Make OS audio buffer size limits visible.

media/audio/mac/audio_manager_mac.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/mac/audio_manager_mac.h"
 
 #include <algorithm>
 #include <limits>
 #include <vector>
 
@@ skipping 16 matching lines @@
 #include "media/base/bind_to_current_loop.h"
 #include "media/base/channel_layout.h"
 #include "media/base/limits.h"
 #include "media/base/media_switches.h"
 
 namespace media {
 
 // Maximum number of output streams that can be open simultaneously.
 static const int kMaxOutputStreams = 50;
 
-// Define bounds for for low-latency input and output streams.
-static const int kMinimumInputOutputBufferSize = 128;
-static const int kMaximumInputOutputBufferSize = 4096;
-
 // Default sample-rate on most Apple hardware.
 static const int kFallbackSampleRate = 44100;
 
 // Helper method to construct AudioObjectPropertyAddress structure given
 // property selector and scope. The property element is always set to
 // kAudioObjectPropertyElementMaster.
 static AudioObjectPropertyAddress GetAudioObjectPropertyAddress(
     AudioObjectPropertySelector selector,
     bool is_input) {
   AudioObjectPropertyScope scope = is_input ? kAudioObjectPropertyScopeInput
@@ skipping 779 matching lines @@
         kFallbackSampleRate, 16, ChooseBufferSize(false, kFallbackSampleRate));
   }
 
   const bool has_valid_input_params = input_params.IsValid();
   const int hardware_sample_rate = HardwareSampleRateForDevice(device);
 
   // Allow pass through buffer sizes.  If concurrent input and output streams
   // exist, they will use the smallest buffer size amongst them.  As such, each
   // stream must be able to FIFO requests appropriately when this happens.
   int buffer_size = ChooseBufferSize(false, hardware_sample_rate);
   if (has_valid_input_params) {

[o1ka, 2017/06/09 10:52:39]

combine into a conditional assignment?
int buffer_size = has_valid_input_params ? .. : ..

[Andrew MacPherson, 2017/06/12 11:23:32]

Done.

-    buffer_size =
-        std::min(kMaximumInputOutputBufferSize,
-                 std::max(input_params.frames_per_buffer(), buffer_size));
+    // If passed in via the input_params we allow buffer sizes to go as low as
+    // the the kMinimumInputOutputBufferSize, ignoring what ChooseBufferSize()
+    // normally returns.
+    buffer_size = std::min(

[o1ka, 2017/06/09 10:52:39]

Add a test to media/audio/audio_manager_unittest.cc to make sure
AudioManager::MakeAudioOutputStreamProxy behaves as we expect?

[Andrew MacPherson, 2017/06/12 11:23:32]

No problem, as soon as we agree that this approach is the right one I'll add a
few tests here. 

+        static_cast<int>(limits::kMaximumInputOutputBufferSize),
+        std::max(input_params.frames_per_buffer(),
+                 static_cast<int>(limits::kMinimumInputOutputBufferSize)));
   }
 
   int hardware_channels;
   if (!GetDeviceChannels(device, AUElement::OUTPUT, &hardware_channels))
     hardware_channels = 2;
 
   // Use the input channel count and channel layout if possible.  Let OSX take
   // care of remapping the channels; this lets user specified channel layouts
   // work correctly.
   int output_channels = input_params.channels();
@@ skipping 34 matching lines @@
 
 int AudioManagerMac::ChooseBufferSize(bool is_input, int sample_rate) {
   // kMinimumInputOutputBufferSize is too small for the output side because
   // CoreAudio can get into under-run if the renderer fails delivering data
   // to the browser within the allowed time by the OS. The workaround is to
   // use 256 samples as the default output buffer size for sample rates
   // smaller than 96KHz.
   // TODO(xians): Remove this workaround after WebAudio supports user defined
   // buffer size.  See https://github.com/WebAudio/web-audio-api/issues/348
   // for details.
-  int buffer_size = is_input ?
-      kMinimumInputOutputBufferSize : 2 * kMinimumInputOutputBufferSize;
+  int buffer_size = is_input ? limits::kMinimumInputOutputBufferSize
+                             : 2 * limits::kMinimumInputOutputBufferSize;
   const int user_buffer_size = GetUserBufferSize();
   if (user_buffer_size) {
     buffer_size = user_buffer_size;
   } else if (sample_rate > 48000) {
     // The default buffer size is too small for higher sample rates and may lead
     // to glitching.  Adjust upwards by multiples of the default size.
     if (sample_rate <= 96000)
-      buffer_size = 2 * kMinimumInputOutputBufferSize;
+      buffer_size = 2 * limits::kMinimumInputOutputBufferSize;
     else if (sample_rate <= 192000)
-      buffer_size = 4 * kMinimumInputOutputBufferSize;
+      buffer_size = 4 * limits::kMinimumInputOutputBufferSize;
   }
 
   return buffer_size;
 }
 
 bool AudioManagerMac::IsSuspending() const {
   DCHECK(GetTaskRunner()->BelongsToCurrentThread());
   return power_observer_->IsSuspending();
 }
 
@@ skipping 263 matching lines @@
 }
 
 std::unique_ptr<AudioManager> CreateAudioManager(
     std::unique_ptr<AudioThread> audio_thread,
     AudioLogFactory* audio_log_factory) {
   return base::MakeUnique<AudioManagerMac>(std::move(audio_thread),
                                            audio_log_factory);
 }
 
 }  // namespace media