Adding experimental exclusive-mode streaming to WASAPIAudioOutputStream

media/audio/win/audio_low_latency_output_win.h

Index: media/audio/win/audio_low_latency_output_win.h
diff --git a/media/audio/win/audio_low_latency_output_win.h b/media/audio/win/audio_low_latency_output_win.h
index 67a5d6c5f0a2c496c6e8828815110ac06f3ad202..30bc8c0d683a232400d825079865a3a6305380fd 100644
--- a/media/audio/win/audio_low_latency_output_win.h
+++ b/media/audio/win/audio_low_latency_output_win.h
@@ -112,6 +112,29 @@
 // - All callback methods from the IMMNotificationClient interface will be
 //   called on a Windows-internal MMDevice thread.
 //
+// Experimental exclusive mode (DO NOT USE IN PRODUCTION CODE):
+//
+// - It is possible to open up a stream in exclusive mode by using the
+//   "--enable-exclusive-mode" command-line flag.
+// - The internal buffering scheme is less flexible for exclusive-mode streams.
+//   Hence, some manual tuning will be required before deciding what frame
+//   size to use. See the WinAudioOutputTest unit test for more details.
+// - If an application opens a stream in exclusive mode, the application has
+//   exclusive use of the audio endpoint device that plays the stream.
+// - Exclusive-mode access to an audio device can block crucial system sounds,

[tommi (sloooow) - chröme, 2012/07/25 11:49:40]

s/can/will
(correct?)
you can also leave out 'crucial' since it will prevent all system sounds.
I think it's probably enough to just say that Chrome will have exclusive access
to the audio device.  Developer should understand what that entails.

[henrika (OOO until Aug 14), 2012/07/25 15:26:30]

Done.

+//   prevent interoperability with other applications, and otherwise degrade
+//   the user experience.
+// - Exclusive-mode should only be utilized when the lowest possible latency
+//   is important.
+// - In exclusive mode, the client can choose to open the stream in any audio
+//   format that the endpoint device supports.
+// - Initial measurements on Windows 7 have shown that the lowest possible
+//   latencies we can achieve are:
+//     o ~3.3333ms @ 48kHz <=> 160 audio frames per buffer.
+//     o ~3.6281ms @ 44.1kHz <=> 160 audio frames per buffer.
+// - See msdn.microsoft.com/en-us/library/windows/desktop/dd370844(v=vs.85).aspx
+//   for more details.
+
 #ifndef MEDIA_AUDIO_WIN_AUDIO_LOW_LATENCY_OUTPUT_WIN_H_
 #define MEDIA_AUDIO_WIN_AUDIO_LOW_LATENCY_OUTPUT_WIN_H_
 
@@ -163,11 +186,16 @@ class MEDIA_EXPORT WASAPIAudioOutputStream
 
   // Retrieves the stream format that the audio engine uses for its internal
   // processing/mixing of shared-mode streams.
+  // This method should not be used in combination with exclusive-mode streams.
   static int HardwareSampleRate(ERole device_role);
 
   bool started() const { return started_; }
 
  private:
+   // Returns AUDCLNT_SHAREMODE_EXCLUSIVE if "enable-exclusive-mode" is used

[tommi (sloooow) - chröme, 2012/07/25 11:49:40]

fix indent

[henrika (OOO until Aug 14), 2012/07/25 15:26:30]

Done.

+   // as command-line flag and AUDCLNT_SHAREMODE_SHARED otherwise (default).
+  static AUDCLNT_SHAREMODE GetShareMode();
+
   // Implementation of IUnknown (trivial in this case). See
   // msdn.microsoft.com/en-us/library/windows/desktop/dd371403(v=vs.85).aspx
   // for details regarding why proper implementations of AddRef(), Release()
@@ -206,7 +234,7 @@ class MEDIA_EXPORT WASAPIAudioOutputStream
   void HandleError(HRESULT err);
 
   // The Open() method is divided into these sub methods.
-  HRESULT SetRenderDevice(ERole device_role);
+  HRESULT SetRenderDevice();
   HRESULT ActivateRenderDevice();
   HRESULT GetAudioEngineStreamFormat();
   bool DesiredFormatIsSupported();
@@ -222,6 +250,8 @@ class MEDIA_EXPORT WASAPIAudioOutputStream
   // new default audio device.
   bool RestartRenderingUsingNewDefaultDevice();
 
+  AUDCLNT_SHAREMODE share_mode() const { return share_mode_; }
+
   // Initializes the COM library for use by the calling thread and sets the
   // thread's concurrency model to multi-threaded.
   base::win::ScopedCOMInitializer com_init_;
@@ -276,6 +306,11 @@ class MEDIA_EXPORT WASAPIAudioOutputStream
   // Defines the role that the system has assigned to an audio endpoint device.
   ERole device_role_;
 
+  // The sharing mode for the connection.
+  // Valid values are AUDCLNT_SHAREMODE_SHARED and AUDCLNT_SHAREMODE_EXCLUSIVE
+  // where AUDCLNT_SHAREMODE_SHARED is the default.
+  AUDCLNT_SHAREMODE share_mode_;
+
   // Counts the number of audio frames written to the endpoint buffer.
   UINT64 num_written_frames_;