Android Audio代码分析4 - AudioSystem::getOutputSamplingRate

 

前面看过的代码中，经常会调用到AudioSystem类中的getOutputSamplingRate函数，getOutputFrameCount函数，getOutputLatency函数等。
这些函数的实现基本类似，今天就细细品味下AudioSystem::getOutputSamplingRate函数。

*****************************************源码*************************************************

status_t AudioSystem::getOutputSamplingRate(int* samplingRate, int streamType)
{
    OutputDescriptor *outputDesc;
    audio_io_handle_t output;
    if (streamType == DEFAULT) {
        streamType = MUSIC;
    }
    output = getOutput((stream_type)streamType);
    if (output == 0) {
        return PERMISSION_DENIED;
    }
    gLock.lock();
    outputDesc = AudioSystem::gOutputs.valueFor(output);
    if (outputDesc == 0) {
        LOGV("getOutputSamplingRate() no output descriptor for output %d in gOutputs", output);
        gLock.unlock();
        const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
        if (af == 0) return PERMISSION_DENIED;
        *samplingRate = af->sampleRate(output);
    } else {
        LOGV("getOutputSamplingRate() reading from output desc");
        *samplingRate = outputDesc->samplingRate;
        gLock.unlock();
    }
    LOGV("getOutputSamplingRate() streamType %d, output %d, sampling rate %d", streamType, output, *samplingRate);
    return NO_ERROR;
}

**********************************************************************************************
源码路径：
frameworks\base\media\libmedia\AudioSystem.cpp

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

status_t AudioSystem::getOutputSamplingRate(int* samplingRate, int streamType)
{
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    // audio output descritor used to cache output configurations in client process to avoid frequent calls
    // through IAudioFlinger
    class OutputDescriptor {
    public:
        OutputDescriptor()
        : samplingRate(0), format(0), channels(0), frameCount(0), latency(0)  {}
        uint32_t samplingRate;
        int32_t format;
        int32_t channels;
        size_t frameCount;
        uint32_t latency;
    };
----------------------------------------------------------------
    OutputDescriptor *outputDesc;
// typedef int audio_io_handle_t;
    audio_io_handle_t output;
// Default类型的stream，强制转为music
    if (streamType == DEFAULT) {
        streamType = MUSIC;
    }
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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;
// 如果是direct模式，或者可能会使用direct模式，就不能使用cache中保存的output
    // 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);
  // gStreamOutputMap中保存的是stream与output之间的对应关系
  // 调用getOutput时，如果获取不到output，就会调用AudioPolicyService的getOutput函数获取output，
  // 并将stream与output的对应关系添加到gStreamOutputMap。（也就是该函数下面的处理）
        output = AudioSystem::gStreamOutputMap.valueFor(stream);
        LOGV_IF((output != 0), "getOutput() read %d from cache for stream %d", output, stream);
    }
 // 如果Cache中找不到stream对应的output
    if (output == 0) {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// establish binder interface to AudioFlinger service
const sp<IAudioPolicyService>& AudioSystem::get_audio_policy_service()
{
    gLock.lock();
 // 如果gAudioPolicyService尚未创建，则创建一个，否则直接将gAudioPolicyService返回
    if (gAudioPolicyService.get() == 0) {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// ServiceManager是一个管理所有service的一个东东
sp<IServiceManager> defaultServiceManager()
{
// 如果gDefaultServiceManager不为空，直接将其返回
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
    
    {
        AutoMutex _l(gDefaultServiceManagerLock);
        if (gDefaultServiceManager == NULL) {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
sp<ProcessState> ProcessState::self()
{
    if (gProcess != NULL) return gProcess;
    
