android音訊hal層簡介

簡單的過客發表於2016-09-03
Android音訊
如下圖為android音訊hal層所處的位置：

這裡寫圖片描述

從上圖可以看出，HAL層下面使用TiniAlsa（Android下一個簡約的Alsa版本）。HAL層分為兩部分，一部分為各種音訊裝置，每種音訊裝置由一個獨立的庫檔案實現：如audio.a2dp.default.so（管理藍芽a2dp音訊），audio.usb.default.so（管理usb外接的音訊），audio.primary.default.so（管理裝置上的大部分音訊）。另一部分為廠家自己實現的音訊策略，Android下提供了預設一套音訊策略，當廠家有特殊的音訊策略時，可以在這部分修改實現。HAL層上面就是音訊系統的核心AudioFlinger，這裡實現了各種輸入、輸出音訊流的管理，管理實時可用的音訊通道，為各種音訊流選擇音訊通道，實現多個音訊流的混音等。
這裡只介紹HAL中各種音訊裝置的管理，如何確定各種音訊裝置支援哪些輸入、輸出音訊等。AudioFlinger在載入音訊裝置庫檔案時，從/system/lib/hw/下查詢以audio開頭的庫，同時根據audio_policy.conf中定義的音訊裝置名（如a2dp、usb、primary）作為庫的第二部分名稱，對於沒有特別指定的，庫的第三部分名稱就是default，所以載入音訊裝置庫的名稱就可以確定了，如：audio.a2dp.default.so。每個音訊庫的實現介面都是一樣的，這樣就可以讓AudioFlinger使用相同的介面呼叫不同音訊裝置。

android\external\bluetooth\bluedroid\audio_a2dp_hw\Audio_a2dp_hw.c
/* 音訊open函式，用於確定音訊輸入輸出流的實現介面，各種音訊資料格式的設定獲取介面 */
static int adev_open(const hw_module_t* module, const char* name,
                     hw_device_t** device)
{
    struct a2dp_audio_device *adev;
    int ret;

    INFO(" adev_open in A2dp_hw module");
    FNLOG();

    if (strcmp(name, AUDIO_HARDWARE_INTERFACE) != 0)
    {
        ERROR("interface %s not matching [%s]", name, AUDIO_HARDWARE_INTERFACE);
        return -EINVAL;
    }

    adev = calloc(1, sizeof(struct a2dp_audio_device));

    if (!adev)
        return -ENOMEM;

    adev->device.common.tag = HARDWARE_DEVICE_TAG;
    adev->device.common.version = AUDIO_DEVICE_API_VERSION_2_0;
    adev->device.common.module = (struct hw_module_t *) module;
    adev->device.common.close = adev_close;

    adev->device.init_check = adev_init_check;
    adev->device.set_voice_volume = adev_set_voice_volume;
    adev->device.set_master_volume = adev_set_master_volume;
    adev->device.set_mode = adev_set_mode;
    adev->device.set_mic_mute = adev_set_mic_mute;
    adev->device.get_mic_mute = adev_get_mic_mute;
    adev->device.set_parameters = adev_set_parameters;
    adev->device.get_parameters = adev_get_parameters;
    adev->device.get_input_buffer_size = adev_get_input_buffer_size;
    adev->device.open_output_stream = adev_open_output_stream;
    adev->device.close_output_stream = adev_close_output_stream;
    adev->device.open_input_stream = adev_open_input_stream;
    adev->device.close_input_stream = adev_close_input_stream;
    adev->device.dump = adev_dump;

    adev->output = NULL;


    *device = &adev->device.common;

    return 0;
}

/* 音訊模組open介面函式 */
static struct hw_module_methods_t hal_module_methods = {
    .open = adev_open,
};

/* 定義的音訊模組 */
struct audio_module HAL_MODULE_INFO_SYM = {
    .common = {
        .tag = HARDWARE_MODULE_TAG,
        .version_major = 1,
        .version_minor = 0,
        .id = AUDIO_HARDWARE_MODULE_ID,
        .name = "A2DP Audio HW HAL",
        .author = "The Android Open Source Project",
        .methods = &hal_module_methods,
    },
};
在音訊裝置庫的實現程式碼裡，沒有看到該音訊裝置庫支援哪些音訊輸入、輸出裝置，音訊裝置支援的輸入、輸出裝置不是在程式碼裡面寫死的，而是通過audio_policy.conf檔案進行配置的。如下面的一份配置檔案。

