android Gui系统之SurfaceFlinger(4)---Vsync(1)

8.Vsync

8.1概论

VSYNC（Vertical Synchronization）是一个相当古老的概念，对于游戏玩家，它有一个更加大名鼎鼎的中文名字—-垂直同步。

“垂直同步(vsync)”指的是显卡的输出帧数和屏幕的垂直刷新率相同，这完全是一个CRT显示器上的概念。其实无论是VSYNC还是垂直同步这个名字，

因为LCD根本就没有垂直扫描的这种东西，因此这个名字本身已经没有意义。但是基于历史的原因，这个名称在图形图像领域被沿袭下来。
在当下，垂直同步的含义我们可以理解为，使得显卡生成帧的速度和屏幕刷新的速度的保持一致。举例来说，如果屏幕的刷新率为60Hz，那么生成帧

的速度就应该被固定在1/60 s。

8.2 VSync信号的产生和分发

VSync信号的产生在android_frameworks_native\services\surfaceflinger\DisplayHardware\HWComposer.cpp里面定义：

HWComposer::HWComposer(
        const sp<SurfaceFlinger>& flinger,
        EventHandler& handler)
    : mFlinger(flinger),
      mFbDev(), mHwc(), mNumDisplays(),
      mCBContext(new cb_context),
      mEventHandler(handler),
      mDebugForceFakeVSync(false),
      mVDSEnabled(false)
{
    for (size_t i = ; i<MAX_HWC_DISPLAYS ; i++) {
        mLists[i] = ;
    }

    for (size_t i= ; i<HWC_NUM_PHYSICAL_DISPLAY_TYPES ; i++) {
        mLastHwVSync[i] = ;
        mVSyncCounts[i] = ;
    }

    char value[PROPERTY_VALUE_MAX];
    property_get("debug.sf.no_hw_vsync", value, "");
    mDebugForceFakeVSync = atoi(value);

    bool needVSyncThread = true;

    // Note: some devices may insist that the FB HAL be opened before HWC.
    int fberr = loadFbHalModule();
    loadHwcModule();

    if (mFbDev && mHwc && hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) {
        // close FB HAL if we don't needed it.
        // FIXME: this is temporary until we're not forced to open FB HAL
        // before HWC.
        framebuffer_close(mFbDev);
        mFbDev = NULL;
    }

    // If we have no HWC, or a pre-1.1 HWC, an FB dev is mandatory.
    if ((!mHwc || !hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))
            && !mFbDev) {
        ALOGE("ERROR: failed to open framebuffer (%s), aborting",
                strerror(-fberr));
        abort();
    }

    // these display IDs are always reserved
    for (size_t i= ; i<NUM_BUILTIN_DISPLAYS ; i++) {
        mAllocatedDisplayIDs.markBit(i);
    }

    if (mHwc) {
        ALOGI("Using %s version %u.%u", HWC_HARDWARE_COMPOSER,
              (hwcApiVersion(mHwc) >> ) & 0xff,
              (hwcApiVersion(mHwc) >> ) & 0xff);
        if (mHwc->registerProcs) {
            mCBContext->hwc = this;
            mCBContext->procs.invalidate = &hook_invalidate;
            mCBContext->procs.vsync = &hook_vsync;
            if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))
                mCBContext->procs.hotplug = &hook_hotplug;
            else
                mCBContext->procs.hotplug = NULL;
            memset(mCBContext->procs.zero, , sizeof(mCBContext->procs.zero));
            mHwc->registerProcs(mHwc, &mCBContext->procs);
        }

        // don't need a vsync thread if we have a hardware composer
        needVSyncThread = false;
        // always turn vsync off when we start
        eventControl(HWC_DISPLAY_PRIMARY, HWC_EVENT_VSYNC, );

        // the number of displays we actually have depends on the
        // hw composer version
        if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_3)) {
            // 1.3 adds support for virtual displays
            mNumDisplays = MAX_HWC_DISPLAYS;
        } else if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) {
            // 1.1 adds support for multiple displays
            mNumDisplays = NUM_BUILTIN_DISPLAYS;
        } else {
            mNumDisplays = ;
        }
    }

    if (mFbDev) {
        ALOG_ASSERT(!(mHwc && hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)),
                "should only have fbdev if no hwc or hwc is 1.0");

