronization common to
- * both PrepareModelCallback and ExecutionCallback. For consistency, any HIDL
- * callback class must inherit from CallbackBase as well as the HIDL callback
- * interface it implements.
- *
- * This class exists to enable synchronization across HIDL. When synchronization
- * is only required in the same process, consider using std::future, std::mutex,
- * std::condition_variable, or std::experimental::latch instead.
- */
-class CallbackBase {
-  public:
-    CallbackBase();
-    ~CallbackBase();
-
-    /**
-     * CallbackBase::wait blocks until notify has been called on the callback
-     * object.
-     */
-    void wait();
-
-    /**
-     * CallbackBase::wait_for blocks until notify has been called on the
-     * callback object or the time duration from the time the wait_for function
-     * was called has expired, whichever comes first.
-     *
-     * @return Status std::cv_status::no_timeout if the callback was notified
-     *                before the time duration expired, std::cv_status::timeout
-     *                otherwise.
-     */
-    template <class Rep, class Period>
-    std::cv_status wait_for(const std::chrono::duration<Rep, Period>& timeout_duration);
-
-    /**
-     * CallbackBase::on_finish binds a function to the callback object. This
-     * bound function will be executed when CallbackBase::notify is called,
-     * before any calls to wait* return. (Note that CallbackBase::wait_for can
-     * return std::cv_status::timeout before CallbackBase::notify is called for
-     * the first time, and hence before the bound function is executed.)
-     *
-     * The bound function must not synchronize with or otherwise access the
-     * callback object it is bound to, as this could cause a deadlock.
-     *
-     * CallbackBase::on_finish can be called at most once on a given callback
-     * object, and the call to CallbackBase::on_finish must finish before
-     * CallbackBase::notify is called.
-     *
-     * @param post_work Function to be invoked the first time
-     *                  CallbackBase::notify is called. Must have a target --
-     *                  i.e., must not compare equal to nullptr. post_work
-     *                  returns true if it successfully completes, false if it
-     *                  fails.
-     * @return bool True if the function was successfully bound, false if
-     *              unsuccessful.
-     *
-     * TODO: Why does the return value of the callback matter?
-     */
-    bool on_finish(std::function<bool(void)> post_work);
-
-    /**
-     * CallbackBase::bind_thread binds a thread to the event for later use by
-     * CallbackBase::join_thread.
-     *
-     * The thread must be passed using std::move.
-     *
-     * Once a thread is bound with CallbackBase::bind_thread, the client code
-     * should ensure that one of the following occurs before the event is
-     * destroyed:
-     * - CallbackBase::join_thread has been called.
-     * - CallbackBase::wait has been called.
-     * - CallbackBase::wait_for has been called and returned other than
-     *   std::cv_status::no_timeout.
-     *
-     * The bound thread shall not call any CallbackBase method with the
-     * exception of CallbackBase::notify, which it must call when the thread has
-     * finished its computation.
-     *
-     * CallbackBase::bind_thread can be called at most once on a given callback
-     * object.
-     *
-     * @param asyncThread Thread to be bound to the callback object. The thread
-     *                    object must represent a thread of execution -- i.e.,
-     *                    asyncThread.joinable() must be true.
-     * @return bool True if successful, false if thread was not properly bound.
-     */
-    bool bind_thread(std::thread&& asyncThread);
-
-    /**
-     * CallbackBase::join_thread ensures that the thread (if any) bound to this
-     * event with CallbackBase::bind_thread has fully finished and cleaned its
-     * resources. It is legal to call this function multiple times, concurrently
-     * or sequentially.
-     */
-    void join_thread();
-
-  protected:
-    /**
-     * CallbackBase::notify enables all prior and future wait* calls on the
-     * callback object to proceed. The call to CallbackBase::notify happens
-     * before any wait* calls on this callback object return (except in the case
-     * of wait_for timing out). The asynchronous call the callback object is
-     * paired with must ensure that any update to state that should be visible
-     * to the caller of wait* happens before the call to CallbackBase::notify.
-     *
-     * CallbackBase::notify must be called exactly once on a given callback
-     * object.
-     */
-    void notify();
-
-  private:
-    // Same as CallbackBase::join_thread but assumes we already hold a lock on
-    // mMutex.
