Android Audio代码分析12 - stream type续

前几天在看stream type的时候，调用函数AudioSystem::getOutput的地方并没有继续往下看。
今天深入看看。
*****************************************源码*************************************************

status_t AudioTrack::set(
        int streamType,
        uint32_t sampleRate,
        int format,
        int channels,
        int frameCount,
        uint32_t flags,
        callback_t cbf,
        void* user,
        int notificationFrames,
        const sp& sharedBuffer,
        bool threadCanCallJava,
        int sessionId)
{


    LOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(), sharedBuffer->size());


    if (mAudioTrack != 0) {
        LOGE("Track already in use");
        return INVALID_OPERATION;
    }


    int afSampleRate;
    if (AudioSystem::getOutputSamplingRate(&afSampleRate, streamType) != NO_ERROR) {
        return NO_INIT;
    }
    uint32_t afLatency;
    if (AudioSystem::getOutputLatency(&afLatency, streamType) != NO_ERROR) {
        return NO_INIT;
    }


    // handle default values first.
    if (streamType == AudioSystem::DEFAULT) {
        streamType = AudioSystem::MUSIC;
    }
    if (sampleRate == 0) {
        sampleRate = afSampleRate;
    }
    // these below should probably come from the audioFlinger too...
    if (format == 0) {
        format = AudioSystem::PCM_16_BIT;
    }
    if (channels == 0) {
        channels = AudioSystem::CHANNEL_OUT_STEREO;
    }


    // validate parameters
    if (!AudioSystem::isValidFormat(format)) {
        LOGE("Invalid format");
        return BAD_VALUE;
    }


    // force direct flag if format is not linear PCM
    if (!AudioSystem::isLinearPCM(format)) {
        flags |= AudioSystem::OUTPUT_FLAG_DIRECT;
    }


    if (!AudioSystem::isOutputChannel(channels)) {
        LOGE("Invalid channel mask");
        return BAD_VALUE;
    }
    uint32_t channelCount = AudioSystem::popCount(channels);


    audio_io_handle_t output = AudioSystem::getOutput((AudioSystem::stream_type)streamType,
            sampleRate, format, channels, (AudioSystem::output_flags)flags);


    if (output == 0) {
        LOGE("Could not get audio output for stream type %d", streamType);
        return BAD_VALUE;
    }


    mVolume[LEFT] = 1.0f;
    mVolume[RIGHT] = 1.0f;
    mSendLevel = 0;
    mFrameCount = frameCount;
    mNotificationFramesReq = notificationFrames;
    mSessionId = sessionId;
    mAuxEffectId = 0;


    // create the IAudioTrack
    status_t status = createTrack(streamType, sampleRate, format, channelCount,
                                  frameCount, flags, sharedBuffer, output, true);


    if (status != NO_ERROR) {
        return status;
    }


    if (cbf != 0) {
        mAudioTrackThread = new AudioTrackThread(*this, threadCanCallJava);
        if (mAudioTrackThread == 0) {
          LOGE("Could not create callback thread");
          return NO_INIT;
        }
    }


    mStatus = NO_ERROR;


    mStreamType = streamType;
    mFormat = format;
    mChannels = channels;
    mChannelCount = channelCount;
    mSharedBuffer = sharedBuffer;
    mMuted = false;
    mActive = 0;
    mCbf = cbf;
    mUserData = user;
    mLoopCount = 0;
    mMarkerPosition = 0;
    mMarkerReached = false;
    mNewPosition = 0;
    mUpdatePeriod = 0;
    mFlags = flags;