        DisplayData& disp(mDisplayData[HWC_DISPLAY_PRIMARY]);
        disp.connected = true;
        disp.format = mFbDev->format;
        DisplayConfig config = DisplayConfig();
        config.width = mFbDev->width;
        config.height = mFbDev->height;
        config.xdpi = mFbDev->xdpi;
        config.ydpi = mFbDev->ydpi;
#ifdef QCOM_BSP
        config.secure = true; //XXX: Assuming primary is always true
#endif
        config.refresh = nsecs_t(1e9 / mFbDev->fps);
        disp.configs.push_back(config);
        disp.currentConfig = ;
    } else if (mHwc) {
        // here we're guaranteed to have at least HWC 1.1
        for (size_t i = ; i<NUM_BUILTIN_DISPLAYS ; i++) {
            queryDisplayProperties(i);
        }
    }

    // read system property for VDS solution
    // This property is expected to be setup once during bootup
    if( (property_get("persist.hwc.enable_vds", value, NULL) > ) &&
        ((!strncmp(value, "", strlen(""))) ||
        !strncasecmp(value, "true", strlen("true")))) {
        //HAL virtual display is using VDS based implementation
        mVDSEnabled = true;
    }

    if (needVSyncThread) {
        // we don't have VSYNC support, we need to fake it
        mVSyncThread = new VSyncThread(*this);
    }
#ifdef QCOM_BSP
    // Threshold Area to enable GPU Tiled Rect.
    property_get("debug.hwc.gpuTiledThreshold", value, "1.9");
    mDynThreshold = atof(value);
#endif
}

HWComposer

bool needVSyncThread = true;

是否需要模拟产生VSync信号，默认是开启的。

  // Note: some devices may insist that the FB HAL be opened before HWC.
    int fberr = loadFbHalModule();
    loadHwcModule();

打开fb & hwc设备的HAL模块。

        if (mHwc->registerProcs) {
            mCBContext->hwc = this;
            mCBContext->procs.invalidate = &hook_invalidate;
            mCBContext->procs.vsync = &hook_vsync;
            if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))
                mCBContext->procs.hotplug = &hook_hotplug;
            else
                mCBContext->procs.hotplug = NULL;
            memset(mCBContext->procs.zero, , sizeof(mCBContext->procs.zero));
            mHwc->registerProcs(mHwc, &mCBContext->procs);
        }

        // don't need a vsync thread if we have a hardware composer
        needVSyncThread = false;

硬件VSync信号启动，不需要软件模拟。

    if (needVSyncThread) {
        // we don't have VSYNC support, we need to fake it
        mVSyncThread = new VSyncThread(*this);
    }

如果需要，启动VSyncThread线程来开启软件模拟。

8.2.1硬件产生

mHwc->registerProcs(mHwc, &mCBContext->procs);

hwc会产生回调：procs.vsync & procs.invalidate 信号。

void HWComposer::vsync(int disp, int64_t timestamp) {
    if (uint32_t(disp) < HWC_NUM_PHYSICAL_DISPLAY_TYPES) {
        {
            Mutex::Autolock _l(mLock);

            // There have been reports of HWCs that signal several vsync events
            // with the same timestamp when turning the display off and on. This
            // is a bug in the HWC implementation, but filter the extra events
            // out here so they don't cause havoc downstream.
            if (timestamp == mLastHwVSync[disp]) {
                ALOGW("Ignoring duplicate VSYNC event from HWC (t=%" PRId64 ")",
                        timestamp);
                return;
            }

            mLastHwVSync[disp] = timestamp;
        }

        char tag[];
        snprintf(tag, sizeof(tag), "HW_VSYNC_%1u", disp);
        ATRACE_INT(tag, ++mVSyncCounts[disp] & );

        mEventHandler.onVSyncReceived(disp, timestamp);
    }
}

最终会通知mEventHandler的消息，这个handler从那里来的呢？

void SurfaceFlinger::init(){

 mHwc = new HWComposer(this,
            *static_cast<HWComposer::EventHandler *>(this));
}

Yes，handler就是SurfaceFlinger，对嘛。SurfaceFlinger就是Surface合成的总管，所以这个信号一定会被它接收。

class SurfaceFlinger : public BnSurfaceComposer,
                       private IBinder::DeathRecipient,
                       private HWComposer::EventHandler

 