-    void join_thread_locked();
-
-    bool mNotified;
-    std::mutex mMutex;
-    std::condition_variable mCondition;
-    std::function<bool(void)> mPostWork;
-    std::thread mThread;
-};
-
-/**
  * The PreparedModelCallback class is used to receive the error status of
  * preparing a model as well as the prepared model from a task executing
- * asynchronously with respect to the runtime. If a calling thread calls wait*
+ * asynchronously with respect to the runtime. If a calling thread calls wait
  * or get* on a PreparedModelCallback object and the corresponding asynchronous
  * task has not finished preparing the model, the calling thread will block
- * until the asynchronous task has either called notify or notify_1_2. For more
- * information on the synchronization behavior, refer to the CallbackBase class.
+ * until the asynchronous task has either called notify or notify_1_2.
  *
- * This class inherits the basic blocking and signaling calls from
- * CallbackBase, and implements the HIDL notify and notify_1_2 calls from
- * IPreparedModelCallback. This callback object is passed as an argument to
- * IDevice::prepareModel.
+ * If the callback object is notified more than once, only the results of the
+ * first call to notify* are used, and the results from subsequent calls are
+ * discarded.
+ *
+ * This callback object is passed as an argument to IDevice::prepareModel*.
  */
-class PreparedModelCallback : public CallbackBase, public IPreparedModelCallback {
+class PreparedModelCallback : public IPreparedModelCallback {
   public:
-    PreparedModelCallback();
-    ~PreparedModelCallback() override;
-
     /**
-     * IPreparedModelCallback::notify and IPreparedModelCallback::notify_1_2
-     * mark the callback object with the return status of the asynchronous
-     * model preparation along with the prepared model, and call
-     * CallbackBase::notify, enabling all prior and future wait* calls on the
-     * PreparedModelCallback object to proceed. For more information on the
-     * synchronization behavior, refer to the CallbackBase class.
+     * IPreparedModelCallback::notify marks the callback object with the return
+     * status of the asynchronous model preparation along with the prepared
+     * model, and allows all prior and future wait calls on the
+     * PreparedModelCallback object to proceed.
      *
-     * Either IPreparedModelCallback::notify or IPreparedModelCallback::notify_1_2
-     * must be called exactly once on a given PreparedModelCallback object.
+     * Either IPreparedModelCallback::notify or
+     * IPreparedModelCallback::notify_1_2 must be called on a given
+     * PreparedModelCallback object.
+     *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify* are used, and the results from subsequent calls
+     * are discarded.
      *
      * @param status Error status returned from asynchronously preparing the
-     *               model; will be:
-     *               - NONE if the asynchronous preparation was successful
-     *               - DEVICE_UNAVAILABLE if driver is offline or busy
-     *               - GENERAL_FAILURE if there is an unspecified error
-     *               - INVALID_ARGUMENT if the input model is invalid
+     *     model; will be:
+     *     - NONE if the asynchronous preparation was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - INVALID_ARGUMENT if the input model is invalid
      * @param preparedModel Returned model that has been prepared for execution,
-     *                      nullptr if the model was unable to be prepared.
+     *     nullptr if the model was unable to be prepared.
      */
     Return<void> notify(ErrorStatus status, const sp<V1_0::IPreparedModel>& preparedModel) override;
+
+    /**
+     * IPreparedModelCallback::notify_1_2 marks the callback object with the
+     * return status of the asynchronous model preparation along with the
+     * prepared model, and allows all prior and future wait calls on the
+     * PreparedModelCallback object to proceed.
+     *
+     * Either IPreparedModelCallback::notify or
+     * IPreparedModelCallback::notify_1_2 must be called on a given
+     * PreparedModelCallback object.
+     *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify* are used, and the results from subsequent calls
+     * are discarded.
+     *
+     * @param status Error status returned from asynchronously preparing the
+     *     model; will be:
+     *     - NONE if the asynchronous preparation was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - INVALID_ARGUMENT if the input model is invalid
+     * @param preparedModel Returned model that has been prepared for execution,
+     *     nullptr if the model was unable to be prepared.
+     */
     Return<void> notify_1_2(ErrorStatus status,
                             const sp<V1_2::IPreparedModel>& preparedModel) override;
 
