Android 11 Audio框架探索

Posted 2021-12-10 一只会飞的猪_fly

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<ProcessState> proc(ProcessState::self());
        MediaLogService::instantiate();
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
        ...
        ...
        ...
        android::hardware::configureRpcThreadpool(4, false /*callerWillJoin*/);
        sp<ProcessState> proc(ProcessState::self());
        sp<IServiceManager> 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<typename SERVICE>
class BinderService
{
public:
    static status_t publish(bool allowIsolated = false,
                            int dumpFlags = IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT) {
        sp<IServiceManager> 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<ProcessState> 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> audioPolicyEffects = new AudioPolicyEffects();
    sp<UidPolicy> uidPolicy = new UidPolicy(this);
    sp<SensorPrivacyPolicy> 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 AudioPolicyManagerObserver

AudioPolicyFactory.cpp中实现了createAudioPolicyManager().

#include <AudioPolicyManager.h>

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<DeviceDescriptor> 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<SwAudioOutputDescriptor> 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<AudioInputDescriptor> 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<SwAudioOutputDescriptor>& output1,
                                 const sp<SwAudioOutputDescriptor>& 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<DeviceDescriptor> 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<SwAudioOutputDescriptor> outputDesc = new SwAudioOutputDescriptor(outProfile,
                                                                                 mpClientInterface);

  AudioPolicyClientInterface *mpClientInterface;  // audio policy client interface
        sp<SwAudioOutputDescriptor> 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<DeviceDescriptorBase>& 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<DeviceDescriptorBase>& device,
                                                           uint32_t *latencyMs,
                                                           audio_output_flags_t flags)
{
    sp<IAudioFlinger> 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<DeviceDescriptorBase>& 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<ThreadBase> 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::ThreadBase> 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<MmapPlaybackThread> 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<PlaybackThread> 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....