Android UI架构(五)--探秘刷新动力Vsync(1)之EventThread.md

参考资料

1. Android SurfaceFlinger SW Vsync模型

一. SFEventThread

EventControlThread: 控制硬件vsync的开关
DispSyncThread: 软件产生vsync的线程
SF EventThread: 该线程用于SurfaceFlinger接收vsync信号用于渲染
App EventThread: 该线程用于接收vsync信号并且上报给App进程，App开始画图

从这4个线程，可以将vsync分为4种不同的类型
HW vsync, 真实由硬件产生的vsync信号
SW vsync, 由DispSync产生的vsync信号
SF vsync, SF接收到的vsync信号
App vsync, App接收到的vsync信号

这里我们着重看看SF EventThread.

1.1 BitTube

先大致了解下BitTube，其实现是socketpairt套接字，用于传递消息。
Buffer大小是4KB。

二. EventThread启动

2.1 SurfaceFlinger.init

void SurfaceFlinger::init() {
    ...
    // start the EventThread
    mEventThreadSource =
            std::make_unique(&mPrimaryDispSync, SurfaceFlinger::vsyncPhaseOffsetNs,
                                             true, "app");
    mEventThread = std::make_unique(mEventThreadSource.get(),
                                                       [this]() { resyncWithRateLimit(); },
                                                       impl::EventThread::InterceptVSyncsCallback(),
                                                       "appEventThread");
    mSfEventThreadSource =
            std::make_unique(&mPrimaryDispSync,
                                             SurfaceFlinger::sfVsyncPhaseOffsetNs, true, "sf");

    mSFEventThread =
            std::make_unique(mSfEventThreadSource.get(),
                                                [this]() { resyncWithRateLimit(); },
                                                [this](nsecs_t timestamp) {
                                                    mInterceptor->saveVSyncEvent(timestamp);
                                                },
                                                "sfEventThread");
    ......
}

2.2 EventThread初始化

EventThread::EventThread(VSyncSource* src, ResyncWithRateLimitCallback resyncWithRateLimitCallback,
                         InterceptVSyncsCallback interceptVSyncsCallback, const char* threadName)
      : mVSyncSource(src),
        mResyncWithRateLimitCallback(resyncWithRateLimitCallback),
        mInterceptVSyncsCallback(interceptVSyncsCallback) {
    for (auto& event : mVSyncEvent) {
        event.header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;
        event.header.id = 0;
        event.header.timestamp = 0;
        event.vsync.count = 0;
    }

    // 绑定thread函数
    mThread = std::thread(&EventThread::threadMain, this);
    // 设置thread名称
    pthread_setname_np(mThread.native_handle(), threadName);

    pid_t tid = pthread_gettid_np(mThread.native_handle());

    // Use SCHED_FIFO to minimize jitter
    constexpr int EVENT_THREAD_PRIORITY = 2;
    struct sched_param param = {0};
    param.sched_priority = EVENT_THREAD_PRIORITY;
    if (pthread_setschedparam(mThread.native_handle(), SCHED_FIFO, ¶m) != 0) {
        ALOGE("Couldn't set SCHED_FIFO for EventThread");
    }

    set_sched_policy(tid, SP_FOREGROUND);
}

2.3 EventThread.threadMain

void EventThread::threadMain() NO_THREAD_SAFETY_ANALYSIS {
    std::unique_lock lock(mMutex);
    while (mKeepRunning) {
        DisplayEventReceiver::Event event;
        Vector > signalConnections;
        // 2.4 阻塞等待事件
        signalConnections = waitForEventLocked(&lock, &event);

        // 分发事件给connection
        const size_t count = signalConnections.size();
        for (size_t i = 0; i < count; i++) {
            const sp& conn(signalConnections[i]);
            // 2.5 分发事件
            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.
                removeDisplayEventConnectionLocked(signalConnections[i]);
            }
        }
    }
}

