add Vsync 信号机制

blogs/Android/Framework/Interview/UI 体系相关/Vsync 信号机制.md

@@ -0,0 +1,103 @@
+---
+Vsync 信号机制
+---
+
+1. Vsync 信号的生成机制
+2. Vsync 在 SurfaceFlinger 中的分发流程
+3. Vsync 信号的分发原理
+
+![](https://i.loli.net/2020/03/26/XIw6DvOa2MTRKoj.png)
+
+```c++
+void SurfaceFlinger::init(){
+    sp<VsyncSource> vsyncSrc = new DispSyncSource(&mPrimaryDispSync, vsyncPhaseOffsetNs, true, "app");
+    mEventThread = new EventThread(vsyncSrc);
+    sp<VsyncSource> sfVsyncSrc = new DisSyncSource(&mPrimaryDispSync, sfVsyncPhaseOffsetNs, true, "sf");
+    mSFEventThread = new EventThread(sfVsyncSrc);
+    mHwc = new HWComposer(this, *static_cast<HWCompiser::EventHandler *>(this));
+}
+// 主要用于生成 Vsync 信号
+HWComposer::HWComposer(const sp<SurfaceFlinger>& flinger, ...){
+    bool needVsyncThread = true;
+    loadHwcModule();
+    if(mHwc){
+        mCBContext->procs.vsync = &hook_vsync;
+        needVsyncThread = false;
+    }
+    if(needVsyncThread){
+        mVsyncThread = new VsyncThread(*this);
+    }
+}
+// hook_vsync
+void HWComposer::vsync(int disp, int64_t timestamp){
+    mEventHandler.onVSyncReceived(disp, timestamp);
+}
+// 软件分发
+bool HWComposer::VSyncThread::threadLoop(){
+    clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &spec, NULL);
+    mHwc.mEventHandler.onVSyncReceived(0, next_vsync);
+    return true;
+}
+
+void SurfaceFlinger::onVSyncReceived(int type, nsece_t timestamp){
+    mPrimaryDispSync.addResyncSample(timestamp);
+}
+bool DispSync::addResyncSample(nsecs_t timestamp){
+    mResyncSamples[idx] = timestamp;
+    updateModelLocked();
+}
+void DispSync::updateModelLocked(){
+    mThread->updateModel(mPeriod, mPhase);
+}
+DispSync::DispSync():mThread(new DispSyncThread()){
+    mThread->run("DispSync", ...);
+}
+// 工作线程的处理流程
+virtual bool threadLoop(){
+    while(true){
+        if(mPeriod == 0){
+            mCond.wait(mMutex);
+            continue;
+        }
+        now = systemTime(SYSTEM_TIME_MONOTONIC);
+        callbackInvocations = gatherCallbackInvocationsLocked(now);
+        fireCallbackInvocations(callbackInvocations);
+    }
+    return false;
+}
+```
+
+```c++
+void EventThread::onVSyncEvent(nsecs_t timestamp){
+    mVSyncEvent[0].header.timestamp = timestamp;
+    mCondition.broadcast();
+}
+Vector<sp<EventThread::Connection>> EventThread::waitForEvent(...){
+    do{
+        // 检查 mVSyncEvent 数组，看有没有 VSync 触发
+        // 如果有就遍历所有注册的 connection
+        // 如果 count>=0 就加到 signalConnections 里，加的时候重置为 -1
+        if(!timestamp){
+            if(waitForVSync){
+                mCondition.waitRelative(mLock, timeout);
+            }
+        }
+    }while(signalConnections.isEmpty());
+    return signalConnections;
+}
+```
+
+```c++
+status_t EventThread::Connection::postEvent(...){
+	DisplayEventReceiver::sendEvents(mChannel, &event, 1);
+}
+ssize_t DisplayEventReceiver::sendEvents(const sp<BitTube>&dataChannel, Event const* events, size_t count){
+    return BitTube::sendObjects(dataChannel, events, count);
+}
+ssize_t BitTube::sendObjects(const sp<BitTube>&tube, void const* events, size_t count, size_t objSize){
+    const char* vaddr = reinterpret_cast<const char*>(events);
+    sszie_t size = tube->write(vaddr, count*objSize);
+}
+```
+
+![](https://i.loli.net/2020/03/26/gsG1Jlv8cxmCzIE.png)