diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index 6f1f1f2..a4a60d5 100644
--- a/services/surfaceflinger/tests/unittests/Android.bp
+++ b/services/surfaceflinger/tests/unittests/Android.bp
@@ -39,6 +39,7 @@
         "CompositionTest.cpp",
         "DispSyncSourceTest.cpp",
         "DisplayIdentificationTest.cpp",
+        "DisplayIdGeneratorTest.cpp",
         "DisplayTransactionTest.cpp",
         "EventThreadTest.cpp",
         "FrameTimelineTest.cpp",
diff --git a/services/surfaceflinger/tests/unittests/DisplayIdGeneratorTest.cpp b/services/surfaceflinger/tests/unittests/DisplayIdGeneratorTest.cpp
new file mode 100644
index 0000000..be7609a
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/DisplayIdGeneratorTest.cpp
@@ -0,0 +1,82 @@
+/*
+ * Copyright 2020 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 <gtest/gtest.h>
+
+#include <vector>
+
+#include <ui/DisplayId.h>
+#include "DisplayIdGenerator.h"
+
+namespace android {
+
+template <typename T>
+void testNextId(DisplayIdGenerator<T>& generator) {
+    constexpr int kNumIds = 5;
+    std::vector<T> ids;
+    for (int i = 0; i < kNumIds; i++) {
+        const auto id = generator.nextId();
+        ASSERT_TRUE(id);
+        ids.push_back(*id);
+    }
+
+    // All IDs should be different.
+    for (size_t i = 0; i < kNumIds; i++) {
+        for (size_t j = i + 1; j < kNumIds; j++) {
+            EXPECT_NE(ids[i], ids[j]);
+        }
+    }
+}
+
+TEST(DisplayIdGeneratorTest, nextIdGpuVirtual) {
+    RandomDisplayIdGenerator<GpuVirtualDisplayId> generator;
+    testNextId(generator);
+}
+
+TEST(DisplayIdGeneratorTest, nextIdHalVirtual) {
+    RandomDisplayIdGenerator<HalVirtualDisplayId> generator;
+    testNextId(generator);
+}
+
+TEST(DisplayIdGeneratorTest, markUnused) {
+    constexpr size_t kMaxIdsCount = 5;
+    RandomDisplayIdGenerator<GpuVirtualDisplayId> generator(kMaxIdsCount);
+
+    const auto id = generator.nextId();
+    EXPECT_TRUE(id);
+
+    for (int i = 1; i < kMaxIdsCount; i++) {
+        EXPECT_TRUE(generator.nextId());
+    }
+
+    EXPECT_FALSE(generator.nextId());
+
+    generator.markUnused(*id);
+    EXPECT_TRUE(generator.nextId());
+}
+
+TEST(DisplayIdGeneratorTest, maxIdsCount) {
+    constexpr size_t kMaxIdsCount = 5;
+    RandomDisplayIdGenerator<GpuVirtualDisplayId> generator(kMaxIdsCount);
+
+    for (int i = 0; i < kMaxIdsCount; i++) {
+        EXPECT_TRUE(generator.nextId());
+    }
+
+    EXPECT_FALSE(generator.nextId());
+}
+
+} // namespace android
diff --git a/services/surfaceflinger/tests/unittests/DisplayTransactionTest.cpp b/services/surfaceflinger/tests/unittests/DisplayTransactionTest.cpp
index b750d6b..a81d216 100644
--- a/services/surfaceflinger/tests/unittests/DisplayTransactionTest.cpp
+++ b/services/surfaceflinger/tests/unittests/DisplayTransactionTest.cpp
@@ -338,9 +338,9 @@
 struct PhysicalDisplayIdType {};
 
 template <uint64_t displayId>
-using VirtualDisplayIdType = std::integral_constant<uint64_t, displayId>;
+using HalVirtualDisplayIdType = std::integral_constant<uint64_t, displayId>;
 