8.2.2软件模拟信号

bool HWComposer::VSyncThread::threadLoop() {
    { // scope for lock
        Mutex::Autolock _l(mLock);
        while (!mEnabled) {
            mCondition.wait(mLock);
        }
    }

    const nsecs_t period = mRefreshPeriod;
    const nsecs_t now = systemTime(CLOCK_MONOTONIC);
    nsecs_t next_vsync = mNextFakeVSync;
    nsecs_t sleep = next_vsync - now;
    if (sleep < ) {
        // we missed, find where the next vsync should be
        sleep = (period - ((now - next_vsync) % period));
        next_vsync = now + sleep;
    }
    mNextFakeVSync = next_vsync + period;

    struct timespec spec;
    spec.tv_sec  = next_vsync / ;
    spec.tv_nsec = next_vsync % ;

    int err;
    do {
        err = clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &spec, NULL);
    } while (err< && errno == EINTR);

    if (err == ) {
        mHwc.mEventHandler.onVSyncReceived(, next_vsync);
    }

    return true;
}

判断系统Vsync信号开关。然后计算下一次刷新的时间点。

 const nsecs_t period = mRefreshPeriod;

刷新间隔，CLOCK_MONOTONIC是从系统开机后的时间间隔（tick累加）

得到需要等待的时间sleep，和下一次vsync信号的时间点。

然后一个do while的操作，来等待信号时间点的到来。

最后，发出这个信号。

这里还有个情况，就是一开始的地方，mEnable变量，系统可以设置enable来控制vsync信号的产生。

void HWComposer::VSyncThread::setEnabled(bool enabled) 

8.3 SurfaceFlinger处理Vsync信号

在4.4以前，Vsync的处理通过EventThread就可以了。但是KK再次对这段逻辑进行细化和复杂化。Google真是费劲心思为了提升性能。

先来直接看下Surfaceflinger的onVSyncReceived函数：

void SurfaceFlinger::onVSyncReceived(int type, nsecs_t timestamp) {
    bool needsHwVsync = false;

    { // Scope for the lock
        Mutex::Autolock _l(mHWVsyncLock);
        if (type ==  && mPrimaryHWVsyncEnabled) {
            needsHwVsync = mPrimaryDispSync.addResyncSample(timestamp);//mPromaryDisplays是什么？
        }
    }

    if (needsHwVsync) {
        enableHardwareVsync();//做了什么
    } else {
        disableHardwareVsync(false);//做了什么

　　} 

}

虽然很短，但是乍一看还是一头雾水

mPrimaryHWVsyncEnabled是什么时候被赋值的？

mPrimaryDispSync是什么，addResyncSample又做了什么？

enableHardwareVsync &disableHardwareVsync在干什么？

要解答这些问题，就从SurfaceFlinger::init开始看

void SurfaceFlinger::init() {
    ALOGI(  "SurfaceFlinger's main thread ready to run. "
            "Initializing graphics H/W...");

    status_t err;
    Mutex::Autolock _l(mStateLock);

    /* Set the mask bit of the sigset to block the SIGPIPE signal */
    sigset_t sigMask;
    sigemptyset (&sigMask);
    sigaddset(&sigMask, SIGPIPE);
    sigprocmask(SIG_BLOCK, &sigMask, NULL);

    // initialize EGL for the default display
    mEGLDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);
    eglInitialize(mEGLDisplay, NULL, NULL);

    // Initialize the H/W composer object.  There may or may not be an
    // actual hardware composer underneath.
    mHwc = new HWComposer(this,
            *static_cast<HWComposer::EventHandler *>(this));

    // get a RenderEngine for the given display / config (can't fail)
    mRenderEngine = RenderEngine::create(mEGLDisplay, mHwc->getVisualID());

    // retrieve the EGL context that was selected/created
    mEGLContext = mRenderEngine->getEGLContext();

    LOG_ALWAYS_FATAL_IF(mEGLContext == EGL_NO_CONTEXT,
            "couldn't create EGLContext");

