diff --git a/audio/2.0/IStreamOut.hal b/audio/2.0/IStreamOut.hal
index 336684f..2ec080d 100644
--- a/audio/2.0/IStreamOut.hal
+++ b/audio/2.0/IStreamOut.hal
@@ -44,44 +44,57 @@
     setVolume(float left, float right) generates (Result retval);
 
     /*
+     * Commands that can be executed on the driver writer thread.
+     */
+    enum WriteCommand : int32_t {
+        WRITE,
+        GET_PRESENTATION_POSITION,
+        GET_LATENCY
+    };
+
+    /*
      * Data structure passed back to the client via status message queue
      * of 'write' operation.
      *
-     * Possible values of 'writeRetval' field:
+     * Possible values of 'retval' field:
      *  - OK, write operation was successful;
      *  - INVALID_ARGUMENTS, stream was not configured properly;
-     *  - INVALID_STATE, stream is in a state that doesn't allow writes.
-     *
-     * Possible values of 'presentationPositionRetval' field (must only
-     * be considered if 'writeRetval' field is set to 'OK'):
-     *  - OK, presentation position retrieved successfully;
-     *  - INVALID_ARGUMENTS, indicates that the position can't be retrieved;
-     *  - INVALID_OPERATION, retrieving presentation position isn't supported;
+     *  - INVALID_STATE, stream is in a state that doesn't allow writes;
+     *  - INVALID_OPERATION, retrieving presentation position isn't supported.
      */
     struct WriteStatus {
-        Result writeRetval;
-        uint64_t written;
-        Result presentationPositionRetval;
-        uint64_t frames;    // presentation position
-        TimeSpec timeStamp; // presentation position
+        Result retval;
+        WriteCommand replyTo;  // discriminator
+        union Reply {
+            uint64_t written;  // WRITE command, amount of bytes written, >= 0.
+            struct PresentationPosition {  // same as generated by
+                uint64_t frames;           // getPresentationPosition.
+                TimeSpec timeStamp;
+            } presentationPosition;
+            uint32_t latencyMs; // Same as generated by getLatency.
+        } reply;
     };
 
     /*
      * Set up required transports for passing audio buffers to the driver.
      *
-     * The transport consists of two message queues: one is used for passing
-     * audio data from the client to the driver, another is used for reporting
-     * write operation status (amount of bytes actually written or error code),
-     * and the presentation position immediately after the write, see
-     * WriteStatus structure definition.
+     * The transport consists of three message queues:
+     *  -- command queue is used to instruct the writer thread what operation
+     *     to perform;
+     *  -- data queue is used for passing audio data from the client
+     *     to the driver;
+     *  -- status queue is used for reporting operation status
+     *     (e.g. amount of bytes actually written or error code).
+     * The driver operates on a dedicated thread.
      *
      * @param frameSize the size of a single frame, in bytes.
      * @param framesCount the number of frames in a buffer.
-     * @param threadPriority priority of the thread that performs writes.
+     * @param threadPriority priority of the driver thread.
      * @return retval OK if both message queues were created successfully.
      *                INVALID_STATE if the method was already called.
      *                INVALID_ARGUMENTS if there was a problem setting up
      *                                  the queues.
+     * @return commandMQ a message queue used for passing commands.
      * @return dataMQ a message queue used for passing audio data in the format
      *                specified at the stream opening.
      * @return statusMQ a message queue used for passing status from the driver
@@ -92,7 +105,9 @@
             ThreadPriority threadPriority)
     generates (
             Result retval,
-            fmq_sync<uint8_t> dataMQ, fmq_sync<WriteStatus> statusMQ);
+            fmq_sync<WriteCommand> commandMQ,
+            fmq_sync<uint8_t> dataMQ,
+            fmq_sync<WriteStatus> statusMQ);
 
     /*
      * Return the number of audio frames written by the audio DSP to DAC since