diff --git a/audio/6.0/Android.bp b/audio/6.0/Android.bp
index dc6bb98..16abc52 100644
--- a/audio/6.0/Android.bp
+++ b/audio/6.0/Android.bp
@@ -15,6 +15,7 @@
         "IStreamIn.hal",
         "IStreamOut.hal",
         "IStreamOutCallback.hal",
+        "IStreamOutEventCallback.hal",
     ],
     interfaces: [
         "android.hardware.audio.common@6.0",
diff --git a/audio/6.0/IStreamOut.hal b/audio/6.0/IStreamOut.hal
index 13a86ec..9da48fe 100644
--- a/audio/6.0/IStreamOut.hal
+++ b/audio/6.0/IStreamOut.hal
@@ -19,6 +19,7 @@
 import android.hardware.audio.common@6.0;
 import IStream;
 import IStreamOutCallback;
+import IStreamOutEventCallback;
 
 interface IStreamOut extends IStream {
     /**
@@ -168,6 +169,18 @@
     clearCallback() generates (Result retval);
 
     /**
+     * Set the callback interface for notifying about an output stream event.
+     *
+     * Calling this method with a null pointer will result in releasing
+     * the local callback proxy on the server side and thus dereference
+     * the callback implementation on the client side.
+     *
+     * @return retval operation completion status.
+     */
+    setEventCallback(IStreamOutEventCallback callback)
+            generates (Result retval);
+
+    /**
      * Returns whether HAL supports pausing and resuming of streams.
      *
      * @return supportsPause true if pausing is supported.
diff --git a/audio/6.0/IStreamOutEventCallback.hal b/audio/6.0/IStreamOutEventCallback.hal
new file mode 100644
index 0000000..de17d73
--- /dev/null
+++ b/audio/6.0/IStreamOutEventCallback.hal
@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 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.
+ */
+
+package android.hardware.audio@6.0;
+
+/**
+ * Asynchronous stream out event callback interface. The interface provides
+ * a way for the HAL to notify platform when there are changes, e.g. codec
+ * format change, from the lower layer.
+ */
+interface IStreamOutEventCallback {
+    /**
+     * Codec format changed.
+     *
+     * @param audioMetadata is a buffer containing decoded format changes
+     *     reported by codec. The buffer contains data that can be transformed
+     *     to audio metadata, which is a C++ object based map. See
+     *     `system/media/audio_utils/include/audio_utils/Metadata.h` for
+     *     more details.
+     */
+    oneway onCodecFormatChanged(vec<uint8_t> audioMetadata);
+};