    return NO_ERROR;
}

**********************************************************************************************


#######################说明################################

// +++++++++++++++++++++++++AudioSystem::getOutput+++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioSystem::getOutput(stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    output_flags flags)
{
    audio_io_handle_t output = 0;
    // Do not use stream to output map cache if the direct output
    // flag is set or if we are likely to use a direct output
    // (e.g voice call stream @ 8kHz could use BT SCO device and be routed to
    // a direct output on some platforms).
    // TODO: the output cache and stream to output mapping implementation needs to
    // be reworked for proper operation with direct outputs. This code is too specific
    // to the first use case we want to cover (Voice Recognition and Voice Dialer over
    // Bluetooth SCO
    if ((flags & AudioSystem::OUTPUT_FLAG_DIRECT) == 0 &&
        ((stream != AudioSystem::VOICE_CALL && stream != AudioSystem::BLUETOOTH_SCO) ||
         channels != AudioSystem::CHANNEL_OUT_MONO ||
         (samplingRate != 8000 && samplingRate != 16000))) {
        Mutex::Autolock _l(gLock);
        output = AudioSystem::gStreamOutputMap.valueFor(stream);
        LOGV_IF((output != 0), "getOutput() read %d from cache for stream %d", output, stream);
    }
    if (output == 0) {
        const sp& aps = AudioSystem::get_audio_policy_service();
        if (aps == 0) return 0;
        output = aps->getOutput(stream, samplingRate, format, channels, flags);
        if ((flags & AudioSystem::OUTPUT_FLAG_DIRECT) == 0) {
            Mutex::Autolock _l(gLock);
            AudioSystem::gStreamOutputMap.add(stream, output);
        }
// +++++++++++++++++++++++++++++AudioPolicyService::getOutput+++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyService::getOutput(AudioSystem::stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    AudioSystem::output_flags flags)
{
    if (mpPolicyManager == NULL) {
        return 0;
    }
    LOGV("getOutput() tid %d", gettid());
    Mutex::Autolock _l(mLock);
    return mpPolicyManager->getOutput(stream, samplingRate, format, channels, flags);
// +++++++++++++++++++++++++++++AudioPolicyManagerBase::getOutput+++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyManagerBase::getOutput(AudioSystem::stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    AudioSystem::output_flags flags)
{
    audio_io_handle_t output = 0;
    uint32_t latency = 0;
// 此处根据stream获取策略
    routing_strategy strategy = getStrategy((AudioSystem::stream_type)stream);
// 这儿再根据策略获取device
// 这句代码和上句代码结合起来，就是根据stream的类型，获取对应的device
    uint32_t device = getDeviceForStrategy(strategy);
    LOGV("getOutput() stream %d, samplingRate %d, format %d, channels %x, flags %x", stream, samplingRate, format, channels, flags);


#ifdef AUDIO_POLICY_TEST
// 定义了AUDIO_POLICY_TEST的时候，会去打开一个output
    if (mCurOutput != 0) {
        LOGV("getOutput() test output mCurOutput %d, samplingRate %d, format %d, channels %x, mDirectOutput %d",
                mCurOutput, mTestSamplingRate, mTestFormat, mTestChannels, mDirectOutput);


        if (mTestOutputs[mCurOutput] == 0) {
            LOGV("getOutput() opening test output");
            AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
            outputDesc->mDevice = mTestDevice;
            outputDesc->mSamplingRate = mTestSamplingRate;
            outputDesc->mFormat = mTestFormat;
            outputDesc->mChannels = mTestChannels;
            outputDesc->mLatency = mTestLatencyMs;
            outputDesc->mFlags = (AudioSystem::output_flags)(mDirectOutput ? AudioSystem::OUTPUT_FLAG_DIRECT : 0);
            outputDesc->mRefCount[stream] = 0;
            mTestOutputs[mCurOutput] = mpClientInterface->openOutput(&outputDesc->mDevice,
                                            &outputDesc->mSamplingRate,
                                            &outputDesc->mFormat,
                                            &outputDesc->mChannels,
                                            &outputDesc->mLatency,
                                            outputDesc->mFlags);
            if (mTestOutputs[mCurOutput]) {
                AudioParameter outputCmd = AudioParameter();
                outputCmd.addInt(String8("set_id"),mCurOutput);
                mpClientInterface->setParameters(mTestOutputs[mCurOutput],outputCmd.toString());
                addOutput(mTestOutputs[mCurOutput], outputDesc);
            }
        }
        return mTestOutputs[mCurOutput];
    }
#endif //AUDIO_POLICY_TEST


// Direct模式的话，会去打开一个output
    // open a direct output if required by specified parameters
    if (needsDirectOuput(stream, samplingRate, format, channels, flags, device)) {


        LOGV("getOutput() opening direct output device %x", device);
        AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
        outputDesc->mDevice = device;
        outputDesc->mSamplingRate = samplingRate;
        outputDesc->mFormat = format;
        outputDesc->mChannels = channels;
        outputDesc->mLatency = 0;
        outputDesc->mFlags = (AudioSystem::output_flags)(flags | AudioSystem::OUTPUT_FLAG_DIRECT);
        outputDesc->mRefCount[stream] = 0;
        output = mpClientInterface->openOutput(&outputDesc->mDevice,
                                        &outputDesc->mSamplingRate,
                                        &outputDesc->mFormat,
                                        &outputDesc->mChannels,
                                        &outputDesc->mLatency,
                                        outputDesc->mFlags);


