最近在关注蓝牙耳机方面的问题，做下简单的流程分析。
解码后，在AudioFlinger里把音频数据写到设备里。这里主要看看AudioFlinger，AudioPolicyManager和external/bluetooth/bluez/audio里面的android_audio_hw.c和liba2dp.c。
在AudioPolicyManager里有设备连接判断。
status_t AudioPolicyManagerBase::setDeviceConnectionState(AudioSystem::audio_devices device,
AudioSystem::device_connection_state state,
const char *device_address)
{
#ifdef WITH_A2DP
// handle A2DP device connection
if (AudioSystem::isA2dpDevice(device)) {
status_t status = handleA2dpConnection(device, device_address);//这里是执行蓝牙连接
if (status != NO_ERROR) {
mAvailableOutputDevices &= ~device;
return status;
}
} else
#endif
}
。。。。。
status_t AudioPolicyManagerBase::handleA2dpConnection(AudioSystem::audio_devices device,
const char *device_address)
{
...............
mA2dpOutput = mpClientInterface->openOutput(&outputDesc->mDevice,
&outputDesc->mSamplingRate,
&outputDesc->mFormat,
&outputDesc->mChannels,
&outputDesc->mLatency,
outputDesc->mFlags);
if (mA2dpOutput) {
// add A2DP output descriptor
addOutput(mA2dpOutput, outputDesc);
//TODO: configure audio effect output stage here
// set initial stream volume for A2DP device
applyStreamVolumes(mA2dpOutput, device);
if (a2dpUsedForSonification()) {
mDuplicatedOutput = mpClientInterface->openDuplicateOutput(mA2dpOutput, mHardwareOutput);
}
if (mDuplicatedOutput != 0 ||
!a2dpUsedForSonification()) {
// If both A2DP and duplicated outputs are open, send device address to A2DP hardware
// interface
AudioParameter param;
param.add(String8("a2dp_sink_address"), String8(device_address));
mpClientInterface->setParameters(mA2dpOutput, param.toString());
mA2dpDeviceAddress = String8(device_address, MAX_DEVICE_ADDRESS_LEN);
if (a2dpUsedForSonification()) {
// add duplicated output descriptor
AudioOutputDescriptor *dupOutputDesc = new AudioOutputDescriptor();
dupOutputDesc->mOutput1 = mOutputs.valueFor(mHardwareOutput);
dupOutputDesc->mOutput2 = mOutputs.valueFor(mA2dpOutput);
dupOutputDesc->mSamplingRate = outputDesc->mSamplingRate;
dupOutputDesc->mFormat = outputDesc->mFormat;
dupOutputDesc->mChannels = outputDesc->mChannels;
dupOutputDesc->mLatency = outputDesc->mLatency;
addOutput(mDuplicatedOutput, dupOutputDesc);
applyStreamVolumes(mDuplicatedOutput, device);
}
} else {
.........
}
如果只是蓝牙播放，那么mDuplicatedOutput和a2dpUsedForSonification都为0，仅执行 addOutput(mA2dpOutput, outputDesc);，走类似speaker和麦克风的流程。
如果是蓝牙和speaker或麦克风同时播放声音，走mpClientInterface->openDuplicateOutput(mA2dpOutput, mHardwareOutput);注意的是，第一个参数是mA2dpOutput，蓝牙的，第二个参数是mHardwareOutput，speaker或mic等其他输出设备。
那么接下来我们去看看openDuplicateOutput函数。在AudioFlinger.cpp里有定义
int AudioFlinger::openDuplicateOutput(int output1, int output2)
{
Mutex::Autolock _l(mLock);
MixerThread *thread1 = checkMixerThread_l(output1);
MixerThread *thread2 = checkMixerThread_l(output2);
if (thread1 == NULL || thread2 == NULL) {
LOGW("openDuplicateOutput() wrong output mixer type for output %d or %d", output1, output2);
return 0;
}
int id = nextUniqueId();
DuplicatingThread *thread = new DuplicatingThread(this, thread1, id);
thread->addOutputTrack(thread2);
mPlaybackThreads.add(id, thread);
// notify client processes of the new output creation
thread->audioConfigChanged_l(AudioSystem::OUTPUT_OPENED);
return id;
}
这个函数开启2个线程，一个是蓝牙的，另一个是mHardwareOutput，实现两者同时播放，比如来电，短信，闹铃声等。
先暂停在这里，接着
static const char *audio_interfaces[] = {
"primary",
"a2dp",
"usb",
};
4.0的代码有三种audio接口，一个是primary，是codec，我们常用的speaker,mic等；二是a2dp，是蓝牙；三是usb接口。
void AudioFlinger::onFirstRef()
{
int rc = 0;
Mutex::Autolock _l(mLock);
/* TODO: move all this work into an Init() function */
mHardwareStatus = AUDIO_HW_IDLE;
for (size_t i = 0; i < ARRAY_SIZE(audio_interfaces); i++) {
const hw_module_t *mod;
audio_hw_device_t *dev;
rc = load_audio_interface(audio_interfaces[i], &mod, &dev);//加载接口.so文件
if (rc)
continue;
LOGI("Loaded %s audio interface from %s (%s)", audio_interfaces[i],
mod->name, mod->id);
mAudioHwDevs.push(dev);
.........
}
这里会遍历audio接口。
void AudioFlinger::RecordThread::onFirstRef()
{
run(mName, PRIORITY_URGENT_AUDIO);
}
int AudioFlinger::openOutput(uint32_t *pDevices,
uint32_t *pSamplingRate,
uint32_t *pFormat,
uint32_t *pChannels,
uint32_t *pLatencyMs,
uint32_t flags){
......
outHwDev = findSuitableHwDev_l(*pDevices);//获取有效audio接口设备
if (outHwDev == NULL)
return 0;
//获取接口.so库里面的参数信息
status = outHwDev->open_output_stream(outHwDev, *pDevices, (int *)&format,
&channels, &samplingRate, &outStream);
LOGV("openOutput() openOutputStream returned output %p, SamplingRate %d, Format %d, Channels %x, status %d",
outStream,
samplingRate,
format,
channels,
status);
mHardwareStatus = AUDIO_HW_IDLE;
if (outStream != NULL) {
AudioStreamOut *output = new AudioStreamOut(outHwDev, outStream);//建立这种设备的输出
........
}
outHwDev->open_output_stream 来自蓝牙audio里的接口函数。
再看android_audio_hw.c
static int adev_open(const hw_module_t* module, const char* name,
hw_device_t** device)
{
struct adev_a2dp *adev;
int ret;
if (strcmp(name, AUDIO_HARDWARE_INTERFACE) != 0)
return -EINVAL;
adev = calloc(1, sizeof(struct adev_a2dp));
if (!adev)
return -ENOMEM;
adev->bt_enabled = true;
adev->suspended = false;
pthread_mutex_init(&adev->lock, NULL);
adev->output = NULL;
adev->device.common.tag = HARDWARE_DEVICE_TAG;
adev->device.common.version = 0;
adev->device.common.module = (struct hw_module_t *) module;
adev->device.common.close = adev_close;
adev->device.get_supported_devices = adev_get_supported_devices;
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;
*device = &adev->device.common;
return 0;
err_str_parms_create:
free(adev);
return ret;
}
这样AudioFlinger里需要的参数，通过这里设定。接着蓝牙audio进行相关init,config,start,stop等状态操作。当然包括蓝牙读写。