Android系统在4.4之后加入了黄油计划，surfaceflinger对显示的处理也变得复杂起来。由于添加了vsyn虚拟化机制，app将要显示的内容和surfaceflinger对显示内容的合成分成了两个部分，而两者开始的信号都是从vsync发出的。这里就涉及vsync信号的发生和传递，并且考虑到性能原因，信号的发生和传递都是按需进行的。因此，研究vsync信号的虚拟化及其处理，有助于理解线程的休眠唤醒和surfaceflinger的架构。
本文参考三篇文章：
https://blog.csdn.net/jinzhuojun/article/details/17293325
https://blog.csdn.net/houliang120/article/details/50908098
https://blog.csdn.net/kc58236582/article/details/52763534
一：SurfaceFlinger的初始化
init{
// start the EventThread
sp<VSyncSource> vsyncSrc = new DispSyncSource(&mPrimaryDispSync,
vsyncPhaseOffsetNs, true);
mEventThread = new EventThread(vsyncSrc);
sp<VSyncSource> sfVsyncSrc = new DispSyncSource(&mPrimaryDispSync,
sfVsyncPhaseOffsetNs, false);
mSFEventThread = new EventThread(sfVsyncSrc);//先开线程
mEventQueue.setEventThread(mSFEventThread);//然后在设置connection
mEventControlThread = new EventControlThread(this);
mEventControlThread->run("EventControl", PRIORITY_URGENT_DISPLAY);
}Vsync发出后，调用SF的方法：
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 < 0) {
// 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 / 1000000000;
spec.tv_nsec = next_vsync % 1000000000;
int err;
do {
err = clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &spec, NULL);
} while (err<0 && errno == EINTR);
if (err == 0) {
mHwc.mEventHandler.onVSyncReceived(0, next_vsync);
}
return true;
}
进入SF的方法：
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);
}
}进入
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 (runningWithoutSyncFramework) {
// 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 > errorThreshold;
}
进入
扫描二维码关注公众号，回复：
3387214 查看本文章
void DispSync::updateModelLocked() {
if (mNumResyncSamples >= MIN_RESYNC_SAMPLES_FOR_UPDATE) {
nsecs_t durationSum = 0;
for (size_t i = 1; i < mNumResyncSamples; i++) {
size_t idx = (mFirstResyncSample + i) % MAX_RESYNC_SAMPLES;
size_t prev = (idx + MAX_RESYNC_SAMPLES - 1) % MAX_RESYNC_SAMPLES;
durationSum += mResyncSamples[idx] - mResyncSamples[prev];
}
mPeriod = durationSum / (mNumResyncSamples - 1);
double sampleAvgX = 0;
double sampleAvgY = 0;
double scale = 2.0 * M_PI / double(mPeriod);
for (size_t i = 0; i < mNumResyncSamples; i++) {
size_t idx = (mFirstResyncSample + i) % MAX_RESYNC_SAMPLES;
nsecs_t sample = mResyncSamples[idx];
double samplePhase = double(sample % mPeriod) * scale;
sampleAvgX += cos(samplePhase);
sampleAvgY += sin(samplePhase);
}
sampleAvgX /= double(mNumResyncSamples);
sampleAvgY /= double(mNumResyncSamples);
mPhase = nsecs_t(atan2(sampleAvgY, sampleAvgX) / scale);
if (mPhase < 0) {
mPhase += mPeriod;
}
if (traceDetailedInfo) {
ATRACE_INT64("DispSync:Period", mPeriod);
ATRACE_INT64("DispSync:Phase", mPhase);
}
mThread->updateModel(mPeriod, mPhase);
}
}
进入DispSyncThread的updateModel方法：
void updateModel(nsecs_t period, nsecs_t phase) {
Mutex::Autolock lock(mMutex);
mPeriod = period;
mPhase = phase;
mCond.signal();//唤醒线程
}唤醒线程后，进入threadloop方法：
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);
if (mStop) {
return false;
}
//TODO:这个方法中如果设置了mperiod后，就不会在阻塞等待vsync信号了，这个阻塞是如何实现的？
if (mPeriod == 0) {//初始化后一直阻塞在这里，直到SF调用updateModel方法设置mPeriod，并唤醒线程
err = mCond.wait(mMutex);
if (err != NO_ERROR) {
ALOGE("error waiting for new events: %s (%d)",
strerror(-err), err);
return false;
}
continue;
}
//计算虚拟化vsync的时间
nextEventTime = computeNextEventTimeLocked(now);
targetTime = nextEventTime;
bool isWakeup = false;
if (now < targetTime) {//阻塞一段时间后，就可以调用需要的callback了
