Android SurfaceFlinger服務（六） ----- VSync訊號的分發

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HWComposer模組產生VSync訊號後要經過分發才能送達到關心VSync事件的模組中去。VSync訊號分發大致流程為HWComposer->SurfaceFlinger->DispSync->DispSyncSource->各個具體模組。下面具體分析下這個流程。

在上一篇文章中分析到硬體或者軟體VSync訊號產生時，會回調SurfaceFlinger的onVSyncReceived函數，來看看這個函數：

void SurfaceFlinger::onVSyncReceived(int type, nsecs_t timestamp) {    bool needsHwVsync = false;    { // Scope for the lock        Mutex::Autolock _l(mHWVsyncLock);         if (type == 0 && mPrimaryHWVsyncEnabled) {            needsHwVsync = mPrimaryDispSync.addResyncSample(timestamp);        }    }      if (needsHwVsync) {        enableHardwareVsync();    } else {        disableHardwareVsync(false);    }  }

• mPrimaryDispSync類型為DispSync,在構造的時候會建立DispSyncThread

DispSync::DispSync() :        mRefreshSkipCount(0),        mThread(new DispSyncThread()) {    mThread->run("DispSync", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);    reset();    beginResync();    if (kTraceDetailedInfo) {        // If we‘re not getting present fences then the ZeroPhaseTracer        // would prevent HW vsync event from ever being turned off.        // Even if we‘re just ignoring the fences, the zero-phase tracing is        // not needed because any time there is an event registered we will        // turn on the HW vsync events.        if (!kIgnorePresentFences) {               addEventListener(0, new ZeroPhaseTracer());        }    }}

• 調用DispSync的addResyncSample方法，最終會去喚醒DispSyncThread線程

bool DispSync::addResyncSample(nsecs_t timestamp) {    Mutex::Autolock lock(mMutex);    size_t idx = (mFirstResyncSample + mNumResyncSamples) % MAX_RESYNC_SAMPLES;    mResyncSamples[idx] = timestamp;    if (mNumResyncSamples < MAX_RESYNC_SAMPLES) {        mNumResyncSamples++;    } else {        mFirstResyncSample = (mFirstResyncSample + 1) % MAX_RESYNC_SAMPLES;    }    updateModelLocked();    if (mNumResyncSamplesSincePresent++ > MAX_RESYNC_SAMPLES_WITHOUT_PRESENT) {        resetErrorLocked();    }    if (kIgnorePresentFences) {        // If we don‘t have the sync framework we will never have        // addPresentFence called.  This means we have no way to know whether        // or not we‘re synchronized with the HW vsyncs, so we just request        // that the HW vsync events be turned on whenever we need to generate        // SW vsync events.        return mThread->hasAnyEventListeners();    }    return mPeriod == 0 || mError > kErrorThreshold;}

• 調用updateModelLocked函數
• 計算是否需要開啟硬體VSync，進行同步

void DispSync::updateModelLocked() {    if (mNumResyncSamples >= MIN_RESYNC_SAMPLES_FOR_UPDATE) {                ......                mThread->updateModel(mPeriod, mPhase);    }}

• 成員變數mThread為DispSyncThread類的對象
• 調用DispSyncThread類pdateModel方法

void updateModel(nsecs_t period, nsecs_t phase) {        Mutex::Autolock lock(mMutex);        mPeriod = period;        mPhase = phase;        mCond.signal();    }

• 該方法主要設定成員變數mPeriod、mPhase
• 然後在喚醒線程迴圈

virtual bool threadLoop() {        status_t err;        nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);        nsecs_t nextEventTime = 0;        while (true) {            Vector<CallbackInvocation> callbackInvocations;            nsecs_t targetTime = 0;            { // Scope for lock                Mutex::Autolock lock(mMutex);                ......                callbackInvocations = gatherCallbackInvocationsLocked(now);            }            if (callbackInvocations.size() > 0) {                fireCallbackInvocations(callbackInvocations);            }        }        return false;    }

