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如下图为android音频hal层所处的位置：
从上图可以看出，HAL层下面使用TiniAlsa（Android下一个简约的Alsa版本）。HAL层分为两部分，一部分为各种音频设备，每种音频设备由一个独立的库文件实现：如audio.a2dp.default.so（管理蓝牙a2dp音频），audio.usb.default.so（管理usb外接的音频），audio.primary.default.so（管理设备上的大部分音频）。另一部分为厂家自己实现的音频策略，Android下提供了默认一套音频策略，当厂家有特殊的音频策略时，可以在这部分修改实现。HAL层上面就是音频系统的核心AudioFlinger，这里实现了各种输入、输出音频流的管理，管理实时可用的音频通道，为各种音频流选择音频通道，实现多个音频流的混音等。
这里只介绍HAL中各种音频设备的管理，如何确定各种音频设备支持哪些输入、输出音频等。AudioFlinger在加载音频设备库文件时，从/system/lib/hw/下查找以audio开头的库，同时根据audio_policy.conf中定义的音频设备名（如a2dp、usb、primary）作为库的第二部分名称，对于没有特别指定的，库的第三部分名称就是default，所以加载音频设备库的名称就可以确定了，如：audio.a2dp.default.so。每个音频库的实现接口都是一样的，这样就可以让AudioFlinger使用相同的接口调用不同音频设备。
android\external\bluetooth\bluedroid\audio_a2dp_hw\Audio_a2dp_hw.c
/* 音频open函数，用于确定音频输入输出流的实现接口，各种音频数据格式的设置获取接口 */
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;
}
/* 音频模块open接口函数 */
static struct hw_module_methods_t hal_module_methods = {
.open = adev_open,
};
/* 定义的音频模块 */
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,
},
};
在音频设备库的实现代码里，没有看到该音频设备库支持哪些音频输入、输出设备，音频设备支持的输入、输出设备不是在代码里面写死的，而是通过audio_policy.conf文件进行配置的。如下面的一份配置文件。
audio_hw_modules {
primary { /* 主音频配置文件 */
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音频配置文件 */
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音频配置文件 */
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
}
}
}
}
可以看到对于a2dp来说，只支持音频输出，不支持音频输入，所以只有outputs device，没有inputs device，而对于主音频设备、就有inputs device。然后在device中就会配置该音频设备支持的输入、输出音频，如a2dp的输出音频就支持：AUDIO_DEVICE_OUT_BLUETOOTH_A2DP|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES|AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER。支持的输入音频设备定义也类似，如主音频中inputs device中的定义。 Android下各种音频输入、输出设备定义如下：
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),
};
这样，有了音频设备库及音频配置文件，就可以确定各个音频设备支持哪些输入、输出音频了。这样当上层需要输出一个声音时，就可以根据策略层选择的输出音频设备，选择该输出音频设备对应音频设备库接口。
现在再看一下主音频设备，在主音频设备支持音频输出中，既有AUDIO_DEVICE_OUT_SPEAKER（喇叭），又有AUDIO_DEVICE_OUT_WIRED_HEADSET （有线耳机）AUDIO_DEVICE_OUT_ALL_SCO（蓝牙通话PCM），而在音频设备库中只有一个adev_open_output_stream接口，这时，就需要在adev_open_output_stream中根据实际的音频输出设备进行不同的操作了。
如下面一个例子代码：
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(喇叭输出时处理) */
/* 省略代码 */
} else if (out->devices & AUDIO_DEVICE_OUT_WIRED_HEADSET) {
/* AUDIO_DEVICE_OUT_WIRED_HEADSET (有线耳机输出时处理) */
/* 省略代码 */
} else if (out->devices & AUDIO_DEVICE_OUT_ALL_SCO) {
/* AUDIO_DEVICE_OUT_ALL_SCO (蓝牙PCM输出时处理) */
/* 省略代码 */
} else {
/* 省略代码 */
}
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