Android 11 Audio框架探索（一）
1、mian_audioservice.cpp文件启动AudioPolicyService与AudioFlinger服务
在面mian函数调用了AudioFlinger，AudioPolicyService的instantiate()函数。
int main(int argc __unused, char **argv)
{
// TODO: update with refined parameters
limitProcessMemory(
"audio.maxmem", /* "ro.audio.maxmem", property that defines limit */
(size_t)512 * (1 << 20), /* SIZE_MAX, upper limit in bytes */
20 /* upper limit as percentage of physical RAM */);
signal(SIGPIPE, SIG_IGN);
bool doLog = (bool) property_get_bool("ro.test_harness", 0);
pid_t childPid;
// FIXME The advantage of making the process containing media.log service the parent process of
// the process that contains the other audio services, is that it allows us to collect more
// detailed information such as signal numbers, stop and continue, resource usage, etc.
// But it is also more complex. Consider replacing this by independent processes, and using
// binder on death notification instead.
if (doLog && (childPid = fork()) != 0) {
// media.log service
//prctl(PR_SET_NAME, (unsigned long) "media.log", 0, 0, 0);
// unfortunately ps ignores PR_SET_NAME for the main thread, so use this ugly hack
strcpy(argv[0], "media.log");
sp proc(ProcessState::self());
MediaLogService::instantiate();
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
...
...
...
android::hardware::configureRpcThreadpool(4, false /*callerWillJoin*/);
sp proc(ProcessState::self());
sp sm = defaultServiceManager();
ALOGI("ServiceManager: %p", sm.get());
AudioFlinger::instantiate();
AudioPolicyService::instantiate();
...
...
...
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
}
} 在BinderService.h中会创建它们的实例，并注册到SystemManager中
template
class BinderService
{
public:
static status_t publish(bool allowIsolated = false,
int dumpFlags = IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT) {
sp sm(defaultServiceManager());
return sm->addService(String16(SERVICE::getServiceName()), new SERVICE(), allowIsolated,
dumpFlags);
}
static void publishAndJoinThreadPool(
bool allowIsolated = false,
int dumpFlags = IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT) {
publish(allowIsolated, dumpFlags);
joinThreadPool();
}
static void instantiate() { publish(); }
static status_t shutdown() { return NO_ERROR; }
private:
static void joinThreadPool() {
sp ps(ProcessState::self());
ps->startThreadPool();
ps->giveThreadPoolName();
IPCThreadState::self()->joinThreadPool();
}
};
} // namespace android 关于如何启动一个Native service这里不赘叙，可参考下面连接：
通过C++实现Android Native Service - 简书
2、AudioProlicyService初始化
AudioPolicyService用于控制音频播放策略（比如插耳机的时候来电用什么设备去播放音乐）、管理音频设备等。
AudioPolicyService的构造函数更简单，只是初始化主要成员。
AudioPolicyService会在onFristRef中做比较多的工作，比如创建command线程，创建AudioPolicyManager。
AudioPolicyService::AudioPolicyService()
: BnAudioPolicyService(),
mAudioPolicyManager(NULL),
mAudioPolicyClient(NULL),
mPhoneState(AUDIO_MODE_INVALID),
mCaptureStateNotifier(false) {
}
void AudioPolicyService::onFirstRef()
{
{
Mutex::Autolock _l(mLock);
// start audio commands thread
mAudioCommandThread = new AudioCommandThread(String8("ApmAudio"), this);
// start output activity command thread
mOutputCommandThread = new AudioCommandThread(String8("ApmOutput"), this);
//创建AudioPolicyClient
mAudioPolicyClient = new AudioPolicyClient(this);
//创建AudioPolicyManager
mAudioPolicyManager = createAudioPolicyManager(mAudioPolicyClient);
}
// load audio processing modules
sp audioPolicyEffects = new AudioPolicyEffects();
sp uidPolicy = new UidPolicy(this);
sp sensorPrivacyPolicy = new SensorPrivacyPolicy(this);
{
Mutex::Autolock _l(mLock);
mAudioPolicyEffects = audioPolicyEffects;
mUidPolicy = uidPolicy;
mSensorPrivacyPolicy = sensorPrivacyPolicy;
}
uidPolicy->registerSelf();
sensorPrivacyPolicy->registerSelf();
} 这里createAudioPolicyManager()方法定义在AudioPolicyInterface.h，AudioPolicyManager.h继承了AudioPolicyInterface。
class AudioPolicyManager : public AudioPolicyInterface, public AudioPolicyManagerObserverAudioPolicyFactory.cpp中实现了createAudioPolicyManager().