     /**
+     * PreparedModelCallback::wait blocks until notify* has been called on the
+     * callback object.
+     */
+    void wait() const;
+
+    /**
      * Retrieves the error status returned from the asynchronous task launched
-     * by IDevice::prepareModel. If IDevice::prepareModel has not finished
+     * by IDevice::prepareModel*. If IDevice::prepareModel* has not finished
      * asynchronously preparing the model, this call will block until the
      * asynchronous task notifies the object.
      *
      * @return status Error status returned from asynchronously preparing the
-     *                model; will be:
-     *                - NONE if the asynchronous preparation was successful
-     *                - DEVICE_UNAVAILABLE if driver is offline or busy
-     *                - GENERAL_FAILURE if there is an unspecified error
-     *                - INVALID_ARGUMENT if the input model is invalid
+     *     model; will be:
+     *     - NONE if the asynchronous preparation was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - INVALID_ARGUMENT if the input model is invalid
      */
-    ErrorStatus getStatus();
+    ErrorStatus getStatus() const;
 
     /**
      * Retrieves the model that has been prepared for execution from the
-     * asynchronous task launched by IDevice::prepareModel. If
-     * IDevice::prepareModel has not finished asynchronously preparing the
+     * asynchronous task launched by IDevice::prepareModel*. If
+     * IDevice::prepareModel* has not finished asynchronously preparing the
      * model, this call will block until the asynchronous task notifies the
      * object.
      *
      * @return preparedModel Returned model that has been prepared for
-     *                       execution, nullptr if the model was unable to be
-     *                       prepared.
+     *     execution, nullptr if the model was unable to be prepared.
      */
-    sp<V1_0::IPreparedModel> getPreparedModel();
+    sp<V1_0::IPreparedModel> getPreparedModel() const;
 
   private:
-    ErrorStatus mErrorStatus;
+    mutable std::mutex mMutex;
+    mutable std::condition_variable mCondition;
+    bool mNotified GUARDED_BY(mMutex) = false;
+    ErrorStatus mErrorStatus = ErrorStatus::GENERAL_FAILURE;
     sp<V1_0::IPreparedModel> mPreparedModel;
 };
 