err = mCond.waitRelative(mMutex, targetTime - now);
if (err == TIMED_OUT) {
isWakeup = true;
} else if (err != NO_ERROR) {
ALOGE("error waiting for next event: %s (%d)",
strerror(-err), err);
return false;
}
}
now = systemTime(SYSTEM_TIME_MONOTONIC);
if (isWakeup) {
mWakeupLatency = ((mWakeupLatency * 63) +
(now - targetTime)) / 64;
if (mWakeupLatency > 500000) {
// Don't correct by more than 500 us
mWakeupLatency = 500000;
}
if (traceDetailedInfo) {
ATRACE_INT64("DispSync:WakeupLat", now - nextEventTime);
ATRACE_INT64("DispSync:AvgWakeupLat", mWakeupLatency);
}
}
//查找需要唤醒对象
callbackInvocations = gatherCallbackInvocationsLocked(now);
}
if (callbackInvocations.size() > 0) {
fireCallbackInvocations(callbackInvocations);//唤醒
}
}
return false;
}可以看看唤醒的方法：
void fireCallbackInvocations(const Vector<CallbackInvocation>& callbacks) {
for (size_t i = 0; i < callbacks.size(); i++) {
callbacks[i].mCallback->onDispSyncEvent(callbacks[i].mEventTime);
}
}遍历callback列表，调用回调方法。
综上，
1.系统使用VSyncThread线程循环生成vsync信号，然后在该线程中调用SF的onVSyncReceived方法，在该方法中SF调用DispSync的addResyncSample方法，最终唤醒DispSyncThread线程，事件传递到DispSyncThread线程中。
2.在DispSyncThread线程中，唤醒后threadloop计算最近一次需要回调的时间，然后发出事件。
所以现在要找到是什么时候注册的callback。
EventThread：
首先看起构造方法：
EventThread::EventThread(const sp<VSyncSource>& src)
: mVSyncSource(src),
mUseSoftwareVSync(false),
mVsyncEnabled(false),
mDebugVsyncEnabled(false) {
for (int32_t i=0 ; i<DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES ; i++) {
mVSyncEvent[i].header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;
mVSyncEvent[i].header.id = 0;
mVSyncEvent[i].header.timestamp = 0;
mVSyncEvent[i].vsync.count = 0;
}
}
void EventThread::onFirstRef() {//可知，构造后线程就跑起来了
run("EventThread", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);
}进入
void MessageQueue::setEventThread(const sp<EventThread>& eventThread)
{
mEventThread = eventThread;
mEvents = eventThread->createEventConnection();
mEventTube = mEvents->getDataChannel();
mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,
MessageQueue::cb_eventReceiver, this);
}sp<EventThread::Connection> EventThread::createEventConnection() const {
return new Connection(const_cast<EventThread*>(this));
}
EventThread::Connection::Connection(
const sp<EventThread>& eventThread)
: count(-1), mEventThread(eventThread), mChannel(new BitTube())
{
}
void EventThread::Connection::onFirstRef() {
// NOTE: mEventThread doesn't hold a strong reference on us
mEventThread->registerDisplayEventConnection(this);
}
status_t EventThread::registerDisplayEventConnection(
const sp<EventThread::Connection>& connection) {
Mutex::Autolock _l(mLock);
mDisplayEventConnections.add(connection);
mCondition.broadcast();//唤醒EventThread
return NO_ERROR;
}即，构造了EventThread后，线程就跑了起来，但是应该是阻塞的状态；当mEventQueue.setEventThread(mSFEventThread)后，线程被唤醒，现在看看线程是如何休眠和唤醒的
bool EventThread::threadLoop() {
DisplayEventReceiver::Event event;
Vector< sp<EventThread::Connection> > signalConnections;
signalConnections = waitForEvent(&event);//阻塞在这里
// 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 report this event
//唤醒后，将事件发送到connection的另一端，即messagequeue中
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 < 0) {
// 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;
}
进入关键函数waitForEvent：
// This will return when (1) a vsync event has been received, and (2) there was
// at least one connection interested in receiving it when we started waiting.