-struct NoDisplayId {};
+struct GpuVirtualDisplayIdType {};
 
 template <typename>
 struct IsPhysicalDisplayId : std::bool_constant<false> {};
@@ -353,7 +353,7 @@
 
 template <typename PhysicalDisplay>
 struct DisplayIdGetter<PhysicalDisplayIdType<PhysicalDisplay>> {
-    static std::optional<PhysicalDisplayId> get() {
+    static PhysicalDisplayId get() {
         if (!PhysicalDisplay::HAS_IDENTIFICATION_DATA) {
             return PhysicalDisplayId::fromPort(static_cast<bool>(PhysicalDisplay::PRIMARY)
                                                        ? LEGACY_DISPLAY_TYPE_PRIMARY
@@ -363,19 +363,18 @@
         const auto info =
                 parseDisplayIdentificationData(PhysicalDisplay::PORT,
                                                PhysicalDisplay::GET_IDENTIFICATION_DATA());
-        return info ? std::make_optional(info->id) : std::nullopt;
+        return info ? info->id : PhysicalDisplayId::fromPort(PhysicalDisplay::PORT);
     }
 };
 
 template <uint64_t displayId>
-struct DisplayIdGetter<VirtualDisplayIdType<displayId>> {
-    // TODO(b/160679868) Use VirtualDisplayId
-    static std::optional<PhysicalDisplayId> get() { return PhysicalDisplayId{displayId}; }
+struct DisplayIdGetter<HalVirtualDisplayIdType<displayId>> {
+    static HalVirtualDisplayId get() { return HalVirtualDisplayId(displayId); }
 };
 
 template <>
-struct DisplayIdGetter<NoDisplayId> {
-    static std::optional<DisplayId> get() { return {}; }
+struct DisplayIdGetter<GpuVirtualDisplayIdType> {
+    static GpuVirtualDisplayId get() { return GpuVirtualDisplayId(0); }
 };
 
 template <typename>
@@ -396,7 +395,7 @@
 constexpr HWDisplayId HWC_VIRTUAL_DISPLAY_HWC_DISPLAY_ID = 1010;
 
 template <uint64_t displayId>
-struct HwcDisplayIdGetter<VirtualDisplayIdType<displayId>> {
+struct HwcDisplayIdGetter<HalVirtualDisplayIdType<displayId>> {
     static constexpr std::optional<HWDisplayId> value = HWC_VIRTUAL_DISPLAY_HWC_DISPLAY_ID;
 };
 
@@ -407,8 +406,8 @@
 
 // DisplayIdType can be:
 //     1) PhysicalDisplayIdType<...> for generated ID of physical display backed by HWC.
-//     2) VirtualDisplayIdType<...> for hard-coded ID of virtual display backed by HWC.
-//     3) NoDisplayId for virtual display without HWC backing.
+//     2) HalVirtualDisplayIdType<...> for hard-coded ID of virtual display backed by HWC.
+//     3) GpuVirtualDisplayIdType for virtual display without HWC backing.
 template <typename DisplayIdType, int width, int height, Critical critical, Async async,
           Secure secure, Primary primary, int grallocUsage>
 struct DisplayVariant {
@@ -440,17 +439,36 @@
 
     static auto makeFakeExistingDisplayInjector(DisplayTransactionTest* test) {
         auto ceDisplayArgs = compositionengine::DisplayCreationArgsBuilder();
-        if (auto displayId = DISPLAY_ID::get()) {
+        if (auto displayId = PhysicalDisplayId::tryCast(DISPLAY_ID::get())) {
             ceDisplayArgs.setPhysical({*displayId, DisplayConnectionType::Internal});
         } else {
+            // We turn off the use of HwcVirtualDisplays, to prevent Composition Engine
+            // from calling into HWComposer. This way all virtual displays will get
+            // a GpuVirtualDisplayId, even if we are in the HwcVirtualDisplayVariant.
+            // In this case we later override it by calling display.setDisplayIdForTesting().
             ceDisplayArgs.setUseHwcVirtualDisplays(false);
+
+            GpuVirtualDisplayId desiredDisplayId = GpuVirtualDisplayId::tryCast(DISPLAY_ID::get())
+                                                           .value_or(GpuVirtualDisplayId(0));
+
+            ON_CALL(test->mFlinger.gpuVirtualDisplayIdGenerator(), nextId())
+                    .WillByDefault(Return(desiredDisplayId));
+
+            auto& generator = test->mFlinger.gpuVirtualDisplayIdGenerator();
+            ceDisplayArgs.setGpuVirtualDisplayIdGenerator(generator);
         }
-        ceDisplayArgs.setPixels({WIDTH, HEIGHT}).setPowerAdvisor(&test->mPowerAdvisor).build();
+        ceDisplayArgs.setPixels({WIDTH, HEIGHT}).setPowerAdvisor(&test->mPowerAdvisor);
 
         auto compositionDisplay =
                 compositionengine::impl::createDisplay(test->mFlinger.getCompositionEngine(),
                                                        ceDisplayArgs.build());
 