 /**
- * The ExecutionCallback class is used to receive the error status of the
- * execution from a task executing asynchronously with respect to the runtime.
- * If a calling thread calls wait* or get* on a PreparedModelCallback object and
- * the corresponding asynchronous task has not finished the execution, the
- * calling thread will block until the asynchronous task has either called notify
- * or notify_1_2. For more information on the synchronization behavior, refer to
- * the CallbackBase class.
+ * The ExecutionCallback class is used to receive the results of the execution
+ * from a task executing asynchronously with respect to the runtime. If a
+ * calling thread calls wait or get* on a ExecutionCallback object and the
+ * corresponding asynchronous task has not finished the execution, the calling
+ * thread will block until the asynchronous task has either called notify or
+ * notify_1_2.
  *
- * This class inherits the basic blocking and signaling calls from
- * CallbackBase, and implements the HIDL notify and notify_1_2 calls from
- * IExecutionCallback. This callback object is passed as an argument to
- * IPreparedModel::execute.
+ * If the callback object is notified more than once, only the results of the
+ * first call to notify* are used, and the results from subsequent calls are
+ * discarded.
+ *
+ * This callback object is passed as an argument to IPreparedModel::execute*.
  */
-class ExecutionCallback : public CallbackBase, public IExecutionCallback {
+class ExecutionCallback : public IExecutionCallback {
   public:
-    ExecutionCallback();
-    ~ExecutionCallback() override;
-
     /**
-     * IExecutionCallback::notify and IExecutionCallback::notify_1_2 mark the
-     * callback object with the return status of the asynchronous execution that
-     * held this callback and enable all prior and future wait* calls on the
-     * ExecutionCallback object to proceed. For more information on the
-     * synchronization behavior, refer to the CallbackBase class.
+     * IExecutionCallback::notify marks the callback object with the return
+     * status of the asynchronous execution that held this callback and enables
+     * all prior and future wait calls on the ExecutionCallback object to
+     * proceed.
      *
      * Either IExecutionCallback::notify or IExecutionCallback::notify_1_2 must
-     * be called exactly once on a given ExecutionCallback object.
+     * be called on a given ExecutionCallback object.
+     *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify* are used, and the results from subsequent calls
+     * are discarded.
      *
      * @param status Error status returned from launching the asynchronous task
-     *               (if the launch fails) or from the asynchronous task itself
-     *               (if the launch succeeds). Must be:
-     *               - NONE if the asynchronous execution was successful
-     *               - DEVICE_UNAVAILABLE if driver is offline or busy
-     *               - GENERAL_FAILURE if there is an unspecified error
-     *               - OUTPUT_INSUFFICIENT_SIZE if provided output buffer is
-     *                 not large enough to store the resultant values
-     *               - INVALID_ARGUMENT if the input request is invalid
+     *     (if the launch fails) or from the asynchronous task itself (if the
+     *     launch succeeds). Must be:
+     *     - NONE if the asynchronous execution was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if there is an unspecified error
+     *     - OUTPUT_INSUFFICIENT_SIZE if provided output buffer is not large
+     *         enough to store the resultant values
+     *     - INVALID_ARGUMENT if the input request is invalid
      */
     Return<void> notify(ErrorStatus status) override;
 
     /**
-     * Similar to IExecutionCallback::notify, but for V1_2::IPreparedModel to
-     * also notify output shapes along with error status.
+     * IExecutionCallback::notify_1_2 marks the callback object with the results
+     * (error status, dynamic output shapes, and timing information) of the
+     * asynchronous execution that held this callback and enables all prior and
+     * future wait calls on the ExecutionCallback object to proceed.
+     *
+     * Either IExecutionCallback::notify or IExecutionCallback::notify_1_2 must
+     * be called on a given ExecutionCallback object.
+     *
+     * If the callback object is notified more than once, only the results of
+     * the first call to notify* are used, and the results from subsequent calls
+     * are discarded.
      *
      * @param status Error status returned from launching the asynchronous task
-     *               (if the launch fails) or from the asynchronous task itself
-     *               (if the launch succeeds). Must be:
-     *               - NONE if the asynchronous execution was successful
-     *               - DEVICE_UNAVAILABLE if driver is offline or busy
-     *               - GENERAL_FAILURE if the asynchronous task resulted in an
-     *                 unspecified error
-     *               - OUTPUT_INSUFFICIENT_SIZE if at least one output
-     *                 operand buffer is not large enough to store the
-     *                 corresponding output
-     *               - INVALID_ARGUMENT if one of the input arguments to
-     *                 prepareModel is invalid
+     *     (if the launch fails) or from the asynchronous task itself (if the
+     *     launch succeeds). Must be:
+     *     - NONE if the asynchronous execution was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if the asynchronous task resulted in an unspecified
+     *         error
+     *     - OUTPUT_INSUFFICIENT_SIZE if at least one output operand buffer is
+     *         not large enough to store the corresponding output
+     *     - INVALID_ARGUMENT if one of the input arguments to prepareModel is
+     *         invalid
      * @param outputShapes A list of shape information of model output operands.
-     *                     The index into "outputShapes" corresponds to the index
-     *                     of the output operand in the Request outputs vector.
-     *                     outputShapes must be empty unless the status is either
-     *                     NONE or OUTPUT_INSUFFICIENT_SIZE.
-     * @return Timing Duration of execution. Unless MeasureTiming::YES was passed when
-     *                launching the execution and status is NONE, all times must
-     *                be reported as UINT64_MAX. A driver may choose to report
-     *                any time as UINT64_MAX, indicating that particular measurement is
-     *                not available.
+     *     The index into "outputShapes" corresponds to the index of the output
+     *     operand in the Request outputs vector. outputShapes must be empty
+     *     unless the status is either NONE or OUTPUT_INSUFFICIENT_SIZE.
+     * @param Timing Duration of execution. Unless MeasureTiming::YES was passed
+     *     when launching the execution and status is NONE, all times must be
+     *     reported as UINT64_MAX. A driver may choose to report any time as
+     *     UINT64_MAX, indicating that particular measurement is not available.
      */
     Return<void> notify_1_2(ErrorStatus status, const hidl_vec<OutputShape>& outputShapes,
                             const Timing& timing) override;
@@ -321,81 +240,87 @@
     }
 