audio_hw_modules {
  primary {   /* 主音訊配置檔案 */
    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_hal_version 3.0
    }
    devices {
      speaker {
        type AUDIO_DEVICE_OUT_SPEAKER
        gains {
          gain_1 {
            mode AUDIO_GAIN_MODE_JOINT
            min_value_mB -8400
            max_value_mB 4000
            default_value_mB 0
            step_value_mB 100
          }
        }
      }
    }
    outputs {
      primary {
        sampling_rates 48000
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_EARPIECE|AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE|AUDIO_DEVICE_OUT_AUX_DIGITAL|AUDIO_DEVICE_OUT_ALL_SCO
        flags AUDIO_OUTPUT_FLAG_PRIMARY
      }
    }
    inputs {
      primary {
        sampling_rates 8000|16000
        channel_masks AUDIO_CHANNEL_IN_MONO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_ALL_SCO 
      }
    }
  }

  a2dp {   /* a2dp音訊配置檔案 */
    global_configuration {
      attached_output_devices AUDIO_DEVICE_OUT_BLUETOOTH_A2DP
      audio_hal_version 2.0
    }  
    outputs {
      a2dp {
        sampling_rates 44100
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_BLUETOOTH_A2DP|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER
      }
    }
  }

  usb {   /* usb音訊配置檔案 */
    global_configuration {
      attached_output_devices AUDIO_DEVICE_OUT_USB_ACCESSORY
      audio_hal_version 2.0
    }  
    outputs {
      usb_accessory {
        sampling_rates 44100
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_USB_ACCESSORY
      }
      usb_device {
        sampling_rates 44100
        channel_masks AUDIO_CHANNEL_OUT_STEREO
        formats AUDIO_FORMAT_PCM_16_BIT
        devices AUDIO_DEVICE_OUT_USB_DEVICE
      }
    }
  }
}

可以看到對於a2dp來說，只支援音訊輸出，不支援音訊輸入，所以只有outputs device，沒有inputs device，而對於主音訊裝置、就有inputs device。然後在device中就會配置該音訊裝置支援的輸入、輸出音訊，如a2dp的輸出音訊就支援：AUDIO_DEVICE_OUT_BLUETOOTH_A2DP|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER。支援的輸入音訊裝置定義也類似，如主音訊中inputs device中的定義。
Android下各種音訊輸入、輸出裝置定義如下：

android/system/core/include/system/audio.h

enum {
    AUDIO_DEVICE_NONE                          = 0x0,
    /* reserved bits */
    AUDIO_DEVICE_BIT_IN                        = 0x80000000,
    AUDIO_DEVICE_BIT_DEFAULT                   = 0x40000000,
    /* output devices */
    AUDIO_DEVICE_OUT_EARPIECE                  = 0x1,
    AUDIO_DEVICE_OUT_SPEAKER                   = 0x2,
    AUDIO_DEVICE_OUT_WIRED_HEADSET             = 0x4,
    AUDIO_DEVICE_OUT_WIRED_HEADPHONE           = 0x8,
    AUDIO_DEVICE_OUT_BLUETOOTH_SCO             = 0x10,
    AUDIO_DEVICE_OUT_BLUETOOTH_SCO_HEADSET     = 0x20,
    AUDIO_DEVICE_OUT_BLUETOOTH_SCO_CARKIT      = 0x40,
    AUDIO_DEVICE_OUT_BLUETOOTH_A2DP            = 0x80,
    AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES = 0x100,
    AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER    = 0x200,
    AUDIO_DEVICE_OUT_AUX_DIGITAL               = 0x400,
    AUDIO_DEVICE_OUT_HDMI                      = AUDIO_DEVICE_OUT_AUX_DIGITAL,
    /* uses an analog connection (multiplexed over the USB connector pins for instance) */
    AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET         = 0x800,
    AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET         = 0x1000,
    /* USB accessory mode: your Android device is a USB device and the dock is a USB host */
    AUDIO_DEVICE_OUT_USB_ACCESSORY             = 0x2000,
    /* USB host mode: your Android device is a USB host and the dock is a USB device */
    AUDIO_DEVICE_OUT_USB_DEVICE                = 0x4000,
    AUDIO_DEVICE_OUT_REMOTE_SUBMIX             = 0x8000,
    /* Telephony voice TX path */
    AUDIO_DEVICE_OUT_TELEPHONY_TX              = 0x10000,
    /* Analog jack with line impedance detected */
    AUDIO_DEVICE_OUT_LINE                      = 0x20000,
    /* HDMI Audio Return Channel */
    AUDIO_DEVICE_OUT_HDMI_ARC                  = 0x40000,
    /* S/PDIF out */
    AUDIO_DEVICE_OUT_SPDIF                     = 0x80000,
    /* FM transmitter out */
    AUDIO_DEVICE_OUT_FM                        = 0x100000,
    /* Line out for av devices */
    AUDIO_DEVICE_OUT_AUX_LINE                  = 0x200000,
    /* limited-output speaker device for acoustic safety */
    AUDIO_DEVICE_OUT_SPEAKER_SAFE              = 0x400000,
    AUDIO_DEVICE_OUT_DEFAULT                   = AUDIO_DEVICE_BIT_DEFAULT,
    AUDIO_DEVICE_OUT_ALL      = (AUDIO_DEVICE_OUT_EARPIECE |
                                 AUDIO_DEVICE_OUT_SPEAKER |
                                 AUDIO_DEVICE_OUT_WIRED_HEADSET |
                                 AUDIO_DEVICE_OUT_WIRED_HEADPHONE |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_SCO |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_SCO_HEADSET |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_SCO_CARKIT |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_A2DP |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER |
                                 AUDIO_DEVICE_OUT_HDMI |
                                 AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET |
                                 AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET |
                                 AUDIO_DEVICE_OUT_USB_ACCESSORY |
                                 AUDIO_DEVICE_OUT_USB_DEVICE |
                                 AUDIO_DEVICE_OUT_REMOTE_SUBMIX |
                                 AUDIO_DEVICE_OUT_TELEPHONY_TX |
                                 AUDIO_DEVICE_OUT_LINE |
                                 AUDIO_DEVICE_OUT_HDMI_ARC |
                                 AUDIO_DEVICE_OUT_SPDIF |
                                 AUDIO_DEVICE_OUT_FM |
                                 AUDIO_DEVICE_OUT_AUX_LINE |
                                 AUDIO_DEVICE_OUT_SPEAKER_SAFE |
                                 AUDIO_DEVICE_OUT_DEFAULT),
    AUDIO_DEVICE_OUT_ALL_A2DP = (AUDIO_DEVICE_OUT_BLUETOOTH_A2DP |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER),
    AUDIO_DEVICE_OUT_ALL_SCO  = (AUDIO_DEVICE_OUT_BLUETOOTH_SCO |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_SCO_HEADSET |
                                 AUDIO_DEVICE_OUT_BLUETOOTH_SCO_CARKIT),
    AUDIO_DEVICE_OUT_ALL_USB  = (AUDIO_DEVICE_OUT_USB_ACCESSORY |
                                 AUDIO_DEVICE_OUT_USB_DEVICE),