+        if (HalVirtualDisplayId::tryCast(DISPLAY_ID::get())) {
+            // CompositionEngine has assigned a placeholder GpuVirtualDisplayId and we need to
+            // override it with the correct HalVirtualDisplayId.
+            compositionDisplay->setDisplayIdForTesting(DISPLAY_ID::get());
+        }
+
         auto injector = FakeDisplayDeviceInjector(test->mFlinger, compositionDisplay,
                                                   CONNECTION_TYPE::value, HWC_DISPLAY_ID_OPT::value,
                                                   static_cast<bool>(PRIMARY));
@@ -532,8 +550,8 @@
     // Called by tests to inject a HWC display setup
     static void injectHwcDisplayWithNoDefaultCapabilities(DisplayTransactionTest* test) {
         const auto displayId = DisplayVariant::DISPLAY_ID::get();
-        ASSERT_TRUE(displayId);
-        FakeHwcDisplayInjector(*displayId, HWC_DISPLAY_TYPE,
+        ASSERT_FALSE(GpuVirtualDisplayId::tryCast(displayId));
+        FakeHwcDisplayInjector(displayId, HWC_DISPLAY_TYPE,
                                static_cast<bool>(DisplayVariant::PRIMARY))
                 .setHwcDisplayId(HWC_DISPLAY_ID)
                 .setWidth(DisplayVariant::WIDTH)
@@ -559,7 +577,7 @@
                 ::testing::UnitTest::GetInstance()->current_test_info();
 
         auto ceDisplayArgs = compositionengine::DisplayCreationArgsBuilder()
-                                     .setPhysical({*DisplayVariant::DISPLAY_ID::get(),
+                                     .setPhysical({DisplayVariant::DISPLAY_ID::get(),
                                                    PhysicalDisplay::CONNECTION_TYPE})
                                      .setPixels({DisplayVariant::WIDTH, DisplayVariant::HEIGHT})
                                      .setIsSecure(static_cast<bool>(DisplayVariant::SECURE))
@@ -686,10 +704,11 @@
 
 template <int width, int height, Secure secure>
 struct NonHwcVirtualDisplayVariant
-      : DisplayVariant<NoDisplayId, width, height, Critical::FALSE, Async::TRUE, secure,
+      : DisplayVariant<GpuVirtualDisplayIdType, width, height, Critical::FALSE, Async::TRUE, secure,
                        Primary::FALSE, GRALLOC_USAGE_NONHWC_VIRTUAL_DISPLAY> {
-    using Base = DisplayVariant<NoDisplayId, width, height, Critical::FALSE, Async::TRUE, secure,
-                                Primary::FALSE, GRALLOC_USAGE_NONHWC_VIRTUAL_DISPLAY>;
+    using Base =
+            DisplayVariant<GpuVirtualDisplayIdType, width, height, Critical::FALSE, Async::TRUE,
+                           secure, Primary::FALSE, GRALLOC_USAGE_NONHWC_VIRTUAL_DISPLAY>;
 
     static void injectHwcDisplay(DisplayTransactionTest*) {}
 
@@ -698,12 +717,17 @@
         const ::testing::TestInfo* const test_info =
                 ::testing::UnitTest::GetInstance()->current_test_info();
 
+        ON_CALL(test->mFlinger.gpuVirtualDisplayIdGenerator(), nextId())
+                .WillByDefault(Return(Base::DISPLAY_ID::get()));
+
         auto ceDisplayArgs = compositionengine::DisplayCreationArgsBuilder()
                                      .setPixels({Base::WIDTH, Base::HEIGHT})
                                      .setIsSecure(static_cast<bool>(Base::SECURE))
                                      .setPowerAdvisor(&test->mPowerAdvisor)
                                      .setName(std::string("Injected display for ") +
                                               test_info->test_case_name() + "." + test_info->name())
+                                     .setGpuVirtualDisplayIdGenerator(
+                                             test->mFlinger.gpuVirtualDisplayIdGenerator())
                                      .build();
 