#include
namespace android {
extern "C" AudioPolicyInterface* createAudioPolicyManager(
AudioPolicyClientInterface *clientInterface)
{
AudioPolicyManager *apm = new AudioPolicyManager(clientInterface);
status_t status = apm->initialize();
if (status != NO_ERROR) {
delete apm;
apm = nullptr;
}
return apm;
}
extern "C" void destroyAudioPolicyManager(AudioPolicyInterface *interface)
{
delete interface;
}
} // namespace android 这里创建了AudioPolicyManager并且调用的它的initialize()。
话不多说我们直接上代码分析，AudioPolicyManager.cpp：
AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface,
bool /*forTesting*/)
:
mUidCached(AID_AUDIOSERVER), // no need to call getuid(), there's only one of us running.
mpClientInterface(clientInterface),
mLimitRingtoneVolume(false), mLastVoiceVolume(-1.0f),
mA2dpSuspended(false),
mConfig(mHwModulesAll, mOutputDevicesAll, mInputDevicesAll, mDefaultOutputDevice),
mAudioPortGeneration(1),
mBeaconMuteRefCount(0),
mBeaconPlayingRefCount(0),
mBeaconMuted(false),
mTtsOutputAvailable(false),
mMasterMono(false),
mMusicEffectOutput(AUDIO_IO_HANDLE_NONE)
{
}
AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface)
: AudioPolicyManager(clientInterface, false /*forTesting*/)
{
loadConfig();
}
void AudioPolicyManager::loadConfig() {
if (deserializeAudioPolicyXmlConfig(getConfig()) != NO_ERROR) {
ALOGE("could not load audio policy configuration file, setting defaults");
getConfig().setDefault();
}
}
status_t AudioPolicyManager::initialize() {
{
//代理引擎
auto engLib = EngineLibrary::load(
"libaudiopolicyengine" + getConfig().getEngineLibraryNameSuffix() + ".so");
if (!engLib) {
ALOGE("%s: Failed to load the engine library", __FUNCTION__);
return NO_INIT;
}
//获取引擎
mEngine = engLib->createEngine();
if (mEngine == nullptr) {
ALOGE("%s: Failed to instantiate the APM engine", __FUNCTION__);
return NO_INIT;
}
}
//设为观察者
mEngine->setObserver(this);
status_t status = mEngine->initCheck(); //初始
if (status != NO_ERROR) {
LOG_FATAL("Policy engine not initialized(err=%d)", status);
return status;
}
// after parsing the config, mOutputDevicesAll and mInputDevicesAll contain all known devices;
// open all output streams needed to access attached devices
// 打开访问附加设备所需的所有输出流
onNewAudioModulesAvailableInt(nullptr /*newDevices*/);
// make sure default device is reachable(确保可以访问默认设备)
if (mDefaultOutputDevice == 0 || !mAvailableOutputDevices.contains(mDefaultOutputDevice)) {
ALOGE_IF(mDefaultOutputDevice != 0, "Default device %s is unreachable",
mDefaultOutputDevice->toString().c_str());
