Android--Vsync代码分析(二)

我们来看看其代码的实现.

void SurfaceFlinger::init()

{

…

// start the EventThread

sp<VSyncSource>vsyncSrc = new DispSyncSource(&mPrimaryDispSync,

vsyncPhaseOffsetNs, true,"app");

mEventThread = newEventThread(vsyncSrc);

sp<VSyncSource>sfVsyncSrc = new DispSyncSource(&mPrimaryDispSync,

sfVsyncPhaseOffsetNs, true, "sf");

mSFEventThread= new EventThread(sfVsyncSrc);

mEventQueue.setEventThread(mSFEventThread);

}

当surfaceFlinger 初始化时,其创建了两个DispSyncSource 和两个EventThread.一个用于APP, 而另一个用于SurfaceFlinger 本身.我们知道DispSyncSource和DispSync协同工作, 共同完成Vsyncevent的fire.而EventThread会triggerApp draw frame和surfaceFlinger合成”dirty”layer当SW-VSYNCevent 产生时.

我们在这里列出DispSyncSource和Vsync-sf 有关系的code.

class DispSyncSource : publicVSyncSource, private DispSync::Callback {

public:

DispSyncSource(DispSync* dispSync, nsecs_t phaseOffset, bool traceVsync,

…

virtual void setVSyncEnabled(bool enable) {

Mutex::Autolock lock(mVsyncMutex);

if (enable) {

status_t err =mDispSync->addEventListener(mPhaseOffset,

static_cast<DispSync::Callback*>(this));

….

} else {

status_t err = mDispSync->removeEventListener(

static_cast<DispSync::Callback*>(this));

….

}

virtual void onDispSyncEvent(nsecs_t when) {

sp<VSyncSource::Callback> callback;

{

Mutex::Autolocklock(mCallbackMutex);

callback = mCallback;

if (mTraceVsync) {

mValue = (mValue + 1) % 2;

ATRACE_INT(mVsyncEventLabel.string(), mValue);

}

}

if (callback != NULL) {

callback->onVSyncEvent(when);

}

}

….

}

其中最重要的两个function是setVSyncEnabled(bool enable)和onDispSyncEvent(nsecs_twhen). setVSyncEnabled()的功能是把自己注册到DispSync(enable=true)中或者从DispSync中remove掉自己(enale=false),当DispSync产生了SW-VSYNCevent时,会check对SW-VSYNC感兴趣的DispSyncSource并回调其onDispSyncEvent().onDispSyncEvent()主要的功能一是enableVsync-sf 在sytrace中,一是callback EventThread中的onVSyncEvent(),从而触发SurfaceFlinger 合成所有更新的layer.

下面我们来看看EventThread 的重要code

…

EventThread::Connection::Connection(

constsp<EventThread>& eventThread)

: count(-1),mEventThread(eventThread), mChannel(new BitTube())

{

}

sp<EventThread::Connection> EventThread::createEventConnection()const {

return new Connection(const_cast<EventThread*>(this));

}

status_tEventThread::registerDisplayEventConnection(

constsp<EventThread::Connection>& connection) {

Mutex::Autolock _l(mLock);

mDisplayEventConnections.add(connection);

…

}

void EventThread::requestNextVsync(

…

if (connection->count < 0) {

connection->count = 0;

mCondition.broadcast();

}

}

void EventThread::onVSyncEvent(nsecs_t timestamp){

Mutex::Autolock _l(mLock);

mCondition.broadcast();

}

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=0 ; i<count ; i++) {

const sp<Connection>&conn(signalConnections[i]);

// now see if we still need to reportthis event

status_t err = conn->postEvent(event);

…

}

// This will return when (1) avsync event has been received, and (2) there was

// at least one connectioninterested in receiving it when we started waiting.

Vector<sp<EventThread::Connection> > EventThread::waitForEvent(

DisplayEventReceiver::Event* event)

{

Mutex::Autolock _l(mLock);

Vector< sp<EventThread::Connection>> signalConnections;

do {

….