        // only accept an output with the requeted parameters
        if (output == 0 ||
            (samplingRate != 0 && samplingRate != outputDesc->mSamplingRate) ||
            (format != 0 && format != outputDesc->mFormat) ||
            (channels != 0 && channels != outputDesc->mChannels)) {
            LOGV("getOutput() failed opening direct output: samplingRate %d, format %d, channels %d",
                    samplingRate, format, channels);
            if (output != 0) {
                mpClientInterface->closeOutput(output);
            }
            delete outputDesc;
            return 0;
        }
        addOutput(output, outputDesc);
        return output;
    }


    if (channels != 0 && channels != AudioSystem::CHANNEL_OUT_MONO &&
        channels != AudioSystem::CHANNEL_OUT_STEREO) {
        return 0;
    }
    // open a non direct output
// stream模式的话，直接取成员变量的值
// 下面看看用于取值的这几个成员变量赋值的地方


    // get which output is suitable for the specified stream. The actual routing change will happen
    // when startOutput() will be called
    uint32_t a2dpDevice = device & AudioSystem::DEVICE_OUT_ALL_A2DP;
    if (AudioSystem::popCount((AudioSystem::audio_devices)device) == 2) {
#ifdef WITH_A2DP
        if (a2dpUsedForSonification() && a2dpDevice != 0) {
            // if playing on 2 devices among which one is A2DP, use duplicated output
            LOGV("getOutput() using duplicated output");
            LOGW_IF((mA2dpOutput == 0), "getOutput() A2DP device in multiple %x selected but A2DP output not opened", device);
// mA2dpOutput和mDuplicatedOutput的赋值都在函数AudioPolicyManagerBase::handleA2dpConnection中
            output = mDuplicatedOutput;
// +++++++++++++++++++++++++++++AudioPolicyManagerBase::handleA2dpConnection+++++++++++++++++++++++++++++++++++
status_t AudioPolicyManagerBase::handleA2dpConnection(AudioSystem::audio_devices device,
                                                 const char *device_address)
{
    // when an A2DP device is connected, open an A2DP and a duplicated output
    LOGV("opening A2DP output for device %s", device_address);
    AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
    outputDesc->mDevice = device;
// 这个函数在下面有看
    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()) {
// mA2dpOutput和mHardwareOutput是已经打开的两个output
            mDuplicatedOutput = mpClientInterface->openDuplicateOutput(mA2dpOutput, mHardwareOutput);
// +++++++++++++++++++++++++++++AudioPolicyService::openDuplicateOutput+++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyService::openDuplicateOutput(audio_io_handle_t output1,
                                                          audio_io_handle_t output2)
{
    sp af = AudioSystem::get_audio_flinger();
    if (af == 0) {
        LOGW("openDuplicateOutput() could not get AudioFlinger");
        return 0;
    }
    return af->openDuplicateOutput(output1, output2);
// +++++++++++++++++++++++++++++++AudioFlinger::openDuplicateOutput+++++++++++++++++++++++++++++++++
int AudioFlinger::openDuplicateOutput(int output1, int output2)
{
    Mutex::Autolock _l(mLock);
    MixerThread *thread1 = checkMixerThread_l(output1);
    MixerThread *thread2 = checkMixerThread_l(output2);
// ++++++++++++++++++++++++++++++AudioFlinger::checkMixerThread_l++++++++++++++++++++++++++++++++++
// checkMixerThread_l() must be called with AudioFlinger::mLock held
AudioFlinger::MixerThread *AudioFlinger::checkMixerThread_l(int output) const
{
    PlaybackThread *thread = checkPlaybackThread_l(output);
    if (thread != NULL) {
        if (thread->type() == PlaybackThread::DIRECT) {
            thread = NULL;
        }
    }
// +++++++++++++++++++++++++++++++AudioFlinger::checkPlaybackThread_l+++++++++++++++++++++++++++++++++
// checkPlaybackThread_l() must be called with AudioFlinger::mLock held
AudioFlinger::PlaybackThread *AudioFlinger::checkPlaybackThread_l(int output) const
{
    PlaybackThread *thread = NULL;
    if (mPlaybackThreads.indexOfKey(output) >= 0) {
// AudioFlinger::openOutput函数中会向mPlaybackThreads中追加成员
        thread = (PlaybackThread *)mPlaybackThreads.valueFor(output).get();
    }
    return thread;
}
// -------------------------------AudioFlinger::checkPlaybackThread_l---------------------------------
    return (MixerThread *)thread;
}
// ------------------------------AudioFlinger::checkMixerThread_l----------------------------------