• 通過gatherCallbackInvocationsLocked得到監聽者集合
• 通過fireCallbackInvocations調用監聽者回呼函數

DispSync的回呼函數在哪裡註冊,我們在看看DispSyncSource的setVSyncEnabled函數

virtual void setVSyncEnabled(bool enable) {        Mutex::Autolock lock(mVsyncMutex);        if (enable) {            status_t err = mDispSync->addEventListener(mPhaseOffset,                    static_cast<DispSync::Callback*>(this));            if (err != NO_ERROR) {                             ALOGE("error registering vsync callback: %s (%d)",                        strerror(-err), err);                                                                                                                                                            }            //ATRACE_INT(mVsyncOnLabel.string(), 1);                                                                                                                                                 } else {            status_t err = mDispSync->removeEventListener(                    static_cast<DispSync::Callback*>(this));            if (err != NO_ERROR) {                             ALOGE("error unregistering vsync callback: %s (%d)",                        strerror(-err), err);                                                                                                                                                            }            //ATRACE_INT(mVsyncOnLabel.string(), 0);                                                                                                                                                 }        mEnabled = enable;    }

• DispSyncSource類繼承於DispSync::Callback
• 調用DispSync->addEventListener註冊回調，當DispSyncSource收到DispSync的VSync訊號時，會回調整onDispSyncEvent函數

virtual void onDispSyncEvent(nsecs_t when) {        sp<VSyncSource::Callback> callback;        {            Mutex::Autolock lock(mCallbackMutex);            callback = mCallback;            if (mTraceVsync) {                mValue = (mValue + 1) % 2;                ATRACE_INT(mVsyncEventLabel.string(), mValue);            }        }        if (callback != NULL) {            callback->onVSyncEvent(when);        }    }

• 這個函數最終還是調用VSyncSource::Callback的onVSyncEvent函數。
• VSyncSource::Callback函數的註冊是通過調用DispSyncSource的setCallback註冊。

DispSyncSource的setCallback函數在哪裡調用的呢，要解答這個問題，還是得回到SurfaceFlinger的init函數裡，以sfVsyncSrc為例。

void SurfaceFlinger::init() {        ......        // start the EventThread    sp<VSyncSource> vsyncSrc = new DispSyncSource(&mPrimaryDispSync,            vsyncPhaseOffsetNs, true, "app");    mEventThread = new EventThread(vsyncSrc);    sp<VSyncSource> sfVsyncSrc = new DispSyncSource(&mPrimaryDispSync,            sfVsyncPhaseOffsetNs, true, "sf");    mSFEventThread = new EventThread(sfVsyncSrc);    mEventQueue.setEventThread(mSFEventThread);    ......}

• 以sfVsyncSrc為參數建立EventThread對象
• 在EventThread的線程迴圈中會去調用enableVSyncLocked函數

void EventThread::enableVSyncLocked() {    if (!mUseSoftwareVSync) {        // never enable h/w VSYNC when screen is off        if (!mVsyncEnabled) {            mVsyncEnabled = true;                      mVSyncSource->setCallback(static_cast<VSyncSource::Callback*>(this));            mVSyncSource->setVSyncEnabled(true);        }    }      mDebugVsyncEnabled = true;    sendVsyncHintOnLocked();}

• EventThread繼承VSyncSource::Callback介面，實現了onVSyncEvent函數
• 調用mVSyncSource的setCallback函數設定回調
• 調用mVSyncSource的setVSyncEnabled函數使能VSync訊號的監聽

void EventThread::onVSyncEvent(nsecs_t timestamp) {    Mutex::Autolock _l(mLock);    mVSyncEvent[0].header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;    mVSyncEvent[0].header.id = 0;      mVSyncEvent[0].header.timestamp = timestamp;    mVSyncEvent[0].vsync.count++;      mCondition.broadcast();}

• EventThread收到VSync訊號後，發送廣播訊息，喚醒相關線程
• 最終EventThread將VSnc訊號傳遞給MessageQueue

Android SurfaceFlinger服務（六） ----- VSync訊號的分發