[Android][Audio] audio_policy.conf文件分析

　　不同的Android产品在音频的设计上通常是存在差异的，而这些差异可以同过Audio的配置文件audio_policy.conf来获得。在Android系统中音频配置文件存放路径有两处，存放地址可以从AudioPolicyManagerBase.cpp文件中知道：

 #define AUDIO_POLICY_VENDOR_CONFIG_FILE  "/vendor/etc/audio_policy.conf"

 #define AUDIO_POLICY_CONFIG_FILE         "/system/etc/audio_policy.conf"

　　在AudioPolicyManager.cpp文件中可以知道系统会首先加载vendor/etc目录下的configure文件，再加载system/etc目录下的configure文件。若这两者加载都发生错误的话，系统会加载default配置文件，并命名为primary module，从这可以看出，音频系统中一定必须存在的module就是primary了。

1     if (loadAudioPolicyConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE) != NO_ERROR) {

2         if (loadAudioPolicyConfig(AUDIO_POLICY_CONFIG_FILE) != NO_ERROR) {

3             ALOGE("could not load audio policy configuration file, setting defaults");

4             defaultAudioPolicyConfig();

5         }

6     }

---

　　【摘录audio_policy.conf】　

  1 # Global configuration section: lists input and output devices always present on the device

  2 # as well as the output device selected by default.

  3 # Devices are designated by a string that corresponds to the enum in audio.h

  4 

  5 global_configuration {

  6   attached_output_devices AUDIO_DEVICE_OUT_SPEAKER

  7   default_output_device AUDIO_DEVICE_OUT_SPEAKER

  8   attached_input_devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_VOICE_CALL|AUDIO_DEVICE_IN_REMOTE_SUBMIX

  9   custom_properties {

 10     voice_volume_applied_after_mixing_in_call true

 11     voice_volume_applied_after_mixing_in_communication false

 12     in_call_music_attenuation_dB 6

 13     }

 14 }

 15 

 16 # audio hardware module section: contains descriptors for all audio hw modules present on the

 17 # device. Each hw module node is named after the corresponding hw module library base name.

 18 # For instance, "primary" corresponds to audio.primary.<device>.so.

 19 # The "primary" module is mandatory and must include at least one output with

 20 # AUDIO_OUTPUT_FLAG_PRIMARY flag.

 21 # Each module descriptor contains one or more output profile descriptors and zero or more

 22 # input profile descriptors. Each profile lists all the parameters supported by a given output

 23 # or input stream category.

 24 # The "channel_masks", "formats", "devices" and "flags" are specified using strings corresponding

 25 # to enums in audio.h and audio_policy.h. They are concatenated by use of "|" without space or "\n".

 26 

 27 audio_hw_modules {

 28   primary {

 29     outputs {

 30       primary {

 31         sampling_rates 48000

 32         channel_masks AUDIO_CHANNEL_OUT_STEREO

 33         formats AUDIO_FORMAT_PCM_16_BIT

 34         devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE|AUDIO_DEVICE_OUT_ALL_SCO

 35         flags AUDIO_OUTPUT_FLAG_PRIMARY

 36       }

 37       deep_buffer {

 38         sampling_rates 48000

 39         channel_masks AUDIO_CHANNEL_OUT_STEREO

 40         formats AUDIO_FORMAT_PCM_16_BIT

 41         devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE

 42         flags AUDIO_OUTPUT_FLAG_DEEP_BUFFER

 43       }

 44     }

 45     inputs {

 46       primary {

 47         sampling_rates 8000|11025|12000|16000|22050|24000|32000|44100|48000

 48         channel_masks AUDIO_CHANNEL_IN_MONO|AUDIO_CHANNEL_IN_STEREO

 49         formats AUDIO_FORMAT_PCM_16_BIT

 50         devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET|AUDIO_DEVICE_IN_WIRED_HEADSET|AUDIO_DEVICE_IN_VOICE_CALL

 51       }

 52     }

 53   }

 54   a2dp {

 55     outputs {

 56       a2dp {

 57         sampling_rates 44100

 58         channel_masks AUDIO_CHANNEL_OUT_STEREO

 59         formats AUDIO_FORMAT_PCM_16_BIT

 60         devices AUDIO_DEVICE_OUT_ALL_A2DP

 61       }

 62     }

 63   }

 64   hdmi {

 65     outputs {

 66      hdmi_stereo {

 67         sampling_rates 48000

 68         channel_masks AUDIO_CHANNEL_OUT_STEREO

 69         formats AUDIO_FORMAT_PCM_16_BIT

 70         devices AUDIO_DEVICE_OUT_AUX_DIGITAL

 71       }

 72       hdmi_multi {

 73         sampling_rates 32000|44100|48000|88200|96000

 74        # channel_masks AUDIO_CHANNEL_OUT_5POINT1

 75         channel_masks dynamic

 76         formats AUDIO_FORMAT_PCM_16_BIT

 77         devices AUDIO_DEVICE_OUT_AUX_DIGITAL

 78         flags AUDIO_OUTPUT_FLAG_DIRECT

 79       }

 80     }

 81   }

 82   r_submix {

 83     outputs {

 84       submix {

 85         sampling_rates 48000

 86         channel_masks AUDIO_CHANNEL_OUT_STEREO

 87         formats AUDIO_FORMAT_PCM_16_BIT

 88         devices AUDIO_DEVICE_OUT_REMOTE_SUBMIX

 89       }

 90     }

 91     inputs {

 92       submix {

 93         sampling_rates 48000

 94         channel_masks AUDIO_CHANNEL_IN_STEREO

 95         formats AUDIO_FORMAT_PCM_16_BIT

 96         devices AUDIO_DEVICE_IN_REMOTE_SUBMIX

 97       }

 98     }

 99   }

100   hs_usb {

101     outputs {

102        usb_dgtl {

103          sampling_rates 44100|48000

104          channel_masks AUDIO_CHANNEL_OUT_STEREO

105          formats AUDIO_FORMAT_PCM_16_BIT

106          devices AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET

107         }

108     }

109   }

110   usb {

111     outputs {

112       usb_accessory {

113         sampling_rates 44100

114         channel_masks AUDIO_CHANNEL_OUT_STEREO

115         formats AUDIO_FORMAT_PCM_16_BIT

116         devices AUDIO_DEVICE_OUT_USB_ACCESSORY

117       }

118 #      usb_device {

119 #        sampling_rates 44100

120 #        channel_masks AUDIO_CHANNEL_OUT_STEREO

121 #        formats AUDIO_FORMAT_PCM_16_BIT

122 #        devices AUDIO_DEVICE_OUT_USB_DEVICE

123 #      }

124     }

125   } }

　　从audio_policy.conf文件中可以发现，系统包含了primary、a2dp、usb等音频接口，对应着系统中的audio.<primary/a2dp/usb>.<device>.so。每个音频接口中又包含了若干个outputs & inputs，并且每个output or input又包含了若干个devices，且还有采样频率，声道数等信息。这些devices信息、采样频率信息 & 声道信息等都会保存在各自module的IOProfile中。按上文中audio_policy.conf配置文件所描述，系统最后会生成6个modules(eg.primary,a2dp,hdmi,r_submix,hs_usb & usb)以及7个outputs。以AUDIO_DEVICE_OUT_SPEAKER为例，该device会定义在primary模块中outputs所属的IOProfile1中，其它设备依次类推。

　　根据audio_policy.conf文件可以简化Audio module的架构，如下图1所示：

图1