    if (thread1 == NULL || thread2 == NULL) {
        LOGW("openDuplicateOutput() wrong output mixer type for output %d or %d", output1, output2);
        return 0;
    }


    int id = nextUniqueId();
// 选一个thread作为主thread，来创建DuplicatingThread
    DuplicatingThread *thread = new DuplicatingThread(this, thread1, id);
// ++++++++++++++++++++++++++++AudioFlinger::DuplicatingThread::DuplicatingThread++++++++++++++++++++++++++++++++++++
AudioFlinger::DuplicatingThread::DuplicatingThread(const sp& audioFlinger, AudioFlinger::MixerThread* mainThread, int id)
    :   MixerThread(audioFlinger, mainThread->getOutput(), id, mainThread->device()), mWaitTimeMs(UINT_MAX)
{
    mType = PlaybackThread::DUPLICATING;
    addOutputTrack(mainThread);
// +++++++++++++++++++++++++++++AudioFlinger::DuplicatingThread::addOutputTrack+++++++++++++++++++++++++++++++++++
void AudioFlinger::DuplicatingThread::addOutputTrack(MixerThread *thread)
{
    int frameCount = (3 * mFrameCount * mSampleRate) / thread->sampleRate();
    OutputTrack *outputTrack = new OutputTrack((ThreadBase *)thread,
                                            this,
                                            mSampleRate,
                                            mFormat,
                                            mChannelCount,
                                            frameCount);
    if (outputTrack->cblk() != NULL) {
        thread->setStreamVolume(AudioSystem::NUM_STREAM_TYPES, 1.0f);
        mOutputTracks.add(outputTrack);
        LOGV("addOutputTrack() track %p, on thread %p", outputTrack, thread);
        updateWaitTime();
    }
// +++++++++++++++++++++++++++++AudioFlinger::PlaybackThread::OutputTrack::OutputTrack+++++++++++++++++++++++++++++++++++
AudioFlinger::PlaybackThread::OutputTrack::OutputTrack(
            const wp& thread,
            DuplicatingThread *sourceThread,
            uint32_t sampleRate,
            int format,
            int channelCount,
            int frameCount)
    :   Track(thread, NULL, AudioSystem::NUM_STREAM_TYPES, sampleRate, format, channelCount, frameCount, NULL, 0),
    mActive(false), mSourceThread(sourceThread)
{


    PlaybackThread *playbackThread = (PlaybackThread *)thread.unsafe_get();
    if (mCblk != NULL) {
        mCblk->flags |= CBLK_DIRECTION_OUT;
        mCblk->buffers = (char*)mCblk + sizeof(audio_track_cblk_t);
        mCblk->volume[0] = mCblk->volume[1] = 0x1000;
        mOutBuffer.frameCount = 0;
        playbackThread->mTracks.add(this);
        LOGV("OutputTrack constructor mCblk %p, mBuffer %p, mCblk->buffers %p, mCblk->frameCount %d, mCblk->sampleRate %d, mCblk->channelCount %d mBufferEnd %p",
                mCblk, mBuffer, mCblk->buffers, mCblk->frameCount, mCblk->sampleRate, mCblk->channelCount, mBufferEnd);
    } else {
        LOGW("Error creating output track on thread %p", playbackThread);
    }
}
// -----------------------------AudioFlinger::PlaybackThread::OutputTrack::OutputTrack-----------------------------------
}
// -----------------------------AudioFlinger::DuplicatingThread::addOutputTrack-----------------------------------
}
// ----------------------------AudioFlinger::DuplicatingThread::DuplicatingThread------------------------------------
// 再将另外一个thread也添加进来
    thread->addOutputTrack(thread2);
    mPlaybackThreads.add(id, thread);
    // notify client processes of the new output creation
    thread->audioConfigChanged_l(AudioSystem::OUTPUT_OPENED);
    return id;
}
// -------------------------------AudioFlinger::openDuplicateOutput---------------------------------
}
// -----------------------------AudioPolicyService::openDuplicateOutput-----------------------------------
        }
        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 {
            LOGW("getOutput() could not open duplicated output for %d and %d",
                    mHardwareOutput, mA2dpOutput);
            mpClientInterface->closeOutput(mA2dpOutput);
            mOutputs.removeItem(mA2dpOutput);
            mA2dpOutput = 0;
            delete outputDesc;
            return NO_INIT;
        }
    } else {
        LOGW("setDeviceConnectionState() could not open A2DP output for device %x", device);
        delete outputDesc;
        return NO_INIT;
    }
    AudioOutputDescriptor *hwOutputDesc = mOutputs.valueFor(mHardwareOutput);