         return compositionengine::impl::createDisplay(test->mFlinger.getCompositionEngine(),
@@ -725,13 +749,13 @@
 
 template <int width, int height, Secure secure>
 struct HwcVirtualDisplayVariant
-      : DisplayVariant<VirtualDisplayIdType<42>, width, height, Critical::FALSE, Async::TRUE,
+      : DisplayVariant<HalVirtualDisplayIdType<42>, width, height, Critical::FALSE, Async::TRUE,
                        secure, Primary::FALSE, GRALLOC_USAGE_HWC_VIRTUAL_DISPLAY>,
         HwcDisplayVariant<HWC_VIRTUAL_DISPLAY_HWC_DISPLAY_ID, DisplayType::VIRTUAL,
-                          DisplayVariant<VirtualDisplayIdType<42>, width, height, Critical::FALSE,
-                                         Async::TRUE, secure, Primary::FALSE,
+                          DisplayVariant<HalVirtualDisplayIdType<42>, width, height,
+                                         Critical::FALSE, Async::TRUE, secure, Primary::FALSE,
                                          GRALLOC_USAGE_HWC_VIRTUAL_DISPLAY>> {
-    using Base = DisplayVariant<VirtualDisplayIdType<42>, width, height, Critical::FALSE,
+    using Base = DisplayVariant<HalVirtualDisplayIdType<42>, width, height, Critical::FALSE,
                                 Async::TRUE, secure, Primary::FALSE, GRALLOC_USAGE_HW_COMPOSER>;
     using Self = HwcVirtualDisplayVariant<width, height, secure>;
 
@@ -740,6 +764,14 @@
         const ::testing::TestInfo* const test_info =
                 ::testing::UnitTest::GetInstance()->current_test_info();
 
+        // In order to prevent compostition engine calling into HWComposer, we
+        // 1. turn off the use of HWC virtual displays,
+        // 2. provide a GpuVirtualDisplayIdGenerator which always returns some fake ID
+        // 3. override the ID by calling setDisplayIdForTesting()
+
+        ON_CALL(test->mFlinger.gpuVirtualDisplayIdGenerator(), nextId())
+                .WillByDefault(Return(GpuVirtualDisplayId(0)));
+
         auto ceDisplayArgs = compositionengine::DisplayCreationArgsBuilder()
                                      .setUseHwcVirtualDisplays(false)
                                      .setPixels({Base::WIDTH, Base::HEIGHT})
@@ -747,6 +779,8 @@
                                      .setPowerAdvisor(&test->mPowerAdvisor)
                                      .setName(std::string("Injected display for ") +
                                               test_info->test_case_name() + "." + test_info->name())
+                                     .setGpuVirtualDisplayIdGenerator(
+                                             test->mFlinger.gpuVirtualDisplayIdGenerator())
                                      .build();
 
         auto compositionDisplay =
@@ -755,8 +789,9 @@
         compositionDisplay->setDisplayIdForTesting(Base::DISPLAY_ID::get());
 
         // Insert display data so that the HWC thinks it created the virtual display.
-        if (const auto displayId = Base::DISPLAY_ID::get()) {
-            test->mFlinger.mutableHwcDisplayData().try_emplace(*displayId);
+        if (const auto displayId = Base::DISPLAY_ID::get();
+            HalVirtualDisplayId::tryCast(displayId)) {
+            test->mFlinger.mutableHwcDisplayData().try_emplace(displayId);
         }
 
         return compositionDisplay;
@@ -1776,13 +1811,11 @@
 
     DisplayDeviceState state;
     if (const auto connectionType = Case::Display::CONNECTION_TYPE::value) {
-        const auto displayId = Case::Display::DISPLAY_ID::get();
+        const auto displayId = PhysicalDisplayId::tryCast(Case::Display::DISPLAY_ID::get());
         ASSERT_TRUE(displayId);
         const auto hwcDisplayId = Case::Display::HWC_DISPLAY_ID_OPT::value;
         ASSERT_TRUE(hwcDisplayId);
-        state.physical = {.id = static_cast<PhysicalDisplayId>(*displayId),
-                          .type = *connectionType,
-                          .hwcDisplayId = *hwcDisplayId};
+        state.physical = {.id = *displayId, .type = *connectionType, .hwcDisplayId = *hwcDisplayId};
     }
 
