audio_manager_cras: Set minimum output buffer size per board

Add a map to set different minimum output buffer size per board.
We can use smaller minimum output buffer size for powerful board, and
use larger minimum output buffer size for slower board.

BUG=581679
TEST=On kevin board, check block size is 768 on WebAudio test page.
TEST=On samus board, check block size is 256 on WebAudio test page.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2787433002
Cr-Commit-Position: refs/heads/master@{#464411}

media/audio/cras/audio_manager_cras.cc

@@ -16,6 +16,7 @@
 #include "base/nix/xdg_util.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/sys_info.h"
 #include "chromeos/audio/audio_device.h"
 #include "chromeos/audio/cras_audio_handler.h"
 #include "media/audio/audio_device_description.h"
@@ -295,12 +296,23 @@ AudioInputStream* AudioManagerCras::MakeLowLatencyInputStream(
   return MakeInputStream(params, device_id);
 }
 
+int AudioManagerCras::GetMinimumOutputBufferSizePerBoard() {
+  // On faster boards we can use smaller buffer size for lower latency.
+  // On slower boards we should use larger buffer size to prevent underrun.
+  std::string board = base::SysInfo::GetLsbReleaseBoard();
+  if (board == "kevin")
+    return 768;
+  else if (board == "samus")
+    return 256;
+  return kMinimumOutputBufferSize;
+}
+
 AudioParameters AudioManagerCras::GetPreferredOutputStreamParameters(
     const std::string& output_device_id,
     const AudioParameters& input_params) {
   ChannelLayout channel_layout = CHANNEL_LAYOUT_STEREO;
   int sample_rate = kDefaultSampleRate;
-  int buffer_size = kMinimumOutputBufferSize;
+  int buffer_size = GetMinimumOutputBufferSizePerBoard();
   int bits_per_sample = 16;
   if (input_params.IsValid()) {
     sample_rate = input_params.sample_rate();

media/audio/cras/audio_manager_cras.h

@@ -72,6 +72,9 @@ class MEDIA_EXPORT AudioManagerCras : public AudioManagerBase {
   AudioInputStream* MakeInputStream(const AudioParameters& params,
                                     const std::string& device_id);
 
+  // Get minimum output buffer size for this board.
+  int GetMinimumOutputBufferSizePerBoard();
+
   void GetAudioDeviceNamesImpl(bool is_input, AudioDeviceNames* device_names);
 
   void AddBeamformingDevices(AudioDeviceNames* device_names);