    // initialize our non-virtual displays
    for (size_t i= ; i<DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES ; i++) {
        DisplayDevice::DisplayType type((DisplayDevice::DisplayType)i);
        // set-up the displays that are already connected
        if (mHwc->isConnected(i) || type==DisplayDevice::DISPLAY_PRIMARY) {
#ifdef QCOM_BSP
            // query from hwc if the non-virtual display is secure.
            bool isSecure = mHwc->isSecure(i);;
#else
            // All non-virtual displays are currently considered secure
            bool isSecure = true;
#endif
            createBuiltinDisplayLocked(type, isSecure);
            wp<IBinder> token = mBuiltinDisplays[i];

            sp<IGraphicBufferProducer> producer;
            sp<IGraphicBufferConsumer> consumer;
            BufferQueue::createBufferQueue(&producer, &consumer,
                    new GraphicBufferAlloc());

            sp<FramebufferSurface> fbs = new FramebufferSurface(*mHwc, i,
                    consumer);
            int32_t hwcId = allocateHwcDisplayId(type);
            sp<DisplayDevice> hw = new DisplayDevice(this,
                    type, hwcId, mHwc->getFormat(hwcId), isSecure, token,
                    fbs, producer,
                    mRenderEngine->getEGLConfig());
            if (i > DisplayDevice::DISPLAY_PRIMARY) {
                // FIXME: currently we don't get blank/unblank requests
                // for displays other than the main display, so we always
                // assume a connected display is unblanked.
                ALOGD("marking display %zu as acquired/unblanked", i);
                hw->setPowerMode(HWC_POWER_MODE_NORMAL);
            }
            mDisplays.add(token, hw);
        }
    }

    // make the GLContext current so that we can create textures when creating Layers
    // (which may happens before we render something)
    getDefaultDisplayDevice()->makeCurrent(mEGLDisplay, mEGLContext);

    // 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);

    mEventControlThread = new EventControlThread(this);
    mEventControlThread->run("EventControl", PRIORITY_URGENT_DISPLAY);
    android_set_rt_ioprio(mEventControlThread->getTid(), );

    // set a fake vsync period if there is no HWComposer
    if (mHwc->initCheck() != NO_ERROR) {
        mPrimaryDispSync.setPeriod();
    }

    // initialize our drawing state
    mDrawingState = mCurrentState;

    // set initial conditions (e.g. unblank default device)
    initializeDisplays();

    // start boot animation
    startBootAnim();
}

SurfaceFlinger::init

有2个几乎一样的DispSyncSource，它的目的是提供了Vsync的虚拟化。关于这块的分析，可以参考Android 4.4(KitKat)中VSync信号的虚拟化

在三缓冲的框架下，对于一帧内容，先等APP UI画完了，SurfaceFlinger再出场整合到FrameBuffer

而现在google就是让它们一起跑起来。

然后搞了2个延时，这样就不会有问题。对应vsyncSrc（绘图延时） & sfVsyncSrc（合成延时）

8.3.1 EventThread

bool EventThread::threadLoop() {
    DisplayEventReceiver::Event event;
    Vector< sp<EventThread::Connection> > signalConnections;
    signalConnections = waitForEvent(&event);

    // dispatch events to listeners...
    const size_t count = signalConnections.size();
    for (size_t i= ; i<count ; i++) {
        const sp<Connection>& conn(signalConnections[i]);
        // now see if we still need to report this event
        status_t err = conn->postEvent(event);
        if (err == -EAGAIN || err == -EWOULDBLOCK) {
            // The destination doesn't accept events anymore, it's probably
            // full. For now, we just drop the events on the floor.
            // FIXME: Note that some events cannot be dropped and would have
            // to be re-sent later.
            // Right-now we don't have the ability to do this.
            ALOGW("EventThread: dropping event (%08x) for connection %p",
                    event.header.type, conn.get());
        } else if (err < ) {
            // handle any other error on the pipe as fatal. the only
            // reasonable thing to do is to clean-up this connection.
            // The most common error we'll get here is -EPIPE.
            removeDisplayEventConnection(signalConnections[i]);
        }
    }
    return true;
}

EventThread会发送消息到surfaceflinger里面的MessageQueue。

MessageQueue处理消息：

int MessageQueue::eventReceiver(int /*fd*/, int /*events*/) {
    ssize_t n;
    DisplayEventReceiver::Event buffer[];
    while ((n = DisplayEventReceiver::getEvents(mEventTube, buffer, )) > ) {
        for (int i= ; i<n ; i++) {
            if (buffer[i].header.type == DisplayEventReceiver::DISPLAY_EVENT_VSYNC) {
#if INVALIDATE_ON_VSYNC
                mHandler->dispatchInvalidate();
#else
                mHandler->dispatchRefresh();
#endif
                break;
            }
        }
    }
    return ;
}