    if (!a2dpUsedForSonification()) {
        // mute music on A2DP output if a notification or ringtone is playing
        uint32_t refCount = hwOutputDesc->strategyRefCount(STRATEGY_SONIFICATION);
        for (uint32_t i = 0; i < refCount; i++) {
            setStrategyMute(STRATEGY_MEDIA, true, mA2dpOutput);
        }
    }


    mA2dpSuspended = false;


    return NO_ERROR;
}
// -----------------------------AudioPolicyManagerBase::handleA2dpConnection-----------------------------------
        } else
#endif
        {
            // if playing on 2 devices among which none is A2DP, use hardware output
            output = mHardwareOutput;
// +++++++++++++++++++++++++++++++AudioPolicyManagerBase::AudioPolicyManagerBase+++++++++++++++++++++++++++++++++
mHardwareOutput在AudioPolicyManagerBase的构造函数中被赋值。
AudioPolicyManagerBase::AudioPolicyManagerBase(AudioPolicyClientInterface *clientInterface)
    :
#ifdef AUDIO_POLICY_TEST
    Thread(false),
#endif //AUDIO_POLICY_TEST
    mPhoneState(AudioSystem::MODE_NORMAL), mRingerMode(0),
    mMusicStopTime(0), mLimitRingtoneVolume(false), mLastVoiceVolume(-1.0f),
    mTotalEffectsCpuLoad(0), mTotalEffectsMemory(0),
    mA2dpSuspended(false)
{
    mpClientInterface = clientInterface;


    for (int i = 0; i < AudioSystem::NUM_FORCE_USE; i++) {
        mForceUse[i] = AudioSystem::FORCE_NONE;
    }


    // devices available by default are speaker, ear piece and microphone
    mAvailableOutputDevices = AudioSystem::DEVICE_OUT_EARPIECE |
                        AudioSystem::DEVICE_OUT_SPEAKER;
    mAvailableInputDevices = AudioSystem::DEVICE_IN_BUILTIN_MIC;


#ifdef WITH_A2DP
    mA2dpOutput = 0;
    mDuplicatedOutput = 0;
    mA2dpDeviceAddress = String8("");
#endif
    mScoDeviceAddress = String8("");


    // open hardware output
    AudioOutputDescriptor *outputDesc = new AudioOutputDescriptor();
// 从下面的这个赋值可见，一般情况下，如果不使用A2DP的话，我们用的device其实是固定的：DEVICE_OUT_SPEAKER
    outputDesc->mDevice = (uint32_t)AudioSystem::DEVICE_OUT_SPEAKER;
// 从AudioPolicyManagerBase对象被创建的地方可知，mpClientInterface其实是AudioPolicyService对象。
    mHardwareOutput = mpClientInterface->openOutput(&outputDesc->mDevice,
                                    &outputDesc->mSamplingRate,
                                    &outputDesc->mFormat,
                                    &outputDesc->mChannels,
                                    &outputDesc->mLatency,
                                    outputDesc->mFlags);


    if (mHardwareOutput == 0) {
        LOGE("Failed to initialize hardware output stream, samplingRate: %d, format %d, channels %d",
                outputDesc->mSamplingRate, outputDesc->mFormat, outputDesc->mChannels);
    } else {
        addOutput(mHardwareOutput, outputDesc);
        setOutputDevice(mHardwareOutput, (uint32_t)AudioSystem::DEVICE_OUT_SPEAKER, true);
        //TODO: configure audio effect output stage here
    }
// +++++++++++++++++++++++++++++++AudioPolicyService::openOutput+++++++++++++++++++++++++++++++++
audio_io_handle_t AudioPolicyService::openOutput(uint32_t *pDevices,
                                uint32_t *pSamplingRate,
                                uint32_t *pFormat,
                                uint32_t *pChannels,
                                uint32_t *pLatencyMs,
                                AudioSystem::output_flags flags)
{
    sp af = AudioSystem::get_audio_flinger();
    if (af == 0) {
        LOGW("openOutput() could not get AudioFlinger");
        return 0;
    }