     state.isSecure = static_cast<bool>(Case::Display::SECURE);
@@ -1954,11 +1987,11 @@
 
     std::optional<DisplayDeviceState::Physical> expectedPhysical;
     if (const auto connectionType = Case::Display::CONNECTION_TYPE::value) {
-        const auto displayId = Case::Display::DISPLAY_ID::get();
+        const auto displayId = PhysicalDisplayId::tryCast(Case::Display::DISPLAY_ID::get());
         ASSERT_TRUE(displayId);
         const auto hwcDisplayId = Case::Display::HWC_DISPLAY_ID_OPT::value;
         ASSERT_TRUE(hwcDisplayId);
-        expectedPhysical = {.id = static_cast<PhysicalDisplayId>(*displayId),
+        expectedPhysical = {.id = *displayId,
                             .type = *connectionType,
                             .hwcDisplayId = *hwcDisplayId};
     }
@@ -1983,9 +2016,9 @@
 
     // SF should have a display token.
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(*displayId) == 1);
-    auto& displayToken = mFlinger.mutablePhysicalDisplayTokens()[*displayId];
+    ASSERT_TRUE(PhysicalDisplayId::tryCast(displayId));
+    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(displayId) == 1);
+    auto& displayToken = mFlinger.mutablePhysicalDisplayTokens()[displayId];
 
     verifyDisplayIsConnected<Case>(displayToken);
 }
@@ -2088,8 +2121,8 @@
 
     // SF should not have a display token.
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(*displayId) == 0);
+    ASSERT_TRUE(PhysicalDisplayId::tryCast(displayId));
+    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(displayId) == 0);
 
     // The existing token should have been removed
     verifyDisplayIsNotConnected(existing.token());
@@ -2174,8 +2207,8 @@
 
     // SF should not have a display token.
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(*displayId) == 0);
+    ASSERT_TRUE(PhysicalDisplayId::tryCast(displayId));
+    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(displayId) == 0);
 }
 
 TEST_F(HandleTransactionLockedTest, processesHotplugDisconnectThenConnectPrimary) {
@@ -2213,9 +2246,9 @@
     // The existing token should have been removed
     verifyDisplayIsNotConnected(existing.token());
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(*displayId) == 1);
-    EXPECT_NE(existing.token(), mFlinger.mutablePhysicalDisplayTokens()[*displayId]);
+    ASSERT_TRUE(PhysicalDisplayId::tryCast(displayId));
+    ASSERT_TRUE(mFlinger.mutablePhysicalDisplayTokens().count(displayId) == 1);
+    EXPECT_NE(existing.token(), mFlinger.mutablePhysicalDisplayTokens()[displayId]);
 
     // A new display should be connected in its place
 
@@ -2349,8 +2382,8 @@
 
     // A virtual display is set up but is removed from the current state.
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    mFlinger.mutableHwcDisplayData().try_emplace(*displayId);
+    ASSERT_TRUE(HalVirtualDisplayId::tryCast(displayId));
+    mFlinger.mutableHwcDisplayData().try_emplace(displayId);
     Case::Display::injectHwcDisplay(this);
     auto existing = Case::Display::makeFakeExistingDisplayInjector(this);
     existing.inject();
@@ -3485,8 +3518,8 @@
 
     // Insert display data so that the HWC thinks it created the virtual display.
     const auto displayId = Case::Display::DISPLAY_ID::get();
-    ASSERT_TRUE(displayId);
-    mFlinger.mutableHwcDisplayData().try_emplace(*displayId);
+    ASSERT_TRUE(HalVirtualDisplayId::tryCast(displayId));
+    mFlinger.mutableHwcDisplayData().try_emplace(displayId);
 
     // A virtual display device is set up
     Case::Display::injectHwcDisplay(this);
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index 37f159c..0ea5ddc 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -40,6 +40,7 @@
 #include "SurfaceInterceptor.h"
 #include "TestableScheduler.h"
 #include "mock/DisplayHardware/MockDisplay.h"
+#include "mock/MockDisplayIdGenerator.h"
 
 namespace android {
 
@@ -171,6 +172,9 @@
 
     SurfaceFlinger* flinger() { return mFlinger.get(); }
     TestableScheduler* scheduler() { return mScheduler; }
+    mock::DisplayIdGenerator<GpuVirtualDisplayId>& gpuVirtualDisplayIdGenerator() {
+        return mGpuVirtualDisplayIdGenerator;
+    }
 