如果Event的类型是

DisplayEventReceiver::DISPLAY_EVENT_VSYNC

正是我们需要的类型，所以，就有2中处理方式：

UI进程需要先准备好数据（invalidate），然后Vsync信号来了以后，就开始刷新屏幕。

SurfaceFlinger是在Vsync来临之际准备数据然后刷新，还是平常就准备当VSync来临是再刷新。

先来看dispatchInvalidate,最后处理的地方就是这里。

        case MessageQueue::INVALIDATE: {
            bool refreshNeeded = handleMessageTransaction();
            refreshNeeded |= handleMessageInvalidate();
            refreshNeeded |= mRepaintEverything;
            if (refreshNeeded) {
                // Signal a refresh if a transaction modified the window state,
                // a new buffer was latched, or if HWC has requested a full
                // repaint
                signalRefresh();
            }
            break;
        }

我们来看下handleMessageRefresh：

void SurfaceFlinger::handleMessageRefresh() {
    ATRACE_CALL();
    preComposition();　　//合成前的准备
    rebuildLayerStacks();//重新建立layer堆栈
    setUpHWComposer();//HWComposer的设定
#ifdef QCOM_BSP
    setUpTiledDr();
#endif
    doDebugFlashRegions();
    doComposition();　　//正式合成工作
    postComposition();　//合成的后期工作
}

8.3.2handleMessageTransaction

handleMessageTransaction在简单判断后直接调用handlerTransaction。

可以看到里面的handleTransactionLocked才是代码真正处理的地方。

void SurfaceFlinger::handleTransactionLocked(uint32_t transactionFlags)
{
    const LayerVector& currentLayers(mCurrentState.layersSortedByZ);
    const size_t count = currentLayers.size();

    /*
     * Traversal of the children
     * (perform the transaction for each of them if needed)
     */

    if (transactionFlags & eTraversalNeeded) {
        for (size_t i= ; i<count ; i++) {
            const sp<Layer>& layer(currentLayers[i]);
            uint32_t trFlags = layer->getTransactionFlags(eTransactionNeeded);
            if (!trFlags) continue;

            const uint32_t flags = layer->doTransaction();
            if (flags & Layer::eVisibleRegion)
                mVisibleRegionsDirty = true;
        }
    }

    /*
     * Perform display own transactions if needed
     */

    if (transactionFlags & eDisplayTransactionNeeded) {
        // here we take advantage of Vector's copy-on-write semantics to
        // improve performance by skipping the transaction entirely when
        // know that the lists are identical
        const KeyedVector<  wp<IBinder>, DisplayDeviceState>& curr(mCurrentState.displays);
        const KeyedVector<  wp<IBinder>, DisplayDeviceState>& draw(mDrawingState.displays);
        if (!curr.isIdenticalTo(draw)) {
            mVisibleRegionsDirty = true;
            const size_t cc = curr.size();
                  size_t dc = draw.size();

            // find the displays that were removed
            // (ie: in drawing state but not in current state)
            // also handle displays that changed
            // (ie: displays that are in both lists)
            for (size_t i= ; i<dc ; i++) {
                const ssize_t j = curr.indexOfKey(draw.keyAt(i));
                if (j < ) {
                    // in drawing state but not in current state
                    if (!draw[i].isMainDisplay()) {
                        // Call makeCurrent() on the primary display so we can
                        // be sure that nothing associated with this display
                        // is current.
                        const sp<const DisplayDevice> defaultDisplay(getDefaultDisplayDevice());
                        defaultDisplay->makeCurrent(mEGLDisplay, mEGLContext);
                        sp<DisplayDevice> hw(getDisplayDevice(draw.keyAt(i)));
                        if (hw != NULL)
                            hw->disconnect(getHwComposer());
                        if (draw[i].type < DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES)
                            mEventThread->onHotplugReceived(draw[i].type, false);
                        mDisplays.removeItem(draw.keyAt(i));
                    } else {
                        ALOGW("trying to remove the main display");
                    }
                } else {
                    // this display is in both lists. see if something changed.
                    const DisplayDeviceState& state(curr[j]);
                    const wp<IBinder>& display(curr.keyAt(j));
                    if (state.surface->asBinder() != draw[i].surface->asBinder()) {
                        // changing the surface is like destroying and
                        // recreating the DisplayDevice, so we just remove it
                        // from the drawing state, so that it get re-added
                        // below.
                        sp<DisplayDevice> hw(getDisplayDevice(display));
                        if (hw != NULL)
                            hw->disconnect(getHwComposer());
                        mDisplays.removeItem(display);
                        mDrawingState.displays.removeItemsAt(i);
                        dc--; i--;
                        // at this point we must loop to the next item
                        continue;
                    }

