audio_policy.conf说明（翻译）

自己记着当笔记，水平有限，仅供参考

#
# Audio policy configuration for generic device builds (goldfish audio HAL - emulator)
#这是模拟器的Audio policy配置
#

# Global configuration section: lists input and output devices always present on the device
# as well as the output device selected by default.
# Devices are designated by a string that corresponds to the enum in audio.h

#全局配置部分：列出了设备上总是存在的输入输出设备，它也是系统默认选择的输出设备
#这里的输入输出设备必须是audio.h中列出的枚举类型

global_configuration {
  attached_output_devices AUDIO_DEVICE_OUT_SPEAKER
  default_output_device AUDIO_DEVICE_OUT_SPEAKER
  attached_input_devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_REMOTE_SUBMIX
}

# audio hardware module section: contains descriptors for all audio hw modules present on the
# device. Each hw module node is named after the corresponding hw module library base name.
# For instance, "primary" corresponds to audio.primary.<device>.so.
# The "primary" module is mandatory and must include at least one output with
# AUDIO_OUTPUT_FLAG_PRIMARY flag.
# Each module descriptor contains one or more output profile descriptors and zero or more
# input profile descriptors. Each profile lists all the parameters supported by a given output
# or input stream category.
# The "channel_masks", "formats", "devices" and "flags" are specified using strings corresponding
# to enums in audio.h and audio_policy.h. They are concatenated by use of "|" without space or "\n".

#audio hardware module部分：包含了对设备上的所有的audio hw modules的descriptors，每一个hw module节点
#的名字都与相应的so库名称相对应，例如："primary"对应audio.primary.<device>.so。其中"primary"module是
#必须的，而且它必须包含一个含AUDIO_OUTPUT_FLAG_PRIMARY标志的output，每一个module的descriptor必须至少
#包含一个output配置descriptor和大于等于0个的input配置descriptor，每一个配置都描述了输入输出流类型支持
#的参数，其中的 "channel_masks"、"formats"、"devices"和"flags"必须为audio.h、audio_policy.h中定义的枚
#举值，它们可以通过|连接，但不能包含空格和"\n"。
audio_hw_modules {
  primary {
    outputs {
      primary {
        sampling_rates
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_SPEAKER
        flags AUDIO_OUTPUT_FLAG_PRIMARY
      }
    }
    inputs {
      primary {
        sampling_rates |
        channel_masks AUDIO_CHANNEL_IN_MONO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_IN_BUILTIN_MIC
      }
    }
  }
  r_submix {
    outputs {
      submix {
        sampling_rates
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_REMOTE_SUBMIX
      }
    }
    inputs {
      submix {
        sampling_rates
        channel_masks AUDIO_CHANNEL_IN_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_IN_REMOTE_SUBMIX
      }
    }
  }
}

另外，存储各个audio_hw_module的结构体如下：

        class HwModule {
        public:
                    HwModule(const char *name);
                    ~HwModule();

            void dump(int fd);

            const char *const mName; // base name of the audio HW module (primary, a2dp ...)
            audio_module_handle_t mHandle;
            Vector <IOProfile *> mOutputProfiles; // output profiles exposed by this module
            Vector <IOProfile *> mInputProfiles;  // input profiles exposed by this module
        };

        // the IOProfile class describes the capabilities of an output or input stream.
        // It is currently assumed that all combination of listed parameters are supported.
        // It is used by the policy manager to determine if an output or input is suitable for
        // a given use case,  open/close it accordingly and connect/disconnect audio tracks
        // to/from it.
        class IOProfile
        {
        public:
            IOProfile(HwModule *module);
            ~IOProfile();

            bool isCompatibleProfile(audio_devices_t device,
                                     uint32_t samplingRate,
                                     uint32_t format,
                                     uint32_t channelMask,
                                     audio_output_flags_t flags) const;

            void dump(int fd);

            // by convention, "0' in the first entry in mSamplingRates, mChannelMasks or mFormats
            // indicates the supported parameters should be read from the output stream
            // after it is opened for the first time
            Vector <uint32_t> mSamplingRates; // supported sampling rates
            Vector <audio_channel_mask_t> mChannelMasks; // supported channel masks
            Vector <audio_format_t> mFormats; // supported audio formats
            audio_devices_t mSupportedDevices; // supported devices (devices this output can be
                                               // routed to)
            audio_output_flags_t mFlags; // attribute flags (e.g primary output,
                                                // direct output...). For outputs only.
            HwModule *mModule;                     // audio HW module exposing this I/O stream
        };

在AudioPolicyManagerBase类对象的mHwModules中保存了这些读取的配置。