    AutoMutex _l(gProcessMutex);
    if (gProcess == NULL) gProcess = new ProcessState;
    return gProcess;
}
----------------------------------------------------------------
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& caller)
{
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
bool ProcessState::supportsProcesses() const
{
    return mDriverFD >= 0;
}
----------------------------------------------------------------
    if (supportsProcesses()) {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp<IBinder> result;
    AutoMutex _l(mLock);
    handle_entry* e = lookupHandleLocked(handle);
    if (e != NULL) {
        // We need to create a new BpBinder if there isn't currently one, OR we
        // are unable to acquire a weak reference on this current one.  See comment
        // in getWeakProxyForHandle() for more info about this.
        IBinder* b = e->binder;
        if (b == NULL || !e->refs->attemptIncWeak(this)) {
            b = new BpBinder(handle); 
            e->binder = b;
            if (b) e->refs = b->getWeakRefs();
            result = b;
        } else {
            // This little bit of nastyness is to allow us to add a primary
            // reference to the remote proxy when this team doesn't have one
            // but another team is sending the handle to us.
            result.force_set(b);
            e->refs->decWeak(this);
        }
    }
    return result;
}
----------------------------------------------------------------
        return getStrongProxyForHandle(0);
    } else {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
sp<IBinder> ProcessState::getContextObject(const String16& name, const sp<IBinder>& caller)
{
    mLock.lock();
    sp<IBinder> object(
        mContexts.indexOfKey(name) >= 0 ? mContexts.valueFor(name) : NULL);
    mLock.unlock();
    
    //printf("Getting context object %s for %p\n", String8(name).string(), caller.get());
    
    if (object != NULL) return object;
    // Don't attempt to retrieve contexts if we manage them
    if (mManagesContexts) {
        LOGE("getContextObject(%s) failed, but we manage the contexts!\n",
            String8(name).string());
        return NULL;
    }
    
    IPCThreadState* ipc = IPCThreadState::self();
    {
        Parcel data, reply;
        // no interface token on this magic transaction
        data.writeString16(name);
        data.writeStrongBinder(caller);
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err = data.errorCheck();
    flags |= TF_ACCEPT_FDS;
    IF_LOG_TRANSACTIONS() {
        TextOutput::Bundle _b(alog);
        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "
            << handle << " / code " << TypeCode(code) << ": "
            << indent << data << dedent << endl;
    }
    
    if (err == NO_ERROR) {
        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),
            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");
        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
    }
    
    if (err != NO_ERROR) {
        if (reply) reply->setError(err);
        return (mLastError = err);
    }
    
    if ((flags & TF_ONE_WAY) == 0) {
        #if 0
        if (code == 4) { // relayout
            LOGI(">>>>>> CALLING transaction 4");
        } else {
            LOGI(">>>>>> CALLING transaction %d", code);
        }
        #endif
        if (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        #if 0
        if (code == 4) { // relayout
            LOGI("<<<<<< RETURNING transaction 4");
        } else {
            LOGI("<<<<<< RETURNING transaction %d", code);
        }
        #endif
        
        IF_LOG_TRANSACTIONS() {
            TextOutput::Bundle _b(alog);
            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "
                << handle << ": ";
            if (reply) alog << indent << *reply << dedent << endl;
            else alog << "(none requested)" << endl;
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }
    
    return err;
}
----------------------------------------------------------------
        status_t result = ipc->transact(0 /*magic*/, 0, data, &reply, 0);
        if (result == NO_ERROR) {
            object = reply.readStrongBinder();
        }
    }
 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
void IPCThreadState::flushCommands()
{
    if (mProcess->mDriverFD <= 0)
        return;
// 和驱动对话？
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
status_t IPCThreadState::talkWithDriver(bool doReceive)
{
    LOG_ASSERT(mProcess->mDriverFD >= 0, "Binder driver is not opened");
    
    binder_write_read bwr;
    
    // Is the read buffer empty?
    const bool needRead = mIn.dataPosition() >= mIn.dataSize();
    
    // We don't want to write anything if we are still reading
    // from data left in the input buffer and the caller
    // has requested to read the next data.
    const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;
    
    bwr.write_size = outAvail;
    bwr.write_buffer = (long unsigned int)mOut.data();
    // This is what we'll read.
    if (doReceive && needRead) {
        bwr.read_size = mIn.dataCapacity();
        bwr.read_buffer = (long unsigned int)mIn.data();
    } else {
        bwr.read_size = 0;
    }
    