                    const sp<DisplayDevice> disp(getDisplayDevice(display));
                    if (disp != NULL) {
                        if (state.layerStack != draw[i].layerStack) {
                            disp->setLayerStack(state.layerStack);
                        }
                        if ((state.orientation != draw[i].orientation)
                                || (state.viewport != draw[i].viewport)
                                || (state.frame != draw[i].frame))
                        {
#ifdef QCOM_BSP
                            int orient = state.orientation;
                            // Honor the orientation change after boot
                            // animation completes and make sure boot
                            // animation is shown in panel orientation always.
                            if(mBootFinished){
                                disp->setProjection(state.orientation,
                                        state.viewport, state.frame);
                                orient = state.orientation;
                            }
                            else{
                                char property[PROPERTY_VALUE_MAX];
                                int panelOrientation =
                                        DisplayState::eOrientationDefault;
                                if(property_get("persist.panel.orientation",
                                            property, "") > ){
                                    panelOrientation = atoi(property) / ;
                                }
                                disp->setProjection(panelOrientation,
                                        state.viewport, state.frame);
                                orient = panelOrientation;
                            }
                            // Set the view frame of each display only of its
                            // default orientation.
                            if(orient == DisplayState::eOrientationDefault and
                                    state.frame.isValid()) {
                                qdutils::setViewFrame(disp->getHwcDisplayId(),
                                    state.frame.left, state.frame.top,
                                    state.frame.right, state.frame.bottom);
                            }
#else
                            disp->setProjection(state.orientation,
                                state.viewport, state.frame);
#endif
                        }
                        if (state.width != draw[i].width || state.height != draw[i].height) {
                            disp->setDisplaySize(state.width, state.height);
                        }
                    }
                }
            }

            // find displays that were added
            // (ie: in current state but not in drawing state)
            for (size_t i= ; i<cc ; i++) {
                if (draw.indexOfKey(curr.keyAt(i)) < ) {
                    const DisplayDeviceState& state(curr[i]);

                    sp<DisplaySurface> dispSurface;
                    sp<IGraphicBufferProducer> producer;
                    sp<IGraphicBufferProducer> bqProducer;
                    sp<IGraphicBufferConsumer> bqConsumer;
                    BufferQueue::createBufferQueue(&bqProducer, &bqConsumer,
                            new GraphicBufferAlloc());

                    int32_t hwcDisplayId = -;
                    if (state.isVirtualDisplay()) {
                        // Virtual displays without a surface are dormant:
                        // they have external state (layer stack, projection,
                        // etc.) but no internal state (i.e. a DisplayDevice).
                        if (state.surface != NULL) {
                            configureVirtualDisplay(hwcDisplayId,
                                    dispSurface, producer, state, bqProducer,
                                    bqConsumer);
                        }
                    } else {
                        ALOGE_IF(state.surface!=NULL,
                                "adding a supported display, but rendering "
                                "surface is provided (%p), ignoring it",
                                state.surface.get());
                        hwcDisplayId = allocateHwcDisplayId(state.type);
                        // for supported (by hwc) displays we provide our
                        // own rendering surface
                        dispSurface = new FramebufferSurface(*mHwc, state.type,
                                bqConsumer);
                        producer = bqProducer;
                    }

                    const wp<IBinder>& display(curr.keyAt(i));
                    if (dispSurface != NULL && producer != NULL) {
                        sp<DisplayDevice> hw = new DisplayDevice(this,
                                state.type, hwcDisplayId,
                                mHwc->getFormat(hwcDisplayId), state.isSecure,
                                display, dispSurface, producer,
                                mRenderEngine->getEGLConfig());
                        hw->setLayerStack(state.layerStack);
                        hw->setProjection(state.orientation,
                                state.viewport, state.frame);
                        hw->setDisplayName(state.displayName);
                        // When a new display device is added update the active
                        // config by querying HWC otherwise the default config
                        // (config 0) will be used.
                        int activeConfig = mHwc->getActiveConfig(hwcDisplayId);
                        if (activeConfig >= ) {
                            hw->setActiveConfig(activeConfig);
                        }
                        mDisplays.add(display, hw);
                        if (state.isVirtualDisplay()) {
                            if (hwcDisplayId >= ) {
                                mHwc->setVirtualDisplayProperties(hwcDisplayId,
                                        hw->getWidth(), hw->getHeight(),
                                        hw->getFormat());
                            }
                        } else {
                            mEventThread->onHotplugReceived(state.type, true);
                        }
                    }
                }
            }
        }
    }