status = NO_INIT;
}
// If microphones address is empty, set it according to device type
//如果麦克风地址为空，请根据设备类型进行设置
for (size_t i = 0; i < mAvailableInputDevices.size(); i++) {
if (mAvailableInputDevices[i]->address().empty()) {
if (mAvailableInputDevices[i]->type() == AUDIO_DEVICE_IN_BUILTIN_MIC) {
mAvailableInputDevices[i]->setAddress(AUDIO_BOTTOM_MICROPHONE_ADDRESS);
} else if (mAvailableInputDevices[i]->type() == AUDIO_DEVICE_IN_BACK_MIC) {
mAvailableInputDevices[i]->setAddress(AUDIO_BACK_MICROPHONE_ADDRESS);
}
}
}
if (mPrimaryOutput == 0) {
ALOGE("Failed to open primary output");
status = NO_INIT;
}
// Silence ALOGV statements
property_set("log.tag." LOG_TAG, "D");
updateDevicesAndOutputs();
return status;
}AudioPolicyManager的构造中代码并不多，主要逻都在initialize()函数中。这里有个很重要的函数onNewAudioModulesAvailableInt()。
void AudioPolicyManager::onNewAudioModulesAvailableInt(DeviceVector *newDevices)
{
for (const auto& hwModule : mHwModulesAll) {
if (std::find(mHwModules.begin(), mHwModules.end(), hwModule) != mHwModules.end()) {
continue;
}
//调用ClientInterface加载Audio模块，ClientInterface将调用AudioFlinger的loadHwModule
hwModule->setHandle(mpClientInterface->loadHwModule(hwModule->getName()));
if (hwModule->getHandle() == AUDIO_MODULE_HANDLE_NONE) {
ALOGW("could not open HW module %s", hwModule->getName());
continue;
}
mHwModules.push_back(hwModule);
// open all output streams needed to access attached devices
// except for direct output streams that are only opened when they are actually
// required by an app.
// This also validates mAvailableOutputDevices list
//打开访问连接设备所需的所有输出流
// 除了直接输出流，它们只有在实际打开时才打开
// 应用程序需要。
// 这也验证了 mAvailableOutputDevices 列表
for (const auto& outProfile : hwModule->getOutputProfiles()) {
if (!outProfile->canOpenNewIo()) {
ALOGE("Invalid Output profile max open count %u for profile %s",
outProfile->maxOpenCount, outProfile->getTagName().c_str());
continue;
}
if (!outProfile->hasSupportedDevices()) {
ALOGW("Output profile contains no device on module %s", hwModule->getName());
continue;
}
if ((outProfile->getFlags() & AUDIO_OUTPUT_FLAG_TTS) != 0) {
mTtsOutputAvailable = true;
}
const DeviceVector &supportedDevices = outProfile->getSupportedDevices();
DeviceVector availProfileDevices = supportedDevices.filter(mOutputDevicesAll);
sp supportedDevice = 0;
if (supportedDevices.contains(mDefaultOutputDevice)) {
supportedDevice = mDefaultOutputDevice;
} else {
// choose first device present in profile's SupportedDevices also part of
// mAvailableOutputDevices.
