Resolves "Huge increase in audio latency on Windows following r192046"

media/audio/win/audio_manager_win.cc

 // 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.
 
 #include "media/audio/audio_io.h"
 
 #include <windows.h>
 #include <objbase.h>  // This has to be before initguid.h
 #include <initguid.h>
 #include <mmsystem.h>
@@ skipping 349 matching lines @@
   int input_channels = 0;
   bool use_input_params = !CoreAudioUtil::IsSupported();
   if (cmd_line->HasSwitch(switches::kEnableExclusiveAudio)) {
     // TODO(crogers): tune these values for best possible WebAudio performance.
     // WebRTC works well at 48kHz and a buffer size of 480 samples will be used
     // for this case. Note that exclusive mode is experimental.
     // This sample rate will be combined with a buffer size of 256 samples,
     // which corresponds to an output delay of ~5.33ms.
     sample_rate = 48000;
     buffer_size = 256;
+    if (input_params.IsValid())
+      channel_layout = input_params.channel_layout();
   } else if (!use_input_params) {
     // Hardware sample-rate on Windows can be configured, so we must query.
     // TODO(henrika): improve possibility to specify an audio endpoint.
     // Use the default device (same as for Wave) for now to be compatible.
     int hw_sample_rate = WASAPIAudioOutputStream::HardwareSampleRate();
 
     AudioParameters params;
     HRESULT hr = CoreAudioUtil::GetPreferredAudioParameters(eRender, eConsole,
                                                             &params);
     int hw_buffer_size =
         FAILED(hr) ? kFallbackBufferSize : params.frames_per_buffer();
     channel_layout = WASAPIAudioOutputStream::HardwareChannelLayout();
 
     // TODO(henrika): Figure out the right thing to do here.
     if (hw_sample_rate && hw_buffer_size) {
       sample_rate = hw_sample_rate;
       buffer_size = hw_buffer_size;
     } else {
       use_input_params = true;
     }
   }
 
   if (input_params.IsValid()) {
-    if (CoreAudioUtil::IsSupported() &&
-        CoreAudioUtil::IsChannelLayoutSupported(eRender, eConsole,
-            input_params.channel_layout())) {
-      // Open up using the same channel layout as the source if it is
-      // supported by the hardware.
-      channel_layout = input_params.channel_layout();
-      VLOG(1) << "Hardware channel layout is not used; using same "
-              << "layout as the source instead (" << channel_layout << ")";
+    if (CoreAudioUtil::IsSupported()) {
+      // Check if it is possible to open up at the specified input channel
+      // layout but avoid checking if the specified layout is the same as the
+      // hardware (preferred) layout. We do this extra check to avoid the
+      // CoreAudioUtil::IsChannelLayoutSupported() overhead in most cases.
+      if (input_params.channel_layout() != channel_layout) {
+        if (CoreAudioUtil::IsChannelLayoutSupported(
+                eRender, eConsole, input_params.channel_layout())) {
+          // Open up using the same channel layout as the source if it is
+          // supported by the hardware.
+          channel_layout = input_params.channel_layout();
+          VLOG(1) << "Hardware channel layout is not used; using same layout"
+                  << " as the source instead (" << channel_layout << ")";
+        }
+      }
     }
     input_channels = input_params.input_channels();
     if (use_input_params) {
       // If WASAPI isn't supported we'll fallback to WaveOut, which will take
       // care of resampling and bits per sample changes.  By setting these
       // equal to the input values, AudioOutputResampler will skip resampling
       // and bit per sample differences (since the input parameters will match
       // the output parameters).
       sample_rate = input_params.sample_rate();
       bits_per_sample = input_params.bits_per_sample();
@@ skipping 28 matching lines @@
   return new PCMWaveInAudioInputStream(this, params, kNumInputBuffers,
                                        xp_device_id);
 }
 
 /// static
 AudioManager* CreateAudioManager() {
   return new AudioManagerWin();
 }
 
 }  // namespace media