Android N Audio: Audio Track play
对于AT在应用层开始play，到AF打开输出通道的调用流程如下图示，代码分析就不赘述了，
AT:AudioTrack AF:AudioFlinger AS:AudioSystemAPII:AudioPolicyInterfaceImpl APS:AudioPolicyService APM:AudioPolicyManager
在openOutput之后，
经过艰难的跋涉，终于又回到了AF，在这里，port的类型有所不同，如下将端口类型分为三种，session、混音和物理设备。
/* Audio port type indicates if it is a session (e.g AudioTrack),
* a mix (e.g PlaybackThread output) or a physical device
* (e.g AUDIO_DEVICE_OUT_SPEAKER) */
typedef enum {
AUDIO_PORT_TYPE_NONE,
AUDIO_PORT_TYPE_DEVICE,
AUDIO_PORT_TYPE_MIX,
AUDIO_PORT_TYPE_SESSION,
} audio_port_type_t;
根据source和port的类型不同，有不同的策略，来打开不同的输出通道，对于source为AUDIO_PORT_TYPE_DEVICE的通道，如果source数目为2，则如下，为输出的mPlaybackThread线程和MIC输入的mRecordThread线程各打开一个通道。
status_t AudioFlinger::PatchPanel::createAudioPatch(const struct audio_patch *patch,
audio_patch_handle_t *handle) {
…
switch (patch->sources[0].type) {
case AUDIO_PORT_TYPE_DEVICE: {
…
if ((patch->num_sources == 2) ||
((patch->sinks[0].type == AUDIO_PORT_TYPE_DEVICE) &&
((patch->sinks[0].ext.device.hw_module != srcModule) ||
(audioHwDevice->version() < AUDIO_DEVICE_API_VERSION_3_0)))) {
…
newPatch->mPlaybackThread = audioflinger->openOutput_l(
patch->sinks[0].ext.device.hw_module,
&output,
&config,
device,
address,
AUDIO_OUTPUT_FLAG_NONE);
…
audio_io_handle_t input = AUDIO_IO_HANDLE_NONE;
newPatch->mRecordThread = audioflinger->openInput_l(srcModule,
&input,
&config,
device,
address,
AUDIO_SOURCE_MIC,
AUDIO_INPUT_FLAG_NONE);
} else {
if (patch->sinks[0].type == AUDIO_PORT_TYPE_MIX) {
sp
patch->sinks[0].ext.mix.handle);
if (thread == 0) {
ALOGW("createAudioPatch() bad capture I/O handle %d",
patch->sinks[0].ext.mix.handle);
status = BAD_VALUE;
goto exit;
}
status = thread->sendCreateAudioPatchConfigEvent(patch, &halHandle);
} else {
if (audioHwDevice->version() < AUDIO_DEVICE_API_VERSION_3_0) {
status = INVALID_OPERATION;
goto exit;
}
audio_hw_device_t *hwDevice = audioHwDevice->hwDevice();
status = hwDevice->create_audio_patch(hwDevice,
patch->num_sources,
patch->sources,
patch->num_sinks,
patch->sinks,
&halHandle);
}
}
create_audio_patch在HAL没有被定义，这个分支应该是走不到的。
我们以audioflinger->openOutput_l的分析为主吧。
sp
audio_io_handle_t *output,
audio_config_t *config,
audio_devices_t devices,
const String8& address,
audio_output_flags_t flags)
{
AudioHwDevice *outHwDev = findSuitableHwDev_l(module, devices);
…
AudioStreamOut *outputStream = NULL;
status_t status = outHwDev->openOutputStream(
&outputStream,
*output,
devices,
flags,
config,
address.string());
mHardwareStatus = AUDIO_HW_IDLE;
if (status == NO_ERROR) {
PlaybackThread *thread;
if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) {
thread = new OffloadThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created offload output: ID %d thread %p", *output, thread);
} else if ((flags & AUDIO_OUTPUT_FLAG_DIRECT)
|| !isValidPcmSinkFormat(config->format)
|| !isValidPcmSinkChannelMask(config->channel_mask)) {
thread = new DirectOutputThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created direct output: ID %d thread %p ", *output, thread);
//Check if this is DirectPCM, if so
if (flags & AUDIO_OUTPUT_FLAG_DIRECT_PCM) {
thread->mIsDirectPcm = true;
}
} else {
thread = new MixerThread(this, outputStream, *output, devices, mSystemReady);
ALOGV("openOutput_l() created mixer output: ID %d thread %p", *output, thread);
}
mPlaybackThreads.add(*output, thread);
return thread;
}
return 0;
}
主要就是找到或加载之前提到的3类输入输出设备，说直白一点，就是加载对应的so库文件，便于后面调用其中的api接口；再接着就是调用设备的openOutputStream，字面意思是打开输出流，这里再说直白一点，就是将so库的api和现在framework的中间层管理起来，形成从上向下的一个调用流程；最后，根据flag类型，来决定使用哪一种线程处理音频数据，这一点在AF的线程分析里面另行分析。
对于AudioHwDevice的openOutputStream，逐层向下调用，最终会到平台厂商的hal层audio_hw.c，
status_t AudioHwDevice::openOutputStream(
AudioStreamOut **ppStreamOut,
audio_io_handle_t handle,
audio_devices_t devices,
audio_output_flags_t flags,
struct audio_config *config,
const char *address)
{
struct audio_config originalConfig = *config;
AudioStreamOut *outputStream = new AudioStreamOut(this, flags);
status_t status = outputStream->open(handle, devices, config, address);
…
*ppStreamOut = outputStream;
return status;
}
status_t AudioStreamOut::open(
audio_io_handle_t handle,
audio_devices_t devices,
struct audio_config *config,
const char *address)
{
audio_stream_out_t *outStream;
int status = hwDev()->open_output_stream(
hwDev(),
handle,
devices,
customFlags,
config,
&outStream,
address);
…
return status;
}
在audio_hw.c的adev_open_output_stream看似很复杂，实际也就是根据flag和device的类型，配置stream_out（定义在audio_hw.h文件里面）的成员变量，并将audio_stream_out类型指针赋值，作为后面操作HAL层的接口。
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 ret = 0;
audio_format_t format;
*stream_out = NULL;
…
out->channel_mask = AUDIO_CHANNEL_OUT_STEREO;
out->supported_channel_masks[0] = AUDIO_CHANNEL_OUT_STEREO;
out->handle = handle;
out->bit_width = CODEC_BACKEND_DEFAULT_BIT_WIDTH;
out->non_blocking = 0;
…
/* Init use case and pcm_config */
if ((out->flags & AUDIO_OUTPUT_FLAG_DIRECT) &&
!(out->flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD ||
(out->flags & AUDIO_OUTPUT_FLAG_DIRECT_PCM)) &&
(out->devices & AUDIO_DEVICE_OUT_AUX_DIGITAL ||
out->devices & AUDIO_DEVICE_OUT_PROXY)) {
…
out->stream.common.get_sample_rate = out_get_sample_rate;
out->stream.common.set_sample_rate = out_set_sample_rate;
out->stream.common.get_buffer_size = out_get_buffer_size;
out->stream.common.get_channels = out_get_channels;
out->stream.common.get_format = out_get_format;
out->stream.common.set_format = out_set_format;
…
*stream_out = &out->stream;
}
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