     /**
+     * ExecutionCallback::wait blocks until notify* has been called on the
+     * callback object.
+     */
+    void wait() const;
+
+    /**
      * Retrieves the error status returned from the asynchronous task launched
      * by either IPreparedModel::execute or IPreparedModel::execute_1_2. If
      * IPreparedModel::execute or IPreparedModel::execute_1_2 has not finished
-     * asynchronously executing, this call will block until the asynchronous task
-     * notifies the object.
+     * asynchronously executing, this call will block until the asynchronous
+     * task notifies the object.
      *
      * @return status Error status returned from launching the asynchronous task
-     *                (if the launch fails) or from the asynchronous task itself
-     *                (if the launch succeeds). Must be:
-     *                - NONE if the asynchronous execution was successful
-     *                - DEVICE_UNAVAILABLE if driver is offline or busy
-     *                - GENERAL_FAILURE if the asynchronous task resulted in an
-     *                  unspecified error
-     *                - OUTPUT_INSUFFICIENT_SIZE if at least one output
-     *                  operand buffer is not large enough to store the
-     *                  corresponding output
-     *                - INVALID_ARGUMENT if one of the input arguments to
-     *                  prepareModel is invalid
+     *     (if the launch fails) or from the asynchronous task itself (if the
+     *     launch succeeds). Must be:
+     *     - NONE if the asynchronous execution was successful
+     *     - DEVICE_UNAVAILABLE if driver is offline or busy
+     *     - GENERAL_FAILURE if the asynchronous task resulted in an unspecified
+     *         error
+     *     - OUTPUT_INSUFFICIENT_SIZE if at least one output operand buffer is
+     *         not large enough to store the corresponding output
+     *     - INVALID_ARGUMENT if one of the input arguments to prepareModel is
+     *         invalid
      */
-    ErrorStatus getStatus();
+    ErrorStatus getStatus() const;
 
     /**
      * Retrieves the output shapes returned from the asynchronous task launched
-     * by IPreparedModel::execute_1_2. If IPreparedModel::execute_1_2 has not finished
-     * asynchronously executing, this call will block until the asynchronous task
-     * notifies the object.
+     * by IPreparedModel::execute_1_2. If IPreparedModel::execute_1_2 has not
+     * finished asynchronously executing, this call will block until the
+     * asynchronous task notifies the object.
      *
-     * If the asynchronous task was launched by IPreparedModel::execute, an empty vector
-     * will be returned.
+     * If the asynchronous task was launched by IPreparedModel::execute, an
+     * empty vector will be returned.
      *
-     * @return outputShapes A list of shape information of model output operands.
-     *                      The index into "outputShapes" corresponds to the index
-     *                      of the output operand in the Request outputs vector.
-     *                      outputShapes must be empty unless the status is either
-     *                      NONE or OUTPUT_INSUFFICIENT_SIZE.
+     * @return outputShapes A list of shape information of model output
+     *     operands. The index into "outputShapes" corresponds to the index of
+     *     the output operand in the Request outputs vector. outputShapes must
+     *     be empty unless the status is either NONE or
+     *     OUTPUT_INSUFFICIENT_SIZE. outputShaps may be empty if the status is
+     *     NONE and all model output operands are fully-specified at execution
+     *     time. outputShapes must have the same number of elements as the
+     *     number of model output operands if the status is
+     *     OUTPUT_INSUFFICIENT_SIZE, or if the status is NONE and the model has
+     *     at least one output operand that is not fully-specified.
      */
-    const std::vector<OutputShape>& getOutputShapes();
+    const std::vector<OutputShape>& getOutputShapes() const;
 