    return af->openOutput(pDevices,
                          pSamplingRate,
                          (uint32_t *)pFormat,
                          pChannels,
                          pLatencyMs,
                          flags);
// ++++++++++++++++++++++++++++++AudioFlinger::openOutput++++++++++++++++++++++++++++++++++
int AudioFlinger::openOutput(uint32_t *pDevices,
                                uint32_t *pSamplingRate,
                                uint32_t *pFormat,
                                uint32_t *pChannels,
                                uint32_t *pLatencyMs,
                                uint32_t flags)
{
    status_t status;
    PlaybackThread *thread = NULL;
    mHardwareStatus = AUDIO_HW_OUTPUT_OPEN;
    uint32_t samplingRate = pSamplingRate ? *pSamplingRate : 0;
    uint32_t format = pFormat ? *pFormat : 0;
    uint32_t channels = pChannels ? *pChannels : 0;
    uint32_t latency = pLatencyMs ? *pLatencyMs : 0;


    LOGV("openOutput(), Device %x, SamplingRate %d, Format %d, Channels %x, flags %x",
            pDevices ? *pDevices : 0,
            samplingRate,
            format,
            channels,
            flags);


    if (pDevices == NULL || *pDevices == 0) {
        return 0;
    }
    Mutex::Autolock _l(mLock);


// 调用HAL层的接口，打开一个output stream。
// device类型，被作为参数传给了HAL
// 这个函数前面已经看过，此处只把与device类型相关的东东再复习一下
// ++++++++++++++++++++++++++++AudioHardwareALSA::openOutputStream++++++++++++++++++++++++++++++++++++
AudioStreamOut *
AudioHardwareALSA::openOutputStream(uint32_t devices,
                                    int *format,
                                    uint32_t *channels,
                                    uint32_t *sampleRate,
                                    status_t *status)
{
...


    // 从device 类型的定义可知，每种device类型占一个bit
// 从函数getDeviceForStrategy的实现可知，我们根据stream类型最终取得的device肯定只有一种
// 此处检查device类型是不是只占了一个bit
    if (devices & (devices - 1)) {
        if (status) *status = err;
        LOGD("openOutputStream called with bad devices");
        return out;
    }


    // Find the appropriate alsa device
// ALSAHandleList中的内容其实是从一个数组中copy过来
// 数组中只是列出了一些初始信息，很多有用的东东是在后面赋值的
// 结构体中一个重要的东东module(alsa_device_t * )是在调用函数s_open中赋值的
// +++++++++++++++++++++++++++++_defaults+++++++++++++++++++++++++++++++++++
static alsa_handle_t _defaults[] = {
    {
        module      : 0,
        devices     : IMX51_OUT_DEFAULT,
        curDev      : 0,
        curMode     : 0,
        handle      : 0,
        format      : SND_PCM_FORMAT_S16_LE, // AudioSystem::PCM_16_BIT
        channels    : 2,
        sampleRate  : DEFAULT_SAMPLE_RATE,
        latency     : 200000, // Desired Delay in usec
        bufferSize  : 6144, // Desired Number of samples
        modPrivate  : (void *)&setDefaultControls,
    },
    {
        module      : 0,
        devices     : IMX51_IN_DEFAULT,
        curDev      : 0,
        curMode     : 0,
        handle      : 0,
        format      : SND_PCM_FORMAT_S16_LE, // AudioSystem::PCM_16_BIT
        channels    : 2,
        sampleRate  : DEFAULT_SAMPLE_RATE,
        latency     : 250000, // Desired Delay in usec
        bufferSize  : 6144, // Desired Number of samples
        modPrivate  : (void *)&setDefaultControls,
    },
};
// -----------------------------_defaults-----------------------------------
    for(ALSAHandleList::iterator it = mDeviceList.begin();
        it != mDeviceList.end(); ++it)
// 根据device类型，找到占的坑
        if (it->devices & devices) {
// 在调s_open时，仍然将device类型作为参数传入
            err = mALSADevice->open(&(*it), devices, mode());
            if (err) break;
// +++++++++++++++++++++++++++++s_open+++++++++++++++++++++++++++++++++++
static status_t s_open(alsa_handle_t *handle, uint32_t devices, int mode)
{
...


