[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。

     if (loadAudioPolicyConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE) != NO_ERROR) {
         if (loadAudioPolicyConfig(AUDIO_POLICY_CONFIG_FILE) != NO_ERROR) {
             ALOGE("could not load audio policy configuration file, setting defaults");
             defaultAudioPolicyConfig();
         }
     }

---

　　【摘录audio_policy.conf】　

 # 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

 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_VOICE_CALL|AUDIO_DEVICE_IN_REMOTE_SUBMIX
   custom_properties {
     voice_volume_applied_after_mixing_in_call true
     voice_volume_applied_after_mixing_in_communication false
     in_call_music_attenuation_dB
     }
 }

 # 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_hw_modules {
   primary {
     outputs {
       primary {
         sampling_rates
         channel_masks AUDIO_CHANNEL_OUT_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE|AUDIO_DEVICE_OUT_ALL_SCO
         flags AUDIO_OUTPUT_FLAG_PRIMARY
       }
       deep_buffer {
         sampling_rates
         channel_masks AUDIO_CHANNEL_OUT_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE
         flags AUDIO_OUTPUT_FLAG_DEEP_BUFFER
       }
     }
     inputs {
       primary {
         sampling_rates ||||||||
         channel_masks AUDIO_CHANNEL_IN_MONO|AUDIO_CHANNEL_IN_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET|AUDIO_DEVICE_IN_WIRED_HEADSET|AUDIO_DEVICE_IN_VOICE_CALL
       }
     }
   }
   a2dp {
     outputs {
       a2dp {
         sampling_rates
         channel_masks AUDIO_CHANNEL_OUT_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_ALL_A2DP
       }
     }
   }
   hdmi {
     outputs {
      hdmi_stereo {
         sampling_rates
         channel_masks AUDIO_CHANNEL_OUT_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_AUX_DIGITAL
       }
       hdmi_multi {
         sampling_rates ||||
        # channel_masks AUDIO_CHANNEL_OUT_5POINT1
         channel_masks dynamic
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_AUX_DIGITAL
         flags AUDIO_OUTPUT_FLAG_DIRECT
       }
     }
   }
   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
       }
     }
   }
   hs_usb {
     outputs {
        usb_dgtl {
          sampling_rates |
          channel_masks AUDIO_CHANNEL_OUT_STEREO
          formats AUDIO_FORMAT_PCM_16_BIT
          devices AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET
         }
     }
   }
   usb {
     outputs {
       usb_accessory {
         sampling_rates
         channel_masks AUDIO_CHANNEL_OUT_STEREO
         formats AUDIO_FORMAT_PCM_16_BIT
         devices AUDIO_DEVICE_OUT_USB_ACCESSORY
       }
 #      usb_device {
 #        sampling_rates
 #        channel_masks AUDIO_CHANNEL_OUT_STEREO
 #        formats AUDIO_FORMAT_PCM_16_BIT
 #        devices AUDIO_DEVICE_OUT_USB_DEVICE
 #      }
     }
   } }

　　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