if (availProfileDevices.isEmpty()) {
continue;
}
supportedDevice = availProfileDevices.itemAt(0);
}
if (!mOutputDevicesAll.contains(supportedDevice)) {
continue;
}
//根据outProfile 得到一个描述符，设置mpClientInterface
sp outputDesc = new SwAudioOutputDescriptor(outProfile,
mpClientInterface);
audio_io_handle_t output = AUDIO_IO_HANDLE_NONE;
//AudioOutputDescriptor.cpp 的open方法会调用AF的openOutput方法
status_t status = outputDesc->open(nullptr, DeviceVector(supportedDevice),
AUDIO_STREAM_DEFAULT,
AUDIO_OUTPUT_FLAG_NONE, &output);
if (status != NO_ERROR) {
ALOGW("Cannot open output stream for devices %s on hw module %s",
supportedDevice->toString().c_str(), hwModule->getName());
continue;
}
for (const auto &device : availProfileDevices) {
// give a valid ID to an attached device once confirmed it is reachable
//一旦确认连接的设备可以访问，就为它提供一个有效的 ID
if (!device->isAttached()) {
device->attach(hwModule);
mAvailableOutputDevices.add(device);
device->setEncapsulationInfoFromHal(mpClientInterface);
if (newDevices) newDevices->add(device);
setEngineDeviceConnectionState(device, AUDIO_POLICY_DEVICE_STATE_AVAILABLE);
}
}
if (mPrimaryOutput == 0 &&
outProfile->getFlags() & AUDIO_OUTPUT_FLAG_PRIMARY) {
mPrimaryOutput = outputDesc;
}
if ((outProfile->getFlags() & AUDIO_OUTPUT_FLAG_DIRECT) != 0) {
outputDesc->close();
} else {
//根据output添加outputDesc描述符
addOutput(output, outputDesc);
setOutputDevices(outputDesc,
DeviceVector(supportedDevice),
true,
0,
NULL);
}
}
// open input streams needed to access attached devices to validate
//打开访问连接设备所需的输入流以进行验证
// mAvailableInputDevices list
for (const auto& inProfile : hwModule->getInputProfiles()) {
if (!inProfile->canOpenNewIo()) {
ALOGE("Invalid Input profile max open count %u for profile %s",
inProfile->maxOpenCount, inProfile->getTagName().c_str());
continue;
}
if (!inProfile->hasSupportedDevices()) {
ALOGW("Input profile contains no device on module %s", hwModule->getName());
continue;
}
// chose first device present in profile's SupportedDevices also part of
// available input devices
//选择配置文件的 SupportedDevices 中存在的第一个设备也是可用的输入设备
const DeviceVector &supportedDevices = inProfile->getSupportedDevices();
DeviceVector availProfileDevices = supportedDevices.filter(mInputDevicesAll);
if (availProfileDevices.isEmpty()) {
ALOGE("%s: Input device list is empty!", __FUNCTION__);
continue;
}
sp inputDesc =
new AudioInputDescriptor(inProfile, mpClientInterface);
audio_io_handle_t input = AUDIO_IO_HANDLE_NONE;
status_t status = inputDesc->open(nullptr,
availProfileDevices.itemAt(0),
AUDIO_SOURCE_MIC,
AUDIO_INPUT_FLAG_NONE,
&input);
if (status != NO_ERROR) {
ALOGW("Cannot open input stream for device %s on hw module %s",
availProfileDevices.toString().c_str(),
hwModule->getName());
continue;
}
for (const auto &device : availProfileDevices) {
// give a valid ID to an attached device once confirmed it is reachable
if (!device->isAttached()) {
device->attach(hwModule);
device->importAudioPortAndPickAudioProfile(inProfile, true);
mAvailableInputDevices.add(device);
if (newDevices) newDevices->add(device);
setEngineDeviceConnectionState(device, AUDIO_POLICY_DEVICE_STATE_AVAILABLE);
}
}
inputDesc->close();
}
}
} 这里创建了SwAudioOutputDescriptor指针，这个类定义在AudioOutputDescriptor.h。
// Audio output driven by a software mixer in audio flinger.
class SwAudioOutputDescriptor: public AudioOutputDescriptor
{
public:
...
...
...
status_t open(const audio_config_t *config,
const DeviceVector &devices,
audio_stream_type_t stream,
audio_output_flags_t flags,
audio_io_handle_t *output);
// Called when a stream is about to be started
// Note: called before setClientActive(true);
status_t start();
// Called after a stream is stopped.
// Note: called after setClientActive(false);
void stop();
void close();
status_t openDuplicating(const sp& output1,
const sp& output2,
audio_io_handle_t *ioHandle);
...
...
...
...