/*
*waitforevent会有三种情况：
*1.线程刚启动，进入threadloop循环，此时connection是空的，没有任何对象要求vsync信号，因此线程会一直阻塞
*2.当SF调用mEventQueue.setEventThread后，会在线程中构造connection，并唤醒线程，向DispSyncThread线程，申请得到vsync信号，然后阻塞
*3.当DispSyncThread的vsync到来后，得到event，然后到threadloop中发出该event
*/
Vector< sp<EventThread::Connection> > EventThread::waitForEvent(
DisplayEventReceiver::Event* event)
{
Mutex::Autolock _l(mLock);
Vector< sp<EventThread::Connection> > signalConnections;
do {
bool eventPending = false;
bool waitForVSync = false;
size_t vsyncCount = 0;
nsecs_t timestamp = 0;
for (int32_t i=0 ; i<DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES ; i++) {
timestamp = mVSyncEvent[i].header.timestamp;
if (timestamp) {
// we have a vsync event to dispatch
*event = mVSyncEvent[i];
mVSyncEvent[i].header.timestamp = 0;
vsyncCount = mVSyncEvent[i].vsync.count;
break;
}
}
if (!timestamp) {
// no vsync event, see if there are some other event
eventPending = !mPendingEvents.isEmpty();
if (eventPending) {
// we have some other event to dispatch
*event = mPendingEvents[0];
mPendingEvents.removeAt(0);
}
}
// find out connections waiting for events
size_t count = mDisplayEventConnections.size();
for (size_t i=0 ; i<count ; i++) {
sp<Connection> connection(mDisplayEventConnections[i].promote());
if (connection != NULL) {
bool added = false;
if (connection->count >= 0) {
// we need vsync events because at least
// one connection is waiting for it
waitForVSync = true;
if (timestamp) {
// we consume the event only if it's time
// (ie: we received a vsync event)
if (connection->count == 0) {
// fired this time around
connection->count = -1;
signalConnections.add(connection);
added = true;
} else if (connection->count == 1 ||
(vsyncCount % connection->count) == 0) {
// continuous event, and time to report it
signalConnections.add(connection);
added = true;
}
}
}
if (eventPending && !timestamp && !added) {
// we don't have a vsync event to process
// (timestamp==0), but we have some pending
// messages.
signalConnections.add(connection);
}
} else {
// we couldn't promote this reference, the connection has
// died, so clean-up!
mDisplayEventConnections.removeAt(i);
--i; --count;
}
}
// Here we figure out if we need to enable or disable vsyncs
if (timestamp && !waitForVSync) {
// we received a VSYNC but we have no clients
// don't report it, and disable VSYNC events
disableVSyncLocked();
} else if (!timestamp && waitForVSync) {
// we have at least one client, so we want vsync enabled
// (TODO: this function is called right after we finish
// notifying clients of a vsync, so this call will be made
// at the vsync rate, e.g. 60fps. If we can accurately
// track the current state we could avoid making this call
// so often.)
enableVSyncLocked();//使能vsync，设置回调
}
// note: !timestamp implies signalConnections.isEmpty(), because we
// don't populate signalConnections if there's no vsync pending
if (!timestamp && !eventPending) {
// wait for something to happen
if (waitForVSync) {
// This is where we spend most of our time, waiting
// for vsync events and new client registrations.
//
// If the screen is off, we can't use h/w vsync, so we
// use a 16ms timeout instead. It doesn't need to be
// precise, we just need to keep feeding our clients.