     /**
-     * Retrieves the duration of execution ofthe asynchronous task launched
-     * by IPreparedModel::execute_1_2. If IPreparedModel::execute_1_2 has not finished
-     * asynchronously executing, this call will block until the asynchronous task
-     * notifies the object.
+     * Retrieves the duration of execution of the asynchronous task launched by
+     * IPreparedModel::execute_1_2. If IPreparedModel::execute_1_2 has not
+     * finished asynchronously executing, this call will block until the
+     * asynchronous task notifies the object.
      *
-     * If the asynchronous task was launched by IPreparedModel::execute, every time
-     * must be UINT64_MAX.
+     * If the asynchronous task was launched by IPreparedModel::execute, every
+     * time must be UINT64_MAX.
      *
-     * @return timing Duration of the execution. Every time must be UINT64_MAX unless
-     *                the status is NONE.
+     * @return timing Duration of the execution. Every time must be UINT64_MAX
+     *     unless the status is NONE.
      */
-    Timing getTiming();
+    Timing getTiming() const;
 
   private:
+    /*
+     * ExecutionCallback::notifyInternal stores the results of the execution
+     * (status, output shapes, and timing information) in the ExecutionCallback
+     * object before any call to wait or get* return. It then enables all prior
+     * and future wait calls on the ExecutionCallback object to proceed.
+     */
+    void notifyInternal(ErrorStatus errorStatus, const hidl_vec<OutputShape>& outputShapes,
+                        const Timing& timing);
+
+    // members
+    mutable std::mutex mMutex;
+    mutable std::condition_variable mCondition;
+    bool mNotified GUARDED_BY(mMutex) = false;
     ErrorStatus mErrorStatus = ErrorStatus::GENERAL_FAILURE;
     std::vector<OutputShape> mOutputShapes = {};
     Timing mTiming = {};
 };
 
-// template function implementation(s) below this point
-
-template <class Rep, class Period>
-std::cv_status CallbackBase::wait_for(const std::chrono::duration<Rep, Period>& timeout_duration) {
-    std::unique_lock<std::mutex> lock(mMutex);
-    std::cv_status status =
-            mCondition.wait_for(lock, timeout_duration, [this] { return mNotified; });
-    if (status != std::cv_status::timeout) {
-        join_thread_locked();
-    }
-    return status;
-}
-
-}  // namespace implementation
-}  // namespace V1_2
-}  // namespace neuralnetworks
-}  // namespace hardware
-}  // namespace android
+}  // namespace android::hardware::neuralnetworks::V1_2::implementation
 
 #endif  // ANDROID_HARDWARE_NEURALNETWORKS_V1_2_CALLBACKS_H
diff --git a/wifi/hostapd/1.1/vts/functional/hostapd_hidl_test.cpp b/wifi/hostapd/1.1/vts/functional/hostapd_hidl_test.cpp
index 26a58b2..7aaad02 100644
--- a/wifi/hostapd/1.1/vts/functional/hostapd_hidl_test.cpp
+++ b/wifi/hostapd/1.1/vts/functional/hostapd_hidl_test.cpp
@@ -56,6 +56,8 @@
    protected:
     std::string getPrimaryWlanIfaceName() {
         std::array<char, PROPERTY_VALUE_MAX> buffer;
+        auto res = property_get("wifi.sap.interface", buffer.data(), nullptr);
+        if (res > 0) return buffer.data();
         property_get("wifi.interface", buffer.data(), "wlan0");
         return buffer.data();
     }