Tell CoreAudio how to interpret Chrome's channel layout.

media/audio/mac/audio_auhal_mac.cc

Index: media/audio/mac/audio_auhal_mac.cc
diff --git a/media/audio/mac/audio_auhal_mac.cc b/media/audio/mac/audio_auhal_mac.cc
index d4d6075bd84a2bac8a586b1cb7337beee28a9b7c..6d352349f79f35baf8539cba6c24c87c6a7ac617 100644
--- a/media/audio/mac/audio_auhal_mac.cc
+++ b/media/audio/mac/audio_auhal_mac.cc
@@ -22,6 +22,31 @@
 
 namespace media {
 
+// Mapping from Chrome's channel layout to CoreAudio layout. This must match the
+// layout of the Channels enum in |channel_layout.h|
+static const AudioChannelLabel kCoreAudioChannelMapping[] = {
+    kAudioChannelLabel_Left,
+    kAudioChannelLabel_Right,
+    kAudioChannelLabel_Center,
+    kAudioChannelLabel_LFEScreen,
+    kAudioChannelLabel_LeftSurround,
+    kAudioChannelLabel_RightSurround,
+    kAudioChannelLabel_LeftCenter,
+    kAudioChannelLabel_RightCenter,
+    kAudioChannelLabel_CenterSurround,
+    kAudioChannelLabel_LeftSurroundDirect,
+    kAudioChannelLabel_RightSurroundDirect,
+};
+static_assert(0 == LEFT && 1 == RIGHT && 2 == CENTER && 3 == LFE &&
+                  4 == BACK_LEFT &&
+                  5 == BACK_RIGHT &&
+                  6 == LEFT_OF_CENTER &&
+                  7 == RIGHT_OF_CENTER &&
+                  8 == BACK_CENTER &&
+                  9 == SIDE_LEFT &&
+                  10 == SIDE_RIGHT,
+              "Channel positions must max OSX channel order.");

[Avi (use Gerrit), 2017/02/09 02:44:38]

"must match"? (And it's "macOS" nowadays.)

[DaleCurtis, 2017/02/09 03:18:38]

Done, switched to CoreAudio like we use elsewhere for consistency.

+
 static void WrapBufferList(AudioBufferList* buffer_list,
                            AudioBus* bus,
                            int frames) {
@@ -535,6 +560,8 @@ bool AUHALStream::ConfigureAUHAL() {
     return false;
   }
 
+  SetAudioChannelLayout();
+
   result = AudioUnitInitialize(audio_unit_);
   if (result != noErr) {
     OSSTATUS_DLOG(ERROR, result) << "AudioUnitInitialize() failed.";
@@ -559,4 +586,55 @@ void AUHALStream::CloseAudioUnit() {
   audio_unit_ = 0;
 }
 
+void AUHALStream::SetAudioChannelLayout() {
+  DCHECK(audio_unit_);
+
+// Code modeled after example from Apple documentation here:
+// https://developer.apple.com/library/content/qa/qa1627/_index.html

[Avi (use Gerrit), 2017/02/09 02:44:38]

Can you clarify in the comments why it appears you're doing this in a very weird
way? e.g.

1. We define FIELD_OFFSET rather than use offsetof from <cstddef> because
AudioChannelLayout is a C struct that ends with a variable-length array:

typedef struct AudioChannelLayout {
    // ...
    AudioChannelDescription mChannelDescriptions[1];
} AudioChannelLayout;

and you can't use offsetof in that case.

2. We allocate the AudioChannelLayout dynamically rather than statically because
of this reason.

-----

etc. BTW, _can_ you use offsetof instead? It would be cool if you could.

[DaleCurtis, 2017/02/09 03:18:38]

I thought (1) would prevent this, but offsetof() works! Thanks for the
suggestion!

+#define FIELD_OFFSET(type, field) ((size_t) & ((type*)0)->field)
+  const size_t layout_size = FIELD_OFFSET(
+      AudioChannelLayout, mChannelDescriptions[params_.channels()]);
+#undef FIELD_OFFSET
+
+  std::unique_ptr<uint8_t[]> layout_storage(new uint8_t[layout_size]);
+  memset(layout_storage.get(), 0, layout_size);
+  AudioChannelLayout* channel_layout =
+      reinterpret_cast<AudioChannelLayout*>(layout_storage.get());
+
+  channel_layout->mNumberChannelDescriptions = params_.channels();
+  channel_layout->mChannelLayoutTag =
+      kAudioChannelLayoutTag_UseChannelDescriptions;
+  AudioChannelDescription* descriptions = channel_layout->mChannelDescriptions;
+
+  if (params_.channel_layout() == CHANNEL_LAYOUT_DISCRETE) {
+    // For the discrete case just assume common input mappings.
+    for (int ch = 0; ch <= CHANNELS_MAX; ++ch) {
+      descriptions[ch].mChannelLabel = kCoreAudioChannelMapping[ch];
+      descriptions[ch].mChannelFlags = kAudioChannelFlags_AllOff;
+    }
+  } else if (params_.channel_layout() == CHANNEL_LAYOUT_MONO) {
+    // CoreAudio has a special label for mono.
+    DCHECK_EQ(params_.channels(), 1);
+    descriptions[0].mChannelLabel = kAudioChannelLabel_Mono;
+    descriptions[0].mChannelFlags = kAudioChannelFlags_AllOff;
+  } else {
+    for (int ch = 0; ch <= CHANNELS_MAX; ++ch) {
+      const int order =
+          ChannelOrder(params_.channel_layout(), static_cast<Channels>(ch));
+      if (order == -1)
+        continue;
+      descriptions[order].mChannelLabel = kCoreAudioChannelMapping[ch];
+      descriptions[order].mChannelFlags = kAudioChannelFlags_AllOff;
+    }
+  }
+
+  OSStatus result = AudioUnitSetProperty(
+      audio_unit_, kAudioUnitProperty_AudioChannelLayout, kAudioUnitScope_Input,
+      0, channel_layout, layout_size);
+  if (result != noErr) {
+    OSSTATUS_DLOG(ERROR, result)
+        << "Failed to set audio channel layout. Using default layout.";
+  }
+}
+
 }  // namespace media