//
// We don't want to stall if there's a driver bug, so we
// use a (long) timeout when waiting for h/w vsync, and
// generate fake events when necessary.
bool softwareSync = mUseSoftwareVSync;
nsecs_t timeout = softwareSync ? ms2ns(16) : ms2ns(1000);
if (mCondition.waitRelative(mLock, timeout) == TIMED_OUT) {
if (!softwareSync) {
ALOGW("Timed out waiting for hw vsync; faking it");
}
// FIXME: how do we decide which display id the fake
// vsync came from ?
mVSyncEvent[0].header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;
mVSyncEvent[0].header.id = DisplayDevice::DISPLAY_PRIMARY;
mVSyncEvent[0].header.timestamp = systemTime(SYSTEM_TIME_MONOTONIC);
mVSyncEvent[0].vsync.count++;
}
} else {
// Nobody is interested in vsync, so we just want to sleep.
// h/w vsync should be disabled, so this will wait until we
// get a new connection, or an existing connection becomes
// interested in receiving vsync again.
mCondition.wait(mLock);//线程第一次运行时，会阻塞到这里
}
}
} while (signalConnections.isEmpty());
// here we're guaranteed to have a timestamp and some connections to signal
// (The connections might have dropped out of mDisplayEventConnections
// while we were asleep, but we'll still have strong references to them.)
return signalConnections;
}
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);
mPowerHAL.vsyncHint(true);
}
}
mDebugVsyncEnabled = true;
}在看waitforevent方法时，一直在纠结那个connection.count是如何被设置为0或者1的，后来跟代码到SF发现在收到invalidate等消息时才会使用requestNextVsync方法，将count改为0，然后唤醒线程，向DispSyncThread申请vsync信号。所以一定要清楚一点，所有的信号都是按需发送的，你不去申请就不会发给你；甚至需要的信号都是上层发出的。
从构造函数可知mVSyncSource是DispSyncSource，进入其函数
virtual void setVSyncEnabled(bool enable) {
// Do NOT lock the mutex here so as to avoid any mutex ordering issues
// with locking it in the onDispSyncEvent callback.
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("VsyncOn", 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("VsyncOn", 0);
}
}
virtual void setCallback(const sp<VSyncSource::Callback>& callback) {
Mutex::Autolock lock(mMutex);
mCallback = callback;
}从上面可知，EventThread将自己注册为DispSyncSource中的mCallback对象；然后DispSyncSource将自己在注册到DispSync中，接下来看看DispSync的方法：
status_t DispSync::addEventListener(nsecs_t phase,
const sp<Callback>& callback) {
Mutex::Autolock lock(mMutex);
return mThread->addEventListener(phase, callback);
} status_t addEventListener(nsecs_t phase, const sp<DispSync::Callback>& callback) {
Mutex::Autolock lock(mMutex);
for (size_t i = 0; i < mEventListeners.size(); i++) {
if (mEventListeners[i].mCallback == callback) {
return BAD_VALUE;
}
}
EventListener listener;
listener.mPhase = phase;
listener.mCallback = callback;
// We want to allow the firstmost future event to fire without
// allowing any past events to fire. Because
// computeListenerNextEventTimeLocked filters out events within a half
// a period of the last event time, we need to initialize the last
// event time to a half a period in the past.
listener.mLastEventTime = systemTime(SYSTEM_TIME_MONOTONIC) - mPeriod / 2;
mEventListeners.push(listener);
mCond.signal();
return NO_ERROR;
}至此可以知道，DispSyncSource其实是被注册到了DispSyncThread中的listener.mCallback。所以DispSyncThread在收到vsync后会调用：
virtual void onDispSyncEvent(nsecs_t when) {
sp<VSyncSource::Callback> callback;
{
Mutex::Autolock lock(mMutex);
callback = mCallback;
if (mTraceVsync) {
mValue = (mValue + 1) % 2;
ATRACE_INT("VSYNC", mValue);
}
}
if (callback != NULL) {
callback->onVSyncEvent(when);
}
}这个callback就是eventThread：
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();
}经过DispSyncThread--->DispSyncSource--->EventThread，事件终于传到了EventThread，可知，就是构造事件，然后唤醒线程。
案例：SF有需求后，第一次唤醒的过程。