    IF_LOG_COMMANDS() {
        TextOutput::Bundle _b(alog);
        if (outAvail != 0) {
            alog << "Sending commands to driver: " << indent;
            const void* cmds = (const void*)bwr.write_buffer;
            const void* end = ((const uint8_t*)cmds)+bwr.write_size;
            alog << HexDump(cmds, bwr.write_size) << endl;
            while (cmds < end) cmds = printCommand(alog, cmds);
            alog << dedent;
        }
        alog << "Size of receive buffer: " << bwr.read_size
            << ", needRead: " << needRead << ", doReceive: " << doReceive << endl;
    }
    
    // Return immediately if there is nothing to do.
    if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;
    
    bwr.write_consumed = 0;
    bwr.read_consumed = 0;
    status_t err;
    do {
        IF_LOG_COMMANDS() {
            alog << "About to read/write, write size = " << mOut.dataSize() << endl;
        }
#if defined(HAVE_ANDROID_OS)
// 通过ioctl函数与驱动对话
        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
            err = NO_ERROR;
        else
            err = -errno;
#else
        err = INVALID_OPERATION;
#endif
        IF_LOG_COMMANDS() {
            alog << "Finished read/write, write size = " << mOut.dataSize() << endl;
        }
    } while (err == -EINTR);
    
    IF_LOG_COMMANDS() {
        alog << "Our err: " << (void*)err << ", write consumed: "
            << bwr.write_consumed << " (of " << mOut.dataSize()
   << "), read consumed: " << bwr.read_consumed << endl;
    }
    if (err >= NO_ERROR) {
        if (bwr.write_consumed > 0) {
            if (bwr.write_consumed < (ssize_t)mOut.dataSize())
                mOut.remove(0, bwr.write_consumed);
            else
                mOut.setDataSize(0);
        }
        if (bwr.read_consumed > 0) {
            mIn.setDataSize(bwr.read_consumed);
            mIn.setDataPosition(0);
        }
        IF_LOG_COMMANDS() {
            TextOutput::Bundle _b(alog);
            alog << "Remaining data size: " << mOut.dataSize() << endl;
            alog << "Received commands from driver: " << indent;
            const void* cmds = mIn.data();
            const void* end = mIn.data() + mIn.dataSize();
            alog << HexDump(cmds, mIn.dataSize()) << endl;
            while (cmds < end) cmds = printReturnCommand(alog, cmds);
            alog << dedent;
        }
        return NO_ERROR;
    }
    
    return err;
}
----------------------------------------------------------------
    talkWithDriver(false);
}
---------------------------------------------------------------- 
    ipc->flushCommands();
    
    if (object != NULL) setContextObject(object, name);
    return object;
}
----------------------------------------------------------------
        return getContextObject(String16("default"), caller);
    }
}
----------------------------------------------------------------
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
        }
    }
    
    return gDefaultServiceManager;
}
----------------------------------------------------------------
        sp<IServiceManager> sm = defaultServiceManager();
        sp<IBinder> binder;
        do {
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    public IBinder getService(String name) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IServiceManager.descriptor);
        data.writeString(name);
  // 此处应该是Binder机制。transact最终应该调到以Bn开头的某个类中
  // 果然...在类ServiceManagerNative中
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
    {
        try {
            switch (code) {
            case IServiceManager.GET_SERVICE_TRANSACTION: {
                data.enforceInterface(IServiceManager.descriptor);
                String name = data.readString();
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    /**
     * Returns a reference to a service with the given name.
     * 
     * @param name the name of the service to get
     * @return a reference to the service, or <code>null</code> if the service doesn't exist
     */
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return getIServiceManager().getService(name);
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }
----------------------------------------------------------------
                IBinder service = getService(name);
                reply.writeStrongBinder(service);
                return true;
            }
    