    if (transactionFlags & (eTraversalNeeded|eDisplayTransactionNeeded)) {
        // The transform hint might have changed for some layers
        // (either because a display has changed, or because a layer
        // as changed).
        //
        // Walk through all the layers in currentLayers,
        // and update their transform hint.
        //
        // If a layer is visible only on a single display, then that
        // display is used to calculate the hint, otherwise we use the
        // default display.
        //
        // NOTE: we do this here, rather than in rebuildLayerStacks() so that
        // the hint is set before we acquire a buffer from the surface texture.
        //
        // NOTE: layer transactions have taken place already, so we use their
        // drawing state. However, SurfaceFlinger's own transaction has not
        // happened yet, so we must use the current state layer list
        // (soon to become the drawing state list).
        //
        sp<const DisplayDevice> disp;
        uint32_t currentlayerStack = ;
        for (size_t i=; i<count; i++) {
            // NOTE: we rely on the fact that layers are sorted by
            // layerStack first (so we don't have to traverse the list
            // of displays for every layer).
            const sp<Layer>& layer(currentLayers[i]);
            uint32_t layerStack = layer->getDrawingState().layerStack;
            if (i== || currentlayerStack != layerStack) {
                currentlayerStack = layerStack;
                // figure out if this layerstack is mirrored
                // (more than one display) if so, pick the default display,
                // if not, pick the only display it's on.
                disp.clear();
                for (size_t dpy= ; dpy<mDisplays.size() ; dpy++) {
                    sp<const DisplayDevice> hw(mDisplays[dpy]);
                    if (hw->getLayerStack() == currentlayerStack) {
                        if (disp == NULL) {
                            disp = hw;
                        } else {
                            disp = NULL;
                            break;
                        }
                    }
                }
            }
            if (disp == NULL) {
                // NOTE: TEMPORARY FIX ONLY. Real fix should cause layers to
                // redraw after transform hint changes. See bug 8508397.

                // could be null when this layer is using a layerStack
                // that is not visible on any display. Also can occur at
                // screen off/on times.
                disp = getDefaultDisplayDevice();
            }
            layer->updateTransformHint(disp);
        }
    }

    /*
     * Perform our own transaction if needed
     */

    const LayerVector& layers(mDrawingState.layersSortedByZ);
    if (currentLayers.size() > layers.size()) {
        // layers have been added
        mVisibleRegionsDirty = true;
    }

    // some layers might have been removed, so
    // we need to update the regions they're exposing.
    if (mLayersRemoved) {
        mLayersRemoved = false;
        mVisibleRegionsDirty = true;
        const size_t count = layers.size();
        for (size_t i= ; i<count ; i++) {
            const sp<Layer>& layer(layers[i]);
            if (currentLayers.indexOf(layer) < ) {
                // this layer is not visible anymore
                // TODO: we could traverse the tree from front to back and
                //       compute the actual visible region
                // TODO: we could cache the transformed region
                const Layer::State& s(layer->getDrawingState());
                Region visibleReg = s.transform.transform(
                        Region(Rect(s.active.w, s.active.h)));
                invalidateLayerStack(s.layerStack, visibleReg);
            }
        }
    }

    commitTransaction();

    updateCursorAsync();
}

handleTransactionLocked

这里处理3中情况，过程类似，我们只看eTraversalNeeded这种情况。

 uint32_t trFlags = layer->getTransactionFlags(eTransactionNeeded);

获取各个layer的标志位，然后做const uint32_t flags = layer->doTransaction(0);的操作

各layer计算各自的可见区域，mVisibleRegionsDirty记录可见区域变化。

以下代码：

mCurrentState.layersSortedByZ

surfaceFlinger有2个记录layer变化的全局变量

State mDrawingState;
State mCurrentState;