    /* input devices */
    AUDIO_DEVICE_IN_COMMUNICATION         = AUDIO_DEVICE_BIT_IN | 0x1,
    AUDIO_DEVICE_IN_AMBIENT               = AUDIO_DEVICE_BIT_IN | 0x2,
    AUDIO_DEVICE_IN_BUILTIN_MIC           = AUDIO_DEVICE_BIT_IN | 0x4,
    AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET = AUDIO_DEVICE_BIT_IN | 0x8,
    AUDIO_DEVICE_IN_WIRED_HEADSET         = AUDIO_DEVICE_BIT_IN | 0x10,
    AUDIO_DEVICE_IN_AUX_DIGITAL           = AUDIO_DEVICE_BIT_IN | 0x20,
    AUDIO_DEVICE_IN_HDMI                  = AUDIO_DEVICE_IN_AUX_DIGITAL,
    /* Telephony voice RX path */
    AUDIO_DEVICE_IN_VOICE_CALL            = AUDIO_DEVICE_BIT_IN | 0x40,
    AUDIO_DEVICE_IN_TELEPHONY_RX          = AUDIO_DEVICE_IN_VOICE_CALL,
    AUDIO_DEVICE_IN_BACK_MIC              = AUDIO_DEVICE_BIT_IN | 0x80,
    AUDIO_DEVICE_IN_REMOTE_SUBMIX         = AUDIO_DEVICE_BIT_IN | 0x100,
    AUDIO_DEVICE_IN_ANLG_DOCK_HEADSET     = AUDIO_DEVICE_BIT_IN | 0x200,
    AUDIO_DEVICE_IN_DGTL_DOCK_HEADSET     = AUDIO_DEVICE_BIT_IN | 0x400,
    AUDIO_DEVICE_IN_USB_ACCESSORY         = AUDIO_DEVICE_BIT_IN | 0x800,
    AUDIO_DEVICE_IN_USB_DEVICE            = AUDIO_DEVICE_BIT_IN | 0x1000,
    /* FM tuner input */
    AUDIO_DEVICE_IN_FM_TUNER              = AUDIO_DEVICE_BIT_IN | 0x2000,
    /* TV tuner input */
    AUDIO_DEVICE_IN_TV_TUNER              = AUDIO_DEVICE_BIT_IN | 0x4000,
    /* Analog jack with line impedance detected */
    AUDIO_DEVICE_IN_LINE                  = AUDIO_DEVICE_BIT_IN | 0x8000,
    /* S/PDIF in */
    AUDIO_DEVICE_IN_SPDIF                 = AUDIO_DEVICE_BIT_IN | 0x10000,
    AUDIO_DEVICE_IN_BLUETOOTH_A2DP        = AUDIO_DEVICE_BIT_IN | 0x20000,
    AUDIO_DEVICE_IN_LOOPBACK              = AUDIO_DEVICE_BIT_IN | 0x40000,
    AUDIO_DEVICE_IN_DEFAULT               = AUDIO_DEVICE_BIT_IN | AUDIO_DEVICE_BIT_DEFAULT,