            case IServiceManager.CHECK_SERVICE_TRANSACTION: {
                data.enforceInterface(IServiceManager.descriptor);
                String name = data.readString();
                IBinder service = checkService(name);
                reply.writeStrongBinder(service);
                return true;
            }
    
            case IServiceManager.ADD_SERVICE_TRANSACTION: {
                data.enforceInterface(IServiceManager.descriptor);
                String name = data.readString();
                IBinder service = data.readStrongBinder();
                addService(name, service);
                return true;
            }
    
            case IServiceManager.LIST_SERVICES_TRANSACTION: {
                data.enforceInterface(IServiceManager.descriptor);
                String[] list = listServices();
                reply.writeStringArray(list);
                return true;
            }
            
            case IServiceManager.SET_PERMISSION_CONTROLLER_TRANSACTION: {
                data.enforceInterface(IServiceManager.descriptor);
                IPermissionController controller
                        = IPermissionController.Stub.asInterface(
                                data.readStrongBinder());
                setPermissionController(controller);
                return true;
            }
            }
        } catch (RemoteException e) {
        }
        
        return false;
    }
----------------------------------------------------------------
        mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0);
        IBinder binder = reply.readStrongBinder();
        reply.recycle();
        data.recycle();
        return binder;
    }
----------------------------------------------------------------
            binder = sm->getService(String16("media.audio_policy"));
            if (binder != 0)
                break;
            LOGW("AudioPolicyService not published, waiting...");
            usleep(500000); // 0.5 s
        } while(true);
        if (gAudioPolicyServiceClient == NULL) {
            gAudioPolicyServiceClient = new AudioPolicyServiceClient();
        }
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
status_t BpBinder::linkToDeath(
    const sp<DeathRecipient>& recipient, void* cookie, uint32_t flags)
{
    Obituary ob;
    ob.recipient = recipient;
    ob.cookie = cookie;
    ob.flags = flags;
    LOG_ALWAYS_FATAL_IF(recipient == NULL,
                        "linkToDeath(): recipient must be non-NULL");
    {
        AutoMutex _l(mLock);
        if (!mObitsSent) {
            if (!mObituaries) {
                mObituaries = new Vector<Obituary>;
                if (!mObituaries) {
                    return NO_MEMORY;
                }
                LOGV("Requesting death notification: %p handle %d\n", this, mHandle);
                getWeakRefs()->incWeak(this);
                IPCThreadState* self = IPCThreadState::self();
                self->requestDeathNotification(mHandle, this);
                self->flushCommands();
            }
            ssize_t res = mObituaries->add(ob);
            return res >= (ssize_t)NO_ERROR ? (status_t)NO_ERROR : res;
        }
    }
    return DEAD_OBJECT;
}
----------------------------------------------------------------
        binder->linkToDeath(gAudioPolicyServiceClient);
        gAudioPolicyService = interface_cast<IAudioPolicyService>(binder);
        gLock.unlock();
    } else {
        gLock.unlock();
    }
    return gAudioPolicyService;
}
----------------------------------------------------------------
        const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
        if (aps == 0) return 0;
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 在类BpAudioPolicyService（class BpAudioPolicyService : public BpInterface<IAudioPolicyService>）中
    virtual audio_io_handle_t getOutput(
                                        AudioSystem::stream_type stream,
                                        uint32_t samplingRate,
                                        uint32_t format,
                                        uint32_t channels,
                                        AudioSystem::output_flags flags)
    {
        Parcel data, reply;
        data.writeInterfaceToken(IAudioPolicyService::getInterfaceDescriptor());
        data.writeInt32(static_cast <uint32_t>(stream));
        data.writeInt32(samplingRate);
        data.writeInt32(static_cast <uint32_t>(format));
        data.writeInt32(channels);
        data.writeInt32(static_cast <uint32_t>(flags));
  // 又是Binder，应该是调到Native的类中
  // 在类AudioPolicyService（class AudioPolicyService: public BnAudioPolicyService, 