}; open函数的具体实现在SwAudioOutputDescriptor.cpp
status_t SwAudioOutputDescriptor::open(const audio_config_t *config,
const DeviceVector &devices,
audio_stream_type_t stream,
audio_output_flags_t flags,
audio_io_handle_t *output)
{
mDevices = devices;
sp device = devices.getDeviceForOpening();
LOG_ALWAYS_FATAL_IF(device == nullptr,
"%s failed to get device descriptor for opening "
"with the requested devices, all device types: %s",
__func__, dumpDeviceTypes(devices.types()).c_str());
audio_config_t lConfig;
if (config == nullptr) {
lConfig = AUDIO_CONFIG_INITIALIZER;
lConfig.sample_rate = mSamplingRate;
lConfig.channel_mask = mChannelMask;
lConfig.format = mFormat;
} else {
lConfig = *config;
}
// if the selected profile is offloaded and no offload info was specified,
// create a default one
if ((mProfile->getFlags() & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) &&
lConfig.offload_info.format == AUDIO_FORMAT_DEFAULT) {
flags = (audio_output_flags_t)(flags | AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD);
lConfig.offload_info = AUDIO_INFO_INITIALIZER;
lConfig.offload_info.sample_rate = lConfig.sample_rate;
lConfig.offload_info.channel_mask = lConfig.channel_mask;
lConfig.offload_info.format = lConfig.format;
lConfig.offload_info.stream_type = stream;
lConfig.offload_info.duration_us = -1;
lConfig.offload_info.has_video = true; // conservative
lConfig.offload_info.is_streaming = true; // likely
}
mFlags = (audio_output_flags_t)(mFlags | flags);
ALOGV("opening output for device %s profile %p name %s",
mDevices.toString().c_str(), mProfile.get(), mProfile->getName().c_str());
//熟悉的openOutput
status_t status = mClientInterface->openOutput(mProfile->getModuleHandle(),
output,
&lConfig,
device,
&mLatency,
mFlags);
if (status == NO_ERROR) {
LOG_ALWAYS_FATAL_IF(*output == AUDIO_IO_HANDLE_NONE,
"%s openOutput returned output handle %d for device %s, "
"selected device %s for opening",
__FUNCTION__, *output, devices.toString().c_str(),
device->toString().c_str());
mSamplingRate = lConfig.sample_rate;
mChannelMask = lConfig.channel_mask;
mFormat = lConfig.format;
mId = PolicyAudioPort::getNextUniqueId();
mIoHandle = *output;
mProfile->curOpenCount++;
}
return status;
} 这里调用mClientInterface->openOutput()这个函数最终是在AudioFlicy.cpp中实现。这个mClientInterface是在SwAudioOutputDescriptor的构造中传过来的。
//根据outProfile 得到一个描述符，设置mpClientInterface
sp outputDesc = new SwAudioOutputDescriptor(outProfile,
mpClientInterface); AudioPolicyClientInterface *mpClientInterface; // audio policy client interface
sp mPrimaryOutput; // primary output descriptor
// list of descriptors for outputs currently opened 下面我们看一下AudioPolicyClientInterface这个类。
AudioPolicyClientInterface.h
// Audio Policy client Interface
class AudioPolicyClientInterface
{
public:
virtual ~AudioPolicyClientInterface() {}
//
// Audio HW module functions
//
// loads a HW module.
virtual audio_module_handle_t loadHwModule(const char *name) = 0;
//
// Audio output Control functions
//
// opens an audio output with the requested parameters. The parameter values can indicate to use the default values
// in case the audio policy manager has no specific requirements for the output being opened.
// When the function returns, the parameter values reflect the actual values used by the audio hardware output stream.
