[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|48000104          channel_masks AUDIO_CHANNEL_OUT_STEREO105          formats AUDIO_FORMAT_PCM_16_BIT106          devices AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET107         }108     }109   }110   usb {111     outputs {112       usb_accessory {113         sampling_rates 44100114         channel_masks AUDIO_CHANNEL_OUT_STEREO115         formats AUDIO_FORMAT_PCM_16_BIT116         devices AUDIO_DEVICE_OUT_USB_ACCESSORY117       }118 #      usb_device {119 #        sampling_rates 44100120 #        channel_masks AUDIO_CHANNEL_OUT_STEREO121 #        formats AUDIO_FORMAT_PCM_16_BIT122 #        devices AUDIO_DEVICE_OUT_USB_DEVICE123 #      }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：

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

图1

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