Android SurfaceFlinger服务（六） ----- VSync信号的分发

HWComposer模块产生VSync信号后要经过分发才能送达到关心VSync事件的模块中去。VSync信号分发大致流程为HWComposer->SurfaceFlinger->DispSync->DispSyncSource->各个具体模块。下面具体分析下这个流程。

在上一篇文章中分析到硬件或者软件VSync信号产生时，会回调SurfaceFlinger的onVSyncReceived函数，来看看这个函数：

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类型为DispSync,在构造的时候会创建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());
        }
    }
}

• 调用DispSync的addResyncSample方法，最终会去唤醒DispSyncThread线程

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;
}

• 调用updateModelLocked函数
• 计算是否需要开启硬件VSync，进行同步

void DispSync::updateModelLocked() {
    if (mNumResyncSamples >= MIN_RESYNC_SAMPLES_FOR_UPDATE) {

        ......

        mThread->updateModel(mPeriod, mPhase);
    }
}

• 成员变量mThread为DispSyncThread类的对象
• 调用DispSyncThread类pdateModel方法

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

• 该方法主要设置成员变量mPeriod、mPhase
• 然后在唤醒线程循环

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;
    }

• 通过gatherCallbackInvocationsLocked得到监听者集合
• 通过fireCallbackInvocations调用监听者回调函数

DispSync的回调函数在哪里注册,我们在看看DispSyncSource的setVSyncEnabled函数

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类继承于DispSync::Callback
• 调用DispSync->addEventListener注册回调，当DispSyncSource收到DispSync的VSync信号时，会回调整onDispSyncEvent函数

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);
        }
    }

• 这个函数最终还是调用VSyncSource::Callback的onVSyncEvent函数。
• VSyncSource::Callback函数的注册是通过调用DispSyncSource的setCallback注册。

DispSyncSource的setCallback函数在哪里调用的呢，要解答这个问题，还是得回到SurfaceFlinger的init函数里，以sfVsyncSrc为例。

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为参数创建EventThread对象
• 在EventThread的线程循环中会去调用enableVSyncLocked函数

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继承VSyncSource::Callback接口，实现了onVSyncEvent函数
• 调用mVSyncSource的setCallback函数设置回调
• 调用mVSyncSource的setVSyncEnabled函数使能VSync信号的监听

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信号后，发送广播消息，唤醒相关线程
• 最终EventThread将VSnc信号传递给MessageQueue

原文地址：https://www.cnblogs.com/qzhang1535/p/9285085.html