一个记录上一次的状态，后者记录当前的状态，这样就可以判断layer的变化状态。layersSortedByZ 可见，layer是通过Z-order排列的。

这个变量记录了所有的layer。

我们来看下

uint32_t Layer::doTransaction(uint32_t flags) {
    ATRACE_CALL();

    const Layer::State& s(getDrawingState());
    const Layer::State& c(getCurrentState());

    const bool sizeChanged = (c.requested.w != s.requested.w) ||
                             (c.requested.h != s.requested.h);

    if (sizeChanged) {
        // the size changed, we need to ask our client to request a new buffer
        ALOGD_IF(DEBUG_RESIZE,
                "doTransaction: geometry (layer=%p '%s'), tr=%02x, scalingMode=%d\n"
                "  current={ active   ={ wh={%4u,%4u} crop={%4d,%4d,%4d,%4d} (%4d,%4d) }\n"
                "            requested={ wh={%4u,%4u} crop={%4d,%4d,%4d,%4d} (%4d,%4d) }}\n"
                "  drawing={ active   ={ wh={%4u,%4u} crop={%4d,%4d,%4d,%4d} (%4d,%4d) }\n"
                "            requested={ wh={%4u,%4u} crop={%4d,%4d,%4d,%4d} (%4d,%4d) }}\n",
                this, getName().string(), mCurrentTransform, mCurrentScalingMode,
                c.active.w, c.active.h,
                c.active.crop.left,
                c.active.crop.top,
                c.active.crop.right,
                c.active.crop.bottom,
                c.active.crop.getWidth(),
                c.active.crop.getHeight(),
                c.requested.w, c.requested.h,
                c.requested.crop.left,
                c.requested.crop.top,
                c.requested.crop.right,
                c.requested.crop.bottom,
                c.requested.crop.getWidth(),
                c.requested.crop.getHeight(),
                s.active.w, s.active.h,
                s.active.crop.left,
                s.active.crop.top,
                s.active.crop.right,
                s.active.crop.bottom,
                s.active.crop.getWidth(),
                s.active.crop.getHeight(),
                s.requested.w, s.requested.h,
                s.requested.crop.left,
                s.requested.crop.top,
                s.requested.crop.right,
                s.requested.crop.bottom,
                s.requested.crop.getWidth(),
                s.requested.crop.getHeight());

        // record the new size, form this point on, when the client request
        // a buffer, it'll get the new size.
        mSurfaceFlingerConsumer->setDefaultBufferSize(
                c.requested.w, c.requested.h);
    }

    if (!isFixedSize()) {

        const bool resizePending = (c.requested.w != c.active.w) ||
                                   (c.requested.h != c.active.h);

        if (resizePending) {
            // don't let Layer::doTransaction update the drawing state
            // if we have a pending resize, unless we are in fixed-size mode.
            // the drawing state will be updated only once we receive a buffer
            // with the correct size.
            //
            // in particular, we want to make sure the clip (which is part
            // of the geometry state) is latched together with the size but is
            // latched immediately when no resizing is involved.

            flags |= eDontUpdateGeometryState;
        }
    }

    // always set active to requested, unless we're asked not to
    // this is used by Layer, which special cases resizes.
    if (flags & eDontUpdateGeometryState)  {
    } else {
        Layer::State& editCurrentState(getCurrentState());
        editCurrentState.active = c.requested;
    }

    if (s.active != c.active) {
        // invalidate and recompute the visible regions if needed
        flags |= Layer::eVisibleRegion;
    }

    if (c.sequence != s.sequence) {
        // invalidate and recompute the visible regions if needed
        flags |= eVisibleRegion;
        this->contentDirty = true;

        // we may use linear filtering, if the matrix scales us
        const uint8_t type = c.transform.getType();
        mNeedsFiltering = (!c.transform.preserveRects() ||
                (type >= Transform::SCALE));
    }

    // Commit the transaction
    commitTransaction();
    return flags;
}

首先判断size是否有修改,然后

mSurfaceFlingerConsumer->setDefaultBufferSize

重新获取大小。

if (c.sequence != s.sequence) {
        // invalidate and recompute the visible regions if needed
        flags |= eVisibleRegion;

Sequence是个什么东西？

当Layer的position，Zorder，alpha,matrix,transparent region,flags,crops.等发生变化的时候，sequence就会自增。

也就是，当这些属性发生变化是，页面在Vsync信号触发的时候，根据sequence来判断是否需要属性页面。

新增layer，

对比2个state中的layer队列，就可以知道新增的layer。

移除layer，

也是比较2个layer队列，就可以找到移除的layer。

提交transaction，主要就是同步2个state。然后currentstate继续跟踪layer变化，如此往复。

Vsync 是SurfaceFlinger模块最核心的概念，所以这块将会分多次讲解。