// The audio policy manager can check if the proposed parameters are suitable or not and act accordingly.
virtual status_t openOutput(audio_module_handle_t module,
audio_io_handle_t *output,
audio_config_t *config,
const sp& device,
uint32_t *latencyMs,
audio_output_flags_t flags) = 0;
// creates a special output that is duplicated to the two outputs passed as arguments. The duplication is performed by
// a special mixer thread in the AudioFlinger.
virtual audio_io_handle_t openDuplicateOutput(audio_io_handle_t output1, audio_io_handle_t output2) = 0;
// closes the output stream
virtual status_t closeOutput(audio_io_handle_t output) = 0;
// suspends the output. When an output is suspended, the corresponding audio hardware output stream is placed in
// standby and the AudioTracks attached to the mixer thread are still processed but the output mix is discarded.
virtual status_t suspendOutput(audio_io_handle_t output) = 0;
// restores a suspended output.
virtual status_t restoreOutput(audio_io_handle_t output) = 0;
//
// Audio input Control functions
//
// opens an audio input
virtual status_t openInput(audio_module_handle_t module,
audio_io_handle_t *input,
audio_config_t *config,
audio_devices_t *device,
const String8& address,
audio_source_t source,
audio_input_flags_t flags) = 0;
...
...
...
...
}; openOutput()在AudioPolicyClientImpl.cpp中被实现。
status_t AudioPolicyService::AudioPolicyClient::openOutput(audio_module_handle_t module,
audio_io_handle_t *output,
audio_config_t *config,
const sp& device,
uint32_t *latencyMs,
audio_output_flags_t flags)
{
sp af = AudioSystem::get_audio_flinger();
if (af == 0) {
ALOGW("%s: could not get AudioFlinger", __func__);
return PERMISSION_DENIED;
}
return af->openOutput(module, output, config, device, latencyMs, flags);
} 获取获取AF的代理并调用了openoutput()函数。
status_t AudioFlinger::openOutput(audio_module_handle_t module,
audio_io_handle_t *output,
audio_config_t *config,
const sp& device,
uint32_t *latencyMs,
audio_output_flags_t flags)
{
ALOGI("openOutput() this %p, module %d Device %s, SamplingRate %d, Format %#08x, "
"Channels %#x, flags %#x",
this, module,
device->toString().c_str(),
config->sample_rate,
config->format,
config->channel_mask,
flags);
audio_devices_t deviceType = device->type();
const String8 address = String8(device->address().c_str());
if (deviceType == AUDIO_DEVICE_NONE) {
return BAD_VALUE;
}
Mutex::Autolock _l(mLock);
sp thread = openOutput_l(module, output, config, deviceType, address, flags);
if (thread != 0) {
if ((flags & AUDIO_OUTPUT_FLAG_MMAP_NOIRQ) == 0) {
PlaybackThread *playbackThread = (PlaybackThread *)thread.get();
*latencyMs = playbackThread->latency();
// notify client processes of the new output creation
playbackThread->ioConfigChanged(AUDIO_OUTPUT_OPENED);
// the first primary output opened designates the primary hw device if no HW module
// named "primary" was already loaded.
AutoMutex lock(mHardwareLock);
if ((mPrimaryHardwareDev == nullptr) && (flags & AUDIO_OUTPUT_FLAG_PRIMARY)) {
ALOGI("Using module %d as the primary audio interface", module);
mPrimaryHardwareDev = playbackThread->getOutput()->audioHwDev;
mHardwareStatus = AUDIO_HW_SET_MODE;
mPrimaryHardwareDev->hwDevice()->setMode(mMode);
mHardwareStatus = AUDIO_HW_IDLE;
}
} else {
MmapThread *mmapThread = (MmapThread *)thread.get();
mmapThread->ioConfigChanged(AUDIO_OUTPUT_OPENED);
}
return NO_ERROR;
}
return NO_INIT;
}
接着看下openOutput_l()函数：
sp AudioFlinger::openOutput_l(audio_module_handle_t module,
audio_io_handle_t *output,
audio_config_t *config,
audio_devices_t deviceType,
const String8& address,
audio_output_flags_t flags)
{
//AudioHwDevice 代表一个打开的音频接口设备
AudioHwDevice *outHwDev = findSuitableHwDev_l(module, deviceType);
if (outHwDev == NULL) {
return 0;
}
if (*output == AUDIO_IO_HANDLE_NONE) {
*output = nextUniqueId(AUDIO_UNIQUE_ID_USE_OUTPUT);
} else {
// Audio Policy does not currently request a specific output handle.