    AUDIO_DEVICE_IN_ALL     = (AUDIO_DEVICE_IN_COMMUNICATION |
                               AUDIO_DEVICE_IN_AMBIENT |
                               AUDIO_DEVICE_IN_BUILTIN_MIC |
                               AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET |
                               AUDIO_DEVICE_IN_WIRED_HEADSET |
                               AUDIO_DEVICE_IN_HDMI |
                               AUDIO_DEVICE_IN_TELEPHONY_RX |
                               AUDIO_DEVICE_IN_BACK_MIC |
                               AUDIO_DEVICE_IN_REMOTE_SUBMIX |
                               AUDIO_DEVICE_IN_ANLG_DOCK_HEADSET |
                               AUDIO_DEVICE_IN_DGTL_DOCK_HEADSET |
                               AUDIO_DEVICE_IN_USB_ACCESSORY |
                               AUDIO_DEVICE_IN_USB_DEVICE |
                               AUDIO_DEVICE_IN_FM_TUNER |
                               AUDIO_DEVICE_IN_TV_TUNER |
                               AUDIO_DEVICE_IN_LINE |
                               AUDIO_DEVICE_IN_SPDIF |
                               AUDIO_DEVICE_IN_BLUETOOTH_A2DP |
                               AUDIO_DEVICE_IN_LOOPBACK |
                               AUDIO_DEVICE_IN_DEFAULT),
    AUDIO_DEVICE_IN_ALL_SCO = AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET,
    AUDIO_DEVICE_IN_ALL_USB  = (AUDIO_DEVICE_IN_USB_ACCESSORY |
                                AUDIO_DEVICE_IN_USB_DEVICE),
};
這樣，有了音訊裝置庫及音訊配置檔案，就可以確定各個音訊裝置支援哪些輸入、輸出音訊了。這樣當上層需要輸出一個聲音時，就可以根據策略層選擇的輸出音訊裝置，選擇該輸出音訊裝置對應音訊裝置庫介面。
現在再看一下主音訊裝置，在主音訊裝置支援音訊輸出中，既有AUDIO_DEVICE_OUT_SPEAKER（喇叭），又有AUDIO_DEVICE_OUT_WIRED_HEADSET  （有線耳機）AUDIO_DEVICE_OUT_ALL_SCO（藍芽通話PCM），而在音訊裝置庫中只有一個adev_open_output_stream介面，這時，就需要在adev_open_output_stream中根據實際的音訊輸出裝置進行不同的操作了。
如下面一個例子程式碼：

static int adev_open_output_stream(struct audio_hw_device *dev,
                                   audio_io_handle_t handle,
                                   audio_devices_t devices,
                                   audio_output_flags_t flags,
                                   struct audio_config *config,
                                   struct audio_stream_out **stream_out,
                                   const char *address __unused)
{
    struct audio_device *adev = (struct audio_device *)dev;
    struct stream_out *out;
    int i, ret;

    ALOGV("%s: enter: sample_rate(%d) channel_mask(%#x) devices(%#x) flags(%#x)",
          __func__, config->sample_rate, config->channel_mask, devices, flags);
    *stream_out = NULL;
    out = (struct stream_out *)calloc(1, sizeof(struct stream_out));

    if (devices == AUDIO_DEVICE_NONE)
        devices = AUDIO_DEVICE_OUT_SPEAKER;

    out->flags = flags;
    out->devices = devices;
    out->dev = adev;
    out->format = config->format;
    out->sample_rate = config->sample_rate;
    out->channel_mask = AUDIO_CHANNEL_OUT_STEREO;
    out->supported_channel_masks[0] = AUDIO_CHANNEL_OUT_STEREO;
    out->handle = handle;

    /* Init use case and pcm_config */
if (out->devices & AUDIO_DEVICE_OUT_SPEAKER) {
   /* AUDIO_DEVICE_OUT_SPEAKER(喇叭輸出時處理)  */
       /*  省略程式碼 */
} else if (out->devices & AUDIO_DEVICE_OUT_WIRED_HEADSET) {
/* AUDIO_DEVICE_OUT_WIRED_HEADSET (有線耳機輸出時處理)  */
       /*  省略程式碼 */
} else  if (out->devices & AUDIO_DEVICE_OUT_ALL_SCO) {
   /* AUDIO_DEVICE_OUT_ALL_SCO (藍芽PCM輸出時處理)  */
       /*  省略程式碼 */
    } else {
       /*  省略程式碼 */
    }

相關文章