  // public AudioPolicyClientInterface,public IBinder::DeathRecipient）中
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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);
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// 看样子真正干活的地方是这儿
audio_io_handle_t AudioPolicyManagerBase::getOutput(AudioSystem::stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    AudioSystem::output_flags flags)
{
// 创建一个AudioOutputDescriptor对象，并打开一个output
    audio_io_handle_t output = 0;
    uint32_t latency = 0;
    routing_strategy strategy = getStrategy((AudioSystem::stream_type)stream);
    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
    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;
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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<IAudioFlinger> af = AudioSystem::get_audio_flinger();
    if (af == 0) {
        LOGW("openOutput() could not get AudioFlinger");
        return 0;
    }
// 又调到了AudioFlinger中
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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);
// 最终干活的还是在硬件抽象层
    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)) {
            thread = new DirectOutputThread(this, output, id, *pDevices);
            LOGV("openOutput() created direct output: ID %d thread %p", id, thread);
        } else {
            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
        }
        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;
}
----------------------------------------------------------------
    return af->openOutput(pDevices,
                          pSamplingRate,
                          (uint32_t *)pFormat,
                          pChannels,
                          pLatencyMs,
                          flags);
}
----------------------------------------------------------------
            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
    // 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
    // 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);
            output = mDuplicatedOutput;
        } else
#endif
        {
            // if playing on 2 devices among which none is A2DP, use hardware output
            output = mHardwareOutput;
        }
        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;
}
----------------------------------------------------------------
    return mpPolicyManager->getOutput(stream, samplingRate, format, channels, flags);
}
----------------------------------------------------------------
        remote()->transact(GET_OUTPUT, data, &reply);
        return static_cast <audio_io_handle_t> (reply.readInt32());
    }
----------------------------------------------------------------
        output = aps->getOutput(stream, samplingRate, format, channels, flags);
        if ((flags & AudioSystem::OUTPUT_FLAG_DIRECT) == 0) {
            Mutex::Autolock _l(gLock);
            AudioSystem::gStreamOutputMap.add(stream, output);
        }
    }
    return output;
}
----------------------------------------------------------------
    output = getOutput((stream_type)streamType);
    if (output == 0) {
        return PERMISSION_DENIED;
    }
// 函数名为getOutputSamplingRate，当然要得到我们想要的东西
    gLock.lock();
// 看AudioSystem中是否保存了output对应的描述符
    outputDesc = AudioSystem::gOutputs.valueFor(output);
    if (outputDesc == 0) {
 // 如果没有，通过AudioFlinger获取
        LOGV("getOutputSamplingRate() no output descriptor for output %d in gOutputs", output);
        gLock.unlock();
        const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
        if (af == 0) return PERMISSION_DENIED;
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
uint32_t AudioFlinger::sampleRate(int output) const
{
    Mutex::Autolock _l(mLock);
    PlaybackThread *thread = checkPlaybackThread_l(output);
    if (thread == NULL) {
        LOGW("sampleRate() unknown thread %d", output);
        return 0;
    }
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
uint32_t AudioFlinger::ThreadBase::sampleRate() const
{
// 函数AudioFlinger::PlaybackThread::readOutputParameters中有对mSampleRate赋值
// 函数AudioFlinger::RecordThread::readInputParameters中有对mSampleRate赋值录音时用
    return mSampleRate;
}
----------------------------------------------------------------
    return thread->sampleRate();
}
----------------------------------------------------------------
        *samplingRate = af->sampleRate(output);
    } else {
 // 若有，直接从描述符中取出
        LOGV("getOutputSamplingRate() reading from output desc");
        *samplingRate = outputDesc->samplingRate;
        gLock.unlock();
    }
    LOGV("getOutputSamplingRate() streamType %d, output %d, sampling rate %d", streamType, output, *samplingRate);
    return NO_ERROR;