Track volume cleanup

Always read and write track volumes atomically. In most places this was
already being done, but there were a couple places where the left and
right channels were read independently.

Changed constant MAX_GAIN_INT to be a uint32_t instead of a float.
It is always used as a uint32_t in comparisons and assignments.
Use MAX_GAIN_INT in more places.

Now that volume is always accessed atomically, removed the union
and alias for uint16_t volume[2], and kept only volumeLR.

Removed volatile as it's meaningless.

In AudioFlinger, clamp the track volumes read from shared memory
before applying master and stream volume.

Change-Id: If65e2b27e5bc3db5bf75540479843041b58433f0

1 files changed, 7 insertions, 4 deletions

diff --git a/include/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index 33a92cd..ffc546e 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -71,10 +71,13 @@ struct audio_track_cblk_t
                 uint32_t    loopStart;
                 uint32_t    loopEnd;
                 int         loopCount;
-    volatile    union {
-                    uint16_t    volume[2];
-                    uint32_t    volumeLR;
-                };
+
+                // Channel volumes are fixed point U4.12, so 0x1000 means 1.0.
+                // Left channel is in [0:15], right channel is in [16:31].
+                // Always read and write the combined pair atomically.
+                // For AudioTrack only, not used by AudioRecord.
+                uint32_t    volumeLR;
+
                 uint32_t    sampleRate;
                 // NOTE: audio_track_cblk_t::frameSize is not equal to AudioTrack::frameSize() for
                 // 8 bit PCM data: in this case,  mCblk->frameSize is based on a sample size of