// find out connections waiting forevents

size_t count = mDisplayEventConnections.size();

for (size_t i=0 ; i<count ; i++) {

….

if (connection->count >= 0) {

// we need vsync eventsbecause at least

// one connection iswaiting for it

waitForVSync = true;

if (timestamp) {

if(connection->count == 0) {

// fired this timearound

connection->count = -1;

signalConnections.add(connection);

}

// Here we figure out if we need toenable or disable vsyncs

if (timestamp && !waitForVSync) {

// we received a VSYNC but we haveno clients

// don't report it, and disableVSYNC events

disableVSyncLocked();

} else if (!timestamp &&waitForVSync) {

// we have at least one client, sowe want vsync enabled

// (TODO: this function is calledright after we finish

// notifying clients of a vsync, sothis call will be made

// at the vsync rate, e.g.60fps. If we can accurately

// track the currentstate we could avoid making this call

// so often.)

enableVSyncLocked();

}

if (!timestamp &&!eventPending) {

// wait for something to happen

if (waitForVSync) {

…

if (mCondition.waitRelative(mLock, timeout) ==TIMED_OUT) {

…..

}

} else {

// Nobody is interested invsync, so we just want to sleep.

mCondition.wait(mLock);

}

}

} while (signalConnections.isEmpty());

…

return signalConnections;

}

我们列出了EventThread 的重要function.下面一一说明其功能.

• EventThread::Connection::Connection() Connection的构造函数.用于进程间的通信by BitTube..在此处主要是搭建一个通路(BitTube)来完成client(App 或SurfaceFlinger)对Vsyncevent 事件的请求(通过requestNextVsync())和EventThread把SW-Vsyncevent callback 到其感兴趣的client.需要注意的是App是通过SurfaceFlinger::createDisplayEventConnection()创建此连接的.而sufaceflinge是在其初始化时callEventQueue.setEventThread(mSFEventThread) 创建的. 所以对App 的EventThread 来说可能有多个connection ,也有可能没有.而对sufaceflinger 目前来说有且只有一个.
• sp<EventThread::Connection>EventThread::createEventConnection() 创建Connection 连接.
• status_tEventThread::registerDisplayEventConnection() 如其名所描述.其功能是把创建的Connection注册到一个容器中.当SW-VSYNCevent 发生时,EventThread会从Connection注册的容器中,找到那些对SW-VSYNCevent感兴趣的connection并把vsyncevent 通过BitTube传到client.
• void EventThread::requestNextVsync() Clinet 端通过Connectioncall 这函数通知EventThread,其对SW-SYNCevent的请求.
• voidEventThread::onVSyncEvent(nsecs_t timestamp) 当SW-VSYNCEVENT 发生时,DispSyncSource 会call此函数,告知EventThread,Vsyncevent已经发生,如果此时有connect对Vsync 感兴趣,EventThread便会通过connect->postEvent(event)把Vsync 事件发送到client端(App或surfaceflinger).
• bool EventThread::threadLoop() 线程的主体函数.其完成两件事.一是把对SW-VSYNCevent 有请求并且还没有处理的connect找出来.而是把Vsyncevent通过connect通知到client.
• Vector< sp<EventThread::Connection>> EventThread::waitForEvent() EventThread 的主要功能都在此函数里.此函数由threadLoop()调用.EventThread 在大部分时间里是sleep的.如果系统的性能比较好,那么其sleep的节奏是和SW-VSYNCevent的节奏一致.即16.6mssleep一次.然而由于其App或surfaceflinger没有Vsync的请求,其sleep的时间为更长.此函数的名为waitForEvent,其到底在等什么event?原来此函数在等待的event就是Dispsync产生的SW-SYNCevent. 其功能check所有的connect是否有Vsync事件请求根据不同的情况做如下处理.

1. 所有的connect都没有Vsync请求,则其通过disableVSyncLocked(),disableVsync event,那么此EventThread将不会收到SW-SYNCevent,一直sleep直到有connect有Vsync请求为止.
2. 在所有的connect中,有SW-SYNC event请求,但是当其请求SW-SYNCevent时,SW-SYNCevent还没有fire,则其通过enableVSyncLocked()enable Vsync并进入sleep.当下一个SW-SYNCevent来到时,便把所有有SW-SYNCevent请求的connection返回给threadLoop.

下图是big pic.