分析Android帧缓冲区状态监控过程
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SurfaceFlinger服务在启动的时候,会创建一个线程来监控由内核发出的帧缓冲区硬件事件。每当帧缓冲区要进入睡眠状态时,内核就会发出一个睡眠事件,这时候SurfaceFlinger服务就会执行一个释放屏幕的操作;而当帧缓冲区从睡眠状态唤醒时,内核就会发出一个唤醒事件,这时候SurfaceFlinger服务就会执行一个获取屏幕的操作。
status_t SurfaceFlinger::readyToRun(){ ALOGI( "SurfaceFlinger's main thread ready to run. " "Initializing graphics H/W..."); // we only support one display currently int dpy = 0; { // initialize the main display GraphicPlane& plane(graphicPlane(dpy)); DisplayHardware* const hw = new DisplayHardware(this, dpy); plane.setDisplayHardware(hw); }...//启动显示屏睡眠/唤醒状态监控hw.startSleepManagement();...}在SurfaceFlinger启动过程中,将创建一个DisplayHardware对象,在构造DisplayHardware的父类DisplayHardwareBase对象时,会创建一个线程,用来监控硬件帧缓冲区的睡眠和唤醒事件。我们知道,在构造一个子类对象时,首先会调用父类的构造函数,因此在构造DisplayHardware对象时,首先会执行DisplayHardwareBase的构造函数:
DisplayHardwareBase::DisplayHardwareBase(const sp<SurfaceFlinger>& flinger, uint32_t displayIndex) { mScreenAcquired = true;//创建一个帧缓存区状态监控线程 mDisplayEventThread = new DisplayEventThread(flinger);}DisplayEventThread线程用于监控硬件帧缓存区的状态,其构造函数如下:
DisplayHardwareBase::DisplayEventThread::DisplayEventThread( const sp<SurfaceFlinger>& flinger) : Thread(false), mFlinger(flinger) {}当SurfaceFlinger服务初始化完成后,在SurfaceFlinger的readyToRun()函数最后,将调用DisplayHardware对象Hw的startSleepManagement()函数来启动DisplayEventThread线程
void DisplayHardwareBase::startSleepManagement() const { if (mDisplayEventThread->initCheck() == NO_ERROR) { mDisplayEventThread->run("DisplayEventThread", PRIORITY_URGENT_DISPLAY); } else { ALOGW("/sys/power/wait_for_fb_{wake|sleep} don't exist"); }}当硬件帧缓冲区被打开时,帧缓冲区驱动程序就会创建/sys/power/wait_for_fb_sleep和/sys/power/wait_for_fb_wake文件,用来通知用户空间显示屏即将要进入睡眠/唤醒状态。函数首先调用initCheck()函数来检查/sys/power/wait_for_fb_sleep和/sys/power/wait_for_fb_wake文件是否存在,如果文件存在,则启动DisplayEventThread线程,DisplayEventThread线程的执行过程如下:
bool DisplayHardwareBase::DisplayEventThread::threadLoop() {//等待硬件帧缓冲区fb进入睡眠状态,如果fb进入睡眠,函数返回,否则函数阻塞 if (waitForFbSleep() == NO_ERROR) { sp<SurfaceFlinger> flinger = mFlinger.promote(); ALOGD("About to give-up screen, flinger = %p", flinger.get());//如果硬件帧缓冲区fb进入睡眠状态,则通知SurfaceFlinger释放显示屏 if (flinger != 0) { flinger->screenReleased(); }//等待硬件帧缓冲区fb进入唤醒状态,如果fb被唤醒,函数返回,否则函数阻塞 if (waitForFbWake() == NO_ERROR) { ALOGD("Screen about to return, flinger = %p", flinger.get());//如果硬件帧缓冲区fb被唤醒,则通知SurfaceFlinger获取显示屏 if (flinger != 0) { flinger->screenAcquired(); }//线程循环执行threadLoop()函数 return true; } } // error, exit the thread return false;}在DisplayEventThread线程执行过程中,通过waitForFbSleep()和waitForFbWake()函数分别等待硬件帧缓冲区的睡眠/唤醒,当fb进入睡眠时,调用SurfaceFlinger的screenReleased()函数释放显示屏;当fb被唤醒时,调用SurfaceFlinger的screenAcquired()函数来获取显示屏。首先介绍fb睡眠监控过程:
status_t DisplayHardwareBase::DisplayEventThread::waitForFbSleep() { int err = 0; char buf;//kSleepFileName = "/sys/power/wait_for_fb_sleep"; int fd = open(kSleepFileName, O_RDONLY, 0); // if the file doesn't exist, the error will be caught in read() below do {//读取"/sys/power/wait_for_fb_sleep"文件,如果文件不存在或者正确读取文件内容,该函数退出循环 err = read(fd, &buf, 1); } while (err < 0 && errno == EINTR); close(fd); ALOGE_IF(err<0, "*** ANDROID_WAIT_FOR_FB_SLEEP failed (%s)", strerror(errno)); return err < 0 ? -errno : int(NO_ERROR);}我们知道,当硬件帧缓冲区fb进入睡眠状态时,fb驱动程序会通过写/sys/power/wait_for_fb_sleep文件来告知用户空间的应用程序当前硬件帧缓冲fb的状态,因此,DisplayEventThread线程只需读取/sys/power/wait_for_fb_sleep文件内容就可以判断fb是否进入睡眠状态。当fb进入睡眠状态时,DisplayEventThread线程就可以读取到/sys/power/wait_for_fb_sleep文件的内容,waitForFbSleep函数返回,否则循环读取/sys/power/wait_for_fb_sleep文件。同样,当硬件帧缓冲区fb被唤醒时,fb驱动程序会通过写/sys/power/wait_for_fb_wake文件来告知用户空间的应用程序当前硬件帧缓冲fb的状态:
status_t DisplayHardwareBase::DisplayEventThread::waitForFbWake() { int err = 0; char buf;//kWakeFileName = "/sys/power/wait_for_fb_wake"; int fd = open(kWakeFileName, O_RDONLY, 0); // if the file doesn't exist, the error will be caught in read() below do {//读取"/sys/power/wait_for_fb_wake"文件,如果文件不存在或者正确读取文件内容,该函数退出循环 err = read(fd, &buf, 1); } while (err < 0 && errno == EINTR); close(fd); ALOGE_IF(err<0, "*** ANDROID_WAIT_FOR_FB_WAKE failed (%s)", strerror(errno)); return err < 0 ? -errno : int(NO_ERROR);}当fb被唤醒时,DisplayEventThread线程就可以读取到/sys/power/wait_for_fb_wake文件的内容,waitForFbWake函数返回,否则循环读取/sys/power/wait_for_fb_wake文件。到此我们就知道DisplayEventThread线程是如何获取硬件帧缓冲区fb的状态,一旦DisplayEventThread线程监控到硬件帧缓冲区fb发生唤醒/睡眠状态切换,那么就会它通知SurfaceFlinger来处理。
1. 显示屏释放过程
当fb进入睡眠时,DisplayEventThread线程将调用SurfaceFlinger的screenReleased()函数来释放显示屏:
void SurfaceFlinger::screenReleased() { class MessageScreenReleased : public MessageBase { SurfaceFlinger* flinger; public: MessageScreenReleased(SurfaceFlinger* flinger) : flinger(flinger) { } virtual bool handler() { flinger->onScreenReleased(); return true; } }; sp<MessageBase> msg = new MessageScreenReleased(this); postMessageSync(msg);}在Android SurfaceFlinger服务的消息循环过程源码分析中介绍了SurfaceFlinger维护的消息循环过程。这里定义了一种释放显示屏的消息类型MessageScreenReleased,并通过postMessageSync()函数向SurfaceFlinger的消息队列同步发送一个MessageScreenReleased消息,由于发送的是一个同步消息,因此该函数将等待SurfaceFlinger的消息循环处理完该消息后才返回。当MessageScreenReleased消息被发送到SurfaceFlinger的消息队列后,SurfaceFlinger的消息循环将调用该消息的处理函数handler(),在MessageScreenReleased消息处理函数handler()中又继续调用SurfaceFlinger的onScreenReleased()函数来释放显示屏:
void SurfaceFlinger::onScreenReleased() { const DisplayHardware& hw(graphicPlane(0).displayHardware()); if (hw.isScreenAcquired()) {//将显示屏释放转交给EventThread线程处理 mEventThread->onScreenReleased();//设置DisplayHardwareBase类的成员变量mScreenAcquired为false hw.releaseScreen(); }}函数首先调用EventThread线程的onScreenReleased()函数来唤醒EventThread线程,并且关闭VSync事件。
void EventThread::onScreenReleased() { Mutex::Autolock _l(mLock); if (!mUseSoftwareVSync) { // disable reliance on h/w vsync mUseSoftwareVSync = true; mCondition.broadcast(); }}而当显示屏处于睡眠状态时,DisplayHardwareBase类的成员变量mScreenAcquired的值就会等于false,表示SurfaceFlinger服务不可以访问显示屏。
void DisplayHardwareBase::releaseScreen() const { mScreenAcquired = false;}2. 显示屏获取过程
当fb被唤醒时,DisplayEventThread线程将调用SurfaceFlinger的screenAcquired()函数来获取显示屏:
void SurfaceFlinger::screenAcquired() { class MessageScreenAcquired : public MessageBase { SurfaceFlinger* flinger; public: MessageScreenAcquired(SurfaceFlinger* flinger) : flinger(flinger) { } virtual bool handler() { flinger->onScreenAcquired(); return true; } }; sp<MessageBase> msg = new MessageScreenAcquired(this); postMessageSync(msg);}这里定义了一种获取显示屏的消息类型MessageScreenAcquired,同样往SurfaceFlinger的消息队列中同步发送一个MessageScreenAcquired消息,SurfaceFlinger的消息循环将调用该消息的handler()函数来处理该消息,在该消息的处理函数handler()中,调用SurfaceFlinger的onScreenAcquired()函数来获取显示屏:
void SurfaceFlinger::onScreenAcquired() { const DisplayHardware& hw(graphicPlane(0).displayHardware()); hw.acquireScreen(); mEventThread->onScreenAcquired(); // this is a temporary work-around, eventually this should be called // by the power-manager SurfaceFlinger::turnElectronBeamOn(mElectronBeamAnimationMode); // from this point on, SF will process updates again repaintEverything();}首先调用DisplayHardware对象hw的acquireScreen()函数设置DisplayHardwareBase类的成员变量mScreenAcquired的值就会等于true,表示SurfaceFlinger服务可以访问显示屏。
void DisplayHardwareBase::acquireScreen() const { mScreenAcquired = true;}接着调用EventThread的onScreenAcquired()函数来唤醒EventThread线程,并打开VSync事件发送器。
void EventThread::onScreenAcquired() { Mutex::Autolock _l(mLock); if (mUseSoftwareVSync) { // resume use of h/w vsync mUseSoftwareVSync = false; mCondition.broadcast(); }}然后调用SurfaceFlinger的turnElectronBeamOn()函数点亮显示屏
status_t SurfaceFlinger::turnElectronBeamOn(int32_t mode){ class MessageTurnElectronBeamOn : public MessageBase { SurfaceFlinger* flinger; int32_t mode; status_t result; public: MessageTurnElectronBeamOn(SurfaceFlinger* flinger, int32_t mode) : flinger(flinger), mode(mode), result(PERMISSION_DENIED) { } status_t getResult() const { return result; } virtual bool handler() { Mutex::Autolock _l(flinger->mStateLock); result = flinger->turnElectronBeamOnImplLocked(mode); return true; } }; postMessageAsync( new MessageTurnElectronBeamOn(this, mode) ); return NO_ERROR;}和前面SurfaceFlinger处理显示屏的释放、获取类似,也是为显示屏点亮处理定义类型为MessageTurnElectronBeamOn的消息,并向SurfaceFlinger消息队列中发送一个该类型的消息,有一点不同的是,这里的消息采用异步方式发送。SurfaceFlinger消息循环将调用该消息的处理函数handler来点亮显示屏,在消息处理函数中又调用SurfaceFlinger的turnElectronBeamOnImplLocked函数来完成显示屏的点亮处理。
status_t SurfaceFlinger::turnElectronBeamOnImplLocked(int32_t mode){ DisplayHardware& hw(graphicPlane(0).editDisplayHardware());//通过DisplayHardwareBase类的成员变量mScreenAcquired来判断当前显示屏是否可用 if (hw.canDraw()) { // we're already on return NO_ERROR; }//是否需要显示动画 if (mode & ISurfaceComposer::eElectronBeamAnimationOn) { electronBeamOnAnimationImplLocked(); } //设置脏区域为整个屏幕大小 mDirtyRegion.set(hw.bounds());//请求VSync事件 signalTransaction(); return NO_ERROR;}该函数通过调用signalTransaction()函数来请求下一次VSync事件
void SurfaceFlinger::signalTransaction() { mEventQueue.invalidate();}变量mEventQueue的类型为MessageQueue,因此这里调用MessageQueue的invalidate()函数
void MessageQueue::invalidate() { mEvents->requestNextVsync();}在Android SurfaceFlinger对VSync信号的处理过程分析中介绍了SurfaceFlinger通过Connection来接收EventThread线程分发的VSync事件,这里调用Connection的requestNextVsync()函数请求EventThread线程分发下一次的VSync事件。
void EventThread::Connection::requestNextVsync() { mEventThread->requestNextVsync(this);}通过唤醒EventThread线程来请求下一次VSync事件
void EventThread::requestNextVsync(const sp<EventThread::Connection>& connection) { Mutex::Autolock _l(mLock); if (connection->count < 0) { connection->count = 0; mCondition.broadcast(); }}回到onScreenAcquired()函数,最后调用repaintEverything()函数请求刷新显示屏。
void SurfaceFlinger::repaintEverything() { const DisplayHardware& hw(graphicPlane(0).displayHardware()); const Rect bounds(hw.getBounds()); setInvalidateRegion(Region(bounds)); signalTransaction();}DisplayHardwareBase类用来控制SurfaceFlinger服务是否能够在显示屏上渲染UI。当DisplayHardwareBase类的成员函数canDraw的返回值等于true时,就表示SurfaceFlinger服务可以在显示屏上渲染系统的UI,否则就不可以。DisplayEventThreadBase类的创建一个名称为“DisplayEventThread”的线程,用来监控fb的睡眠/唤醒状态切换事件。这个线程循环调用DisplayEventThread类的成员函数threadLoop来监控fb的睡眠/唤醒状态切换事件,并根据fb的状态通知SurfaceFlinger释放或获取显示屏。
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