     // Extend this as needed for accessing SurfaceFlinger private (and public)
     // functions.
@@ -463,7 +467,8 @@
         static constexpr hal::HWConfigId DEFAULT_ACTIVE_CONFIG = 0;
         static constexpr hal::PowerMode DEFAULT_POWER_MODE = hal::PowerMode::ON;
 
-        FakeHwcDisplayInjector(DisplayId displayId, hal::DisplayType hwcDisplayType, bool isPrimary)
+        FakeHwcDisplayInjector(HalDisplayId displayId, hal::DisplayType hwcDisplayType,
+                               bool isPrimary)
               : mDisplayId(displayId), mHwcDisplayType(hwcDisplayType), mIsPrimary(isPrimary) {}
 
         auto& setHwcDisplayId(hal::HWDisplayId displayId) {
@@ -536,14 +541,16 @@
             flinger->mutableHwcDisplayData()[mDisplayId].hwcDisplay = std::move(display);
 
             if (mHwcDisplayType == hal::DisplayType::PHYSICAL) {
-                flinger->mutableHwcPhysicalDisplayIdMap().emplace(mHwcDisplayId, mDisplayId);
+                const auto physicalId = PhysicalDisplayId::tryCast(mDisplayId);
+                LOG_ALWAYS_FATAL_IF(!physicalId);
+                flinger->mutableHwcPhysicalDisplayIdMap().emplace(mHwcDisplayId, *physicalId);
                 (mIsPrimary ? flinger->mutableInternalHwcDisplayId()
                             : flinger->mutableExternalHwcDisplayId()) = mHwcDisplayId;
             }
         }
 
     private:
-        const DisplayId mDisplayId;
+        const HalDisplayId mDisplayId;
         const hal::DisplayType mHwcDisplayType;
         const bool mIsPrimary;
 
@@ -635,10 +642,10 @@
             DisplayDeviceState state;
             if (const auto type = mCreationArgs.connectionType) {
                 LOG_ALWAYS_FATAL_IF(!displayId);
+                const auto physicalId = PhysicalDisplayId::tryCast(*displayId);
+                LOG_ALWAYS_FATAL_IF(!physicalId);
                 LOG_ALWAYS_FATAL_IF(!mHwcDisplayId);
-                state.physical = {.id = static_cast<PhysicalDisplayId>(*displayId),
-                                  .type = *type,
-                                  .hwcDisplayId = *mHwcDisplayId};
+                state.physical = {.id = *physicalId, .type = *type, .hwcDisplayId = *mHwcDisplayId};
             }
 
             state.isSecure = mCreationArgs.isSecure;
@@ -672,6 +679,7 @@
     sp<SurfaceFlinger> mFlinger = new SurfaceFlinger(mFactory, SurfaceFlinger::SkipInitialization);
     TestableScheduler* mScheduler = nullptr;
     Hwc2::mock::Display mDisplay;
+    mock::DisplayIdGenerator<GpuVirtualDisplayId> mGpuVirtualDisplayIdGenerator;
 };
 
 } // namespace android
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.cpp b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.cpp
index 0780af1..251ab36 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.cpp
@@ -20,17 +20,13 @@
 
 #include "mock/DisplayHardware/MockComposer.h"
 
-namespace android {
-namespace Hwc2 {
-namespace mock {
+namespace android::Hwc2::mock {
 
 // Explicit default instantiation is recommended.
 Composer::Composer() = default;
 Composer::~Composer() = default;
 
-} // namespace mock
-} // namespace Hwc2
-} // namespace android
+} // namespace android::Hwc2::mock
 
 // TODO(b/129481165): remove the #pragma below and fix conversion issues
 #pragma clang diagnostic pop // ignored "-Wconversion"
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.h b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.h
index cd9b87a..1ba3c0f 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.h
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockComposer.h
@@ -24,8 +24,7 @@
 
 class GraphicBuffer;
 