    // device类型是在找device名称的时候
    const char *devName = deviceName(handle, devices, mode, 1);
// +++++++++++++++++++++++++++++deviceName+++++++++++++++++++++++++++++++++++
//card_device =0, return the card name, card_device=1, return the card device name
const char *deviceName(alsa_handle_t *alsa_handle, uint32_t device, int mode, int card_device)
{
...  


    property_get("ro.HDMI_AUDIO_OUTPUT", value, "");
// device类型在此处作判断用
// 因此，device name的取得，很大程度上说依赖于配置文件的
    if((device & AudioSystem::DEVICE_OUT_WIRED_HDMI) && havespdifdevice && (strcmp(value, "1") == 0))
    {
        return spdifcardname;


    }else if(havesgtldevice)
    {
        return sgtlcardname;
    }
    
    return "default";
}
// -----------------------------deviceName-----------------------------------


    // The PCM stream is opened in blocking mode, per ALSA defaults.  The
    // AudioFlinger seems to assume blocking mode too, so asynchronous mode
    // should not be used.
// 根据上面得到的device name，打开一个alsa设备
    int err = snd_pcm_open(&handle->handle, devName, direction(handle), 0);


    if (err < 0) {
        LOGE("Failed to Initialize any ALSA %s device: %s", stream, strerror(err));
        return NO_INIT;
    }


...

    // 将device类型保存到上面占地坑中
    handle->curDev = devices;
    handle->curMode = mode;


    return err;
}
// -----------------------------s_open-----------------------------------
            if (devices & AudioSystem::DEVICE_OUT_WIRED_HDMI){
                strcpy(mCurCard ,SPDIF);
                mMixer = mMixerSpdif;
            } else {
                strcpy(mCurCard,SGTL5000);
                mMixer = mMixerSgtl5000;
            }


            out = new AudioStreamOutALSA(this, &(*it));
            err = out->set(format, channels, sampleRate);
            break;
        }


    if (status) *status = err;
    return out;
}
// ----------------------------AudioHardwareALSA::openOutputStream------------------------------------
    AudioStreamOut *output = mAudioHardware->openOutputStream(*pDevices,
                                                             (int *)&format,
                                                             &channels,
                                                             &samplingRate,
                                                             &status);
    LOGV("openOutput() openOutputStream returned output %p, SamplingRate %d, Format %d, Channels %x, status %d",
            output,
            samplingRate,
            format,
            channels,
            status);


    mHardwareStatus = AUDIO_HW_IDLE;
    if (output != 0) {
        int id = nextUniqueId();
        if ((flags & AudioSystem::OUTPUT_FLAG_DIRECT) ||
            (format != AudioSystem::PCM_16_BIT) ||
            (channels != AudioSystem::CHANNEL_OUT_STEREO)) { 
// 如果是direct模式，创建一个DirectOutputThread
            thread = new DirectOutputThread(this, output, id, *pDevices);
            LOGV("openOutput() created direct output: ID %d thread %p", id, thread);
        } else {
// stream模式的话，创建一个MixerThread
            thread = new MixerThread(this, output, id, *pDevices);
            LOGV("openOutput() created mixer output: ID %d thread %p", id, thread);


#ifdef LVMX
            unsigned bitsPerSample =
                (format == AudioSystem::PCM_16_BIT) ? 16 :
                    ((format == AudioSystem::PCM_8_BIT) ? 8 : 0);
            unsigned channelCount = (channels == AudioSystem::CHANNEL_OUT_STEREO) ? 2 : 1;
            int audioOutputType = LifeVibes::threadIdToAudioOutputType(thread->id());


            LifeVibes::init_aot(audioOutputType, samplingRate, bitsPerSample, channelCount);
            LifeVibes::setDevice(audioOutputType, *pDevices);
#endif


        }
// 将上面创建的thread追加到playback thread列表，并将其与一个unique id关联起来
        mPlaybackThreads.add(id, thread);


        if (pSamplingRate) *pSamplingRate = samplingRate;
        if (pFormat) *pFormat = format;
        if (pChannels) *pChannels = channels;
        if (pLatencyMs) *pLatencyMs = thread->latency();


