Enable renderer side mixing behind a flag.

content/renderer/media/audio_renderer_mixer_manager.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Manages sharing of an AudioRendererMixer among AudioRendererMixerInputs based

[scherkus (not reviewing), 2012/07/13 18:40:16]

nit: we (media code) has typically put these alongside class definitions as
opposed to top of file

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+// on their AudioParameters configuration.  Inputs with the same AudioParameters
+// configuration will share a mixer.  While a new AudioRendererMixer will be

[scherkus (not reviewing), 2012/07/13 18:40:16]

s/.  While/ while/

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+// lazily created if one with the exact AudioParameters does not exist.
+//
+// There should only be one instance of AudioRendererMixerManager per render
+// thread.
+//
+// TODO(dalecurtis): Right now we require AudioParameters to be an exact match
+// when we should be able to ignore bits per channel since we're only dealing
+// with floats.  However, bits per channel is currently used to interleave the
+// audio data by AudioDevice::AudioThreadCallback::Process for consumption via
+// the shared memory.  See http://crbug.com/114700.
+
+#ifndef CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_
+#define CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_
+
+#include <map>
+
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
+#include "content/common/content_export.h"
+#include "media/audio/audio_parameters.h"
+
+namespace media {
+class AudioRendererMixer;
+class AudioRendererMixerInput;
+}
+
+class CONTENT_EXPORT AudioRendererMixerManager {

[jochen (gone - plz use gerrit), 2012/07/13 08:51:03]

all code in content/ should go into namespace content

[DaleCurtis, 2012/07/13 21:28:33]

Done.  I noticed the classes I was interacting with didn't have this
(AudioDevice, RASP), so I left it off.  Should those classes have it too?

+ public:
+  AudioRendererMixerManager();
+  virtual ~AudioRendererMixerManager();

[scherkus (not reviewing), 2012/07/13 18:40:16]

remove virtual -- I'm not seeing any virtualness in this class

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+
+  // Returns an AudioRendererMixerInput.
+  scoped_refptr<media::AudioRendererMixerInput> CreateInput();

[jochen (gone - plz use gerrit), 2012/07/13 08:51:03]

why not just return a raw pointer? that way, the mixer isn't unnecessarily
reffed up and down during return

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+
+ private:
+  friend class AudioRendererMixerManagerTest;

[jochen (gone - plz use gerrit), 2012/07/13 08:51:03]

please use FRIEND_TEST_ALL_PREFIXES

[DaleCurtis, 2012/07/13 21:28:33]

I don't think that's necessary here? I only access the private methods/members
through the base AudioRendererMixerManagerTest class and delegate for the
instantiated test cases.  As such the test names can be tagged with whatever and
the code will compile fine.

+
+  // Returns a mixer instance based on AudioParameters; an existing one if one
+  // with the provided AudioParameters exists or a new one if not.
+  media::AudioRendererMixer* GetMixer(const media::AudioParameters& params);
+
+  // Remove a mixer instance given a mixer if the only other reference is held
+  // by AudioRendererMixerManager.  Every AudioRendererMixer owner must call
+  // this method when it's done with a mixer.
+  void RemoveMixer(const media::AudioParameters& params);
+
+  // Map of AudioParameters to <AudioRendererMixer, Count>.  Count allows
+  // AudioRendererMixerManager to keep track explicitly (v.s. RefCounting which
+  // is implicit) of the number of outstanding AudioRendererMixers.
+  typedef std::map<media::AudioParameters,
+                   std::pair<media::AudioRendererMixer*,int>,

[scherkus (not reviewing), 2012/07/13 18:40:16]

s/,int/, int/

Alternatively can use a struct so code is self documenting. Something like:
struct MixerReferences {
  ARM* mixer;
  int count;
};

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+                   media::AudioParameters::Compare> AudioRendererMixerMap;
+  AudioRendererMixerMap mixer_map_;
+  base::Lock mixer_map_lock_;

[scherkus (not reviewing), 2012/07/13 18:40:16]

which threads interact with this class requiring a lock?

CreateInput() should be on render thread -- what thread is input removal
happening on?

[DaleCurtis, 2012/07/13 21:28:33]

ARMM is constructed on the render thread, GetMixer and RemoveMixer happen on the
pipeline thread for each <audio> source.  Specifically GetMixer ends up being
called from media::Pipeline::InitializeTask() and RemoveMixer from
media::Pipeline::TeardownStateTransitionTask().

+
+  DISALLOW_COPY_AND_ASSIGN(AudioRendererMixerManager);
+};
+
+#endif  // CONTENT_RENDERER_MEDIA_AUDIO_RENDERER_MIXER_MANAGER_H_