-namespace Hwc2 {
-namespace mock {
+namespace Hwc2::mock {
 
 using android::hardware::graphics::common::V1_0::ColorTransform;
 using android::hardware::graphics::common::V1_0::Transform;
@@ -140,6 +139,5 @@
     MOCK_METHOD2(getClientTargetProperty, Error(Display, IComposerClient::ClientTargetProperty*));
 };
 
-} // namespace mock
-} // namespace Hwc2
+} // namespace Hwc2::mock
 } // namespace android
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.cpp b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.cpp
index 2ec37c1..c9788af 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.cpp
@@ -16,14 +16,10 @@
 
 #include "mock/DisplayHardware/MockDisplay.h"
 
-namespace android {
-namespace Hwc2 {
-namespace mock {
+namespace android::Hwc2::mock {
 
 // Explicit default instantiation is recommended.
 Display::Display() = default;
 Display::~Display() = default;
 
-} // namespace mock
-} // namespace Hwc2
-} // namespace android
\ No newline at end of file
+} // namespace android::Hwc2::mock
\ No newline at end of file
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.h b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.h
index fe99e77..a96d9db 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.h
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockDisplay.h
@@ -22,9 +22,7 @@
 
 using android::HWC2::Layer;
 
-namespace android {
-namespace Hwc2 {
-namespace mock {
+namespace android::Hwc2::mock {
 
 namespace hal = android::hardware::graphics::composer::hal;
 
@@ -98,6 +96,4 @@
     MOCK_CONST_METHOD0(isVsyncPeriodSwitchSupported, bool());
 };
 
-} // namespace mock
-} // namespace Hwc2
-} // namespace android
+} // namespace android::Hwc2::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.cpp b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.cpp
index 8be7077..1ba38a8 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.cpp
@@ -16,14 +16,10 @@
 
 #include "MockPowerAdvisor.h"
 
-namespace android {
-namespace Hwc2 {
-namespace mock {
+namespace android::Hwc2::mock {
 
 // Explicit default instantiation is recommended.
 PowerAdvisor::PowerAdvisor() = default;
 PowerAdvisor::~PowerAdvisor() = default;
 
-} // namespace mock
-} // namespace Hwc2
-} // namespace android
+} // namespace android::Hwc2::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.h b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.h
index e22d0cf..7450b5d 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.h
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockPowerAdvisor.h
@@ -20,9 +20,7 @@
 
 #include "DisplayHardware/PowerAdvisor.h"
 
-namespace android {
-namespace Hwc2 {
-namespace mock {
+namespace android::Hwc2::mock {
 
 class PowerAdvisor : public android::Hwc2::PowerAdvisor {
 public:
@@ -34,6 +32,4 @@
     MOCK_METHOD0(notifyDisplayUpdateImminent, void());
 };
 
-} // namespace mock
-} // namespace Hwc2
-} // namespace android
+} // namespace android::Hwc2::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockDisplayIdGenerator.h b/services/surfaceflinger/tests/unittests/mock/MockDisplayIdGenerator.h
new file mode 100644
index 0000000..cfc37ea
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/mock/MockDisplayIdGenerator.h
@@ -0,0 +1,36 @@
+/*
+ * Copyright 2020 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 <gmock/gmock.h>
+
+#include "DisplayIdGenerator.h"
+
+namespace android::mock {
+
+template <typename T>
+class DisplayIdGenerator : public android::DisplayIdGenerator<T> {
+public:
+    // Explicit default instantiation is recommended.
+    DisplayIdGenerator() = default;
+    virtual ~DisplayIdGenerator() = default;
+
+    MOCK_METHOD0(nextId, std::optional<T>());
+    MOCK_METHOD1(markUnused, void(T));
+};
+
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockEventThread.cpp b/services/surfaceflinger/tests/unittests/mock/MockEventThread.cpp
index 408cd35..302dc01 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockEventThread.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/MockEventThread.cpp
@@ -16,12 +16,10 @@
 
 #include "mock/MockEventThread.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 // Explicit default instantiation is recommended.
 EventThread::EventThread() = default;
 EventThread::~EventThread() = default;
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockEventThread.h b/services/surfaceflinger/tests/unittests/mock/MockEventThread.h
index eefdec1..b4594c1 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockEventThread.h
+++ b/services/surfaceflinger/tests/unittests/mock/MockEventThread.h
@@ -20,8 +20,7 @@
 