// If this is ever needed, see openInput_l() for example code.
ALOGE("openOutput_l requested output handle %d is not AUDIO_IO_HANDLE_NONE", *output);
return 0;
}
mHardwareStatus = AUDIO_HW_OUTPUT_OPEN;
// FOR TESTING ONLY:
// This if statement allows overriding the audio policy settings
// and forcing a specific format or channel mask to the HAL/Sink device for testing.
if (!(flags & (AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD | AUDIO_OUTPUT_FLAG_DIRECT))) {
// Check only for Normal Mixing mode
if (kEnableExtendedPrecision) {
// Specify format (uncomment one below to choose)
//config->format = AUDIO_FORMAT_PCM_FLOAT;
//config->format = AUDIO_FORMAT_PCM_24_BIT_PACKED;
//config->format = AUDIO_FORMAT_PCM_32_BIT;
//config->format = AUDIO_FORMAT_PCM_8_24_BIT;
// ALOGV("openOutput_l() upgrading format to %#08x", config->format);
}
if (kEnableExtendedChannels) {
// Specify channel mask (uncomment one below to choose)
//config->channel_mask = audio_channel_out_mask_from_count(4); // for USB 4ch
//config->channel_mask = audio_channel_mask_from_representation_and_bits(
// AUDIO_CHANNEL_REPRESENTATION_INDEX, (1 << 4) - 1); // another 4ch example
}
}
///为设备打开一个输出流，会获得一个audio_stream_out_t *stream; 一个audio_devices_t devices,其中会生成一个AudioStreamOut
//（AudioStreamOut *outputStream = new AudioStreamOut(this, flags);）就是来封装audio_stream_out_t和audio_devices_t的。
AudioStreamOut *outputStream = NULL;
status_t status = outHwDev->openOutputStream(
&outputStream,
*output,
deviceType,
flags,
config,
address.string());
mHardwareStatus = AUDIO_HW_IDLE;
if (status == NO_ERROR) {
if (flags & AUDIO_OUTPUT_FLAG_MMAP_NOIRQ) {
sp thread =
new MmapPlaybackThread(this, *output, outHwDev, outputStream, mSystemReady);
mMmapThreads.add(*output, thread);
ALOGV("openOutput_l() created mmap playback thread: ID %d thread %p",
*output, thread.get());
return thread;
} else {
//创建播放线程
sp thread;
if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) {
thread = new OffloadThread(this, outputStream, *output, mSystemReady);
ALOGV("openOutput_l() created offload output: ID %d thread %p",
*output, thread.get());
} else if ((flags & AUDIO_OUTPUT_FLAG_DIRECT)
|| !isValidPcmSinkFormat(config->format)
|| !isValidPcmSinkChannelMask(config->channel_mask)) {
thread = new DirectOutputThread(this, outputStream, *output, mSystemReady);
ALOGV("openOutput_l() created direct output: ID %d thread %p",
*output, thread.get());
} else {
thread = new MixerThread(this, outputStream, *output, mSystemReady);
ALOGV("openOutput_l() created mixer output: ID %d thread %p",
*output, thread.get());
}
//添加到mPlaybackThreads中
mPlaybackThreads.add(*output, thread);
mPatchPanel.notifyStreamOpened(outHwDev, *output);
return thread;
}
}
return 0;
} 在openOutput_l函数中这个findSuitableHwDev_l函数很重要。
这里暂时不做分析....