        // notify client processes of the new output creation
        thread->audioConfigChanged_l(AudioSystem::OUTPUT_OPENED);
        return id;
    }


    return 0;
}
// ------------------------------AudioFlinger::openOutput----------------------------------
}
// -------------------------------AudioPolicyService::openOutput---------------------------------


    updateDeviceForStrategy();
#ifdef AUDIO_POLICY_TEST
    AudioParameter outputCmd = AudioParameter();
    outputCmd.addInt(String8("set_id"), 0);
    mpClientInterface->setParameters(mHardwareOutput, outputCmd.toString());


    mTestDevice = AudioSystem::DEVICE_OUT_SPEAKER;
    mTestSamplingRate = 44100;
    mTestFormat = AudioSystem::PCM_16_BIT;
    mTestChannels =  AudioSystem::CHANNEL_OUT_STEREO;
    mTestLatencyMs = 0;
    mCurOutput = 0;
    mDirectOutput = false;
    for (int i = 0; i < NUM_TEST_OUTPUTS; i++) {
        mTestOutputs[i] = 0;
    }


    const size_t SIZE = 256;
    char buffer[SIZE];
    snprintf(buffer, SIZE, "AudioPolicyManagerTest");
    run(buffer, ANDROID_PRIORITY_AUDIO);
#endif //AUDIO_POLICY_TEST
}


AudioPolicyManagerBase对象是在AudioPolicyService的构造函数中创建的。
// ++++++++++++++++++++++++++++++AudioPolicyService::AudioPolicyService++++++++++++++++++++++++++++++++++
AudioPolicyService::AudioPolicyService()
    : BnAudioPolicyService() , mpPolicyManager(NULL)
{
    char value[PROPERTY_VALUE_MAX];


    // start tone playback thread
    mTonePlaybackThread = new AudioCommandThread(String8(""));
    // start audio commands thread
    mAudioCommandThread = new AudioCommandThread(String8("ApmCommandThread"));


#if (defined GENERIC_AUDIO) || (defined AUDIO_POLICY_TEST)
    mpPolicyManager = new AudioPolicyManagerBase(this);
    LOGV("build for GENERIC_AUDIO - using generic audio policy");
#else
    // if running in emulation - use the emulator driver
    if (property_get("ro.kernel.qemu", value, 0)) {
        LOGV("Running in emulation - using generic audio policy");
        mpPolicyManager = new AudioPolicyManagerBase(this);
    }
    else {
        LOGV("Using hardware specific audio policy");
// 正常的话，应该是走到这儿来。
// 这个函数以前看过，实际使用的应该是ALSA中的AudioPlicyManager
// Alsa中的AudioPolicyManager继承自AudioPolicyManagerBase，
// 并且基本上没做什么变化
        mpPolicyManager = createAudioPolicyManager(this);
    }
#endif


    // load properties
    property_get("ro.camera.sound.forced", value, "0");
    mpPolicyManager->setSystemProperty("ro.camera.sound.forced", value);
}
// ------------------------------AudioPolicyService::AudioPolicyService----------------------------------
// -------------------------------AudioPolicyManagerBase::AudioPolicyManagerBase---------------------------------
        }
        LOGV("getOutput() using output %d for 2 devices %x", output, device);
    } else {
#ifdef WITH_A2DP
        if (a2dpDevice != 0) {
            // if playing on A2DP device, use a2dp output
            LOGW_IF((mA2dpOutput == 0), "getOutput() A2DP device %x selected but A2DP output not opened", device);
            output = mA2dpOutput;
        } else
#endif
        {
            // if playing on not A2DP device, use hardware output
            output = mHardwareOutput;
        }
    }




    LOGW_IF((output ==0), "getOutput() could not find output for stream %d, samplingRate %d, format %d, channels %x, flags %x",
                stream, samplingRate, format, channels, flags);


    return output;
}
// -----------------------------AudioPolicyManagerBase::getOutput-----------------------------------
}
// -----------------------------AudioPolicyService::getOutput-----------------------------------
    }
    return output;
}
// -------------------------AudioSystem::getOutput---------------------------------------

###########################################################


&&&&&&&&&&&&&&&&&&&&&&&总结&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
根据stream type找到对应的strategy。
根据strategy找到对应的device type。
根据device type找到对应的alsa_handle_t对象。
根据配置和device type找到设备名称，并打开设备获取一个snd_pcm_t对象。
将snd_pcm_t对象保存到alsa_handle_t对象对象中。


我们之前已经看过write操作，
其实就是往snd_pcm_t对象的stopped_areas或者running_areas成员中copy数据；
或者直接调用snd_pcm_t对象的fast_ops的writei函数写数据。
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&