 #include "Scheduler/EventThread.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 class EventThread : public android::EventThread {
 public:
@@ -47,5 +46,4 @@
     MOCK_METHOD0(getEventThreadConnectionCount, size_t());
 };
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.cpp b/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.cpp
index 358dfdb..417dcb0 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.cpp
@@ -16,12 +16,10 @@
 
 #include "mock/MockFrameTracer.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 // Explicit default instantiation is recommended.
 FrameTracer::FrameTracer() = default;
 FrameTracer::~FrameTracer() = default;
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.h b/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.h
index f768b81..305cb1c 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.h
+++ b/services/surfaceflinger/tests/unittests/mock/MockFrameTracer.h
@@ -20,8 +20,7 @@
 
 #include "FrameTracer/FrameTracer.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 class FrameTracer : public android::FrameTracer {
 public:
@@ -39,5 +38,4 @@
     MOCK_METHOD0(miniDump, std::string());
 };
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.cpp b/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.cpp
index 7e925b9..0a0e7b5 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.cpp
@@ -20,15 +20,13 @@
 
 #include "mock/MockSurfaceInterceptor.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 // Explicit default instantiation is recommended.
 SurfaceInterceptor::SurfaceInterceptor() = default;
 SurfaceInterceptor::~SurfaceInterceptor() = default;
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
 
 // TODO(b/129481165): remove the #pragma below and fix conversion issues
 #pragma clang diagnostic pop // ignored "-Wconversion"
diff --git a/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.h b/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.h
index 03a04a9..b085027 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.h
+++ b/services/surfaceflinger/tests/unittests/mock/MockSurfaceInterceptor.h
@@ -20,8 +20,7 @@
 
 #include "SurfaceInterceptor.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 class SurfaceInterceptor : public android::SurfaceInterceptor {
 public:
@@ -48,5 +47,4 @@
     MOCK_METHOD1(saveVSyncEvent, void(nsecs_t));
 };
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockTimeStats.cpp b/services/surfaceflinger/tests/unittests/mock/MockTimeStats.cpp
index d686939..f8e76b2 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockTimeStats.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/MockTimeStats.cpp
@@ -16,12 +16,10 @@
 
 #include "mock/MockTimeStats.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 // Explicit default instantiation is recommended.
 TimeStats::TimeStats() = default;
 TimeStats::~TimeStats() = default;
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockTimeStats.h b/services/surfaceflinger/tests/unittests/mock/MockTimeStats.h
index 4186e2b..ff37ec8 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockTimeStats.h
+++ b/services/surfaceflinger/tests/unittests/mock/MockTimeStats.h
@@ -20,8 +20,7 @@
 
 #include "TimeStats/TimeStats.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 class TimeStats : public android::TimeStats {
 public:
@@ -59,5 +58,4 @@
     MOCK_METHOD1(setPresentFenceGlobal, void(const std::shared_ptr<FenceTime>&));
 };
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockVSyncTracker.cpp b/services/surfaceflinger/tests/unittests/mock/MockVSyncTracker.cpp
index 8a18123..bcccae5 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockVSyncTracker.cpp
+++ b/services/surfaceflinger/tests/unittests/mock/MockVSyncTracker.cpp
@@ -15,15 +15,11 @@
  */
 
 #include "mock/MockVSyncTracker.h"
-#include <thread>
 
-using namespace std::chrono_literals;
-namespace android {
-namespace mock {
+namespace android::mock {
 
 // Explicit default instantiation is recommended.
 VSyncTracker::VSyncTracker() = default;
 VSyncTracker::~VSyncTracker() = default;
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/MockVsyncController.h b/services/surfaceflinger/tests/unittests/mock/MockVsyncController.h
index 1d87546..94d9966 100644
--- a/services/surfaceflinger/tests/unittests/mock/MockVsyncController.h
+++ b/services/surfaceflinger/tests/unittests/mock/MockVsyncController.h
@@ -20,8 +20,7 @@
 
 #include "Scheduler/VsyncController.h"
 
-namespace android {
-namespace mock {
+namespace android::mock {
 
 class VsyncController : public android::scheduler::VsyncController {
 public:
@@ -36,5 +35,4 @@
     MOCK_CONST_METHOD1(dump, void(std::string&));
 };
 
-} // namespace mock
-} // namespace android
+} // namespace android::mock