2.4 EventThread.waitForEventLocked

// 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.
Vector > EventThread::waitForEventLocked(
        std::unique_lock* lock, DisplayEventReceiver::Event* event) {
    Vector > signalConnections;

    while (signalConnections.isEmpty() && mKeepRunning) {
        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) {
                // 当timestamp不为0时，说明有事件发生
                if (mInterceptVSyncsCallback) {
                    mInterceptVSyncsCallback(timestamp);
                }
                *event = mVSyncEvent[i];
                // 置timestamp为0，标记为当前事件被消费
                mVSyncEvent[i].header.timestamp = 0;
                vsyncCount = mVSyncEvent[i].vsync.count;
                break;
            }
        }

        // mDisplayEventConnections保存的是注册的Connection的,
        // SF EventThread线程里只有一个Connection, 而这个Connection主要是用来渲染
        // 而如果是APP EventThread, 这里会有多个connection
        size_t count = mDisplayEventConnections.size();
        if (!timestamp && count) {
            // 没有vsync事件， 来看下是否有其它pending的event, 这里主要是hotplug的事件
            eventPending = !mPendingEvents.isEmpty();
            if (eventPending) {
                // we have some other event to dispatch
                *event = mPendingEvents[0];
                mPendingEvents.removeAt(0);
            }
        }

        for (size_t i = 0; i < count;) {
            sp connection(mDisplayEventConnections[i].promote());
            if (connection != nullptr) {
                bool added = false;
                // Connection->count的值大小含义如下：
                // 1. >=1:  表示持续接收Vsync信号
                // 2. ==0:  只接收一次Vsync信号
                // 3. ==-1: 不接收Vsync信号
                if (connection->count >= 0) {
                    // 如果有 connection->count >= 0，说明需要Vsync信号
                    waitForVSync = true;
                    if (timestamp) { // 大于0.说明有事件
                        // 处理本次事件
                        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);
                }
                ++i;
            } else {
                // we couldn't promote this reference, the connection has
                // died, so clean-up!
                mDisplayEventConnections.removeAt(i);
                --count;
            }
        }

        // Here we figure out if we need to enable or disable vsyncs
        if (timestamp && !waitForVSync) {
            // 收到Vsync信号，但是没有Connection监听，所以关闭Vsync
            disableVSyncLocked();
        } else if (!timestamp && waitForVSync) {
            // 有Connection监听，但是还没有Vsync信号，所以打开Vsync
            enableVSyncLocked();
        }

        // 没有事件发生
        if (!timestamp && !eventPending) {
            if (waitForVSync) { // 如果有connection监听，则需要等待Vsync事件
                // 以防止硬件Driver出问题，设置一个超时时间16ms
                bool softwareSync = mUseSoftwareVSync;
                auto timeout = softwareSync ? 16ms : 1000ms;
                if (mCondition.wait_for(*lock, timeout) == std::cv_status::timeout) {
                    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 {
                // 没有connection监听，也没有收到事件，则一直等待
                mCondition.wait(*lock);
            }
        }
    }

    // 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;
}

当EventThread初始化进入时，由于没有Connection，timestamp也为0，直接进入waitForEventLocked一直等待。

2.5 EventThread.Connection.postEvent

status_t EventThread::Connection::postEvent(const DisplayEventReceiver::Event& event) {
    ssize_t size = DisplayEventReceiver::sendEvents(&mChannel, &event, 1);
    return size < 0 ? status_t(size) : status_t(NO_ERROR);
}

通过BitTube发送事件。

三. SF EventThread 添加Connection

3.1 SurfaceFlinger.init

sfEventThread添加Connection是在这个线程启动后就添加的。

void SurfaceFlinger::init() {
    ......
    mEventQueue->setEventThread(mSFEventThread.get());
    mVsyncModulator.setEventThreads(mSFEventThread.get(), mEventThread.get());
    ......
}

3.2 MessageQueue.setEventThread

void MessageQueue::setEventThread(android::EventThread* eventThread) {
    if (mEventThread == eventThread) {
        return;
    }

    if (mEventTube.getFd() >= 0) {
        mLooper->removeFd(mEventTube.getFd());
    }

    mEventThread = eventThread;
    // 3.3 创建Connection
    mEvents = eventThread->createEventConnection();
    // 3.4 建立BitTube连接
    mEvents->stealReceiveChannel(&mEventTube);
    // 3.5 接受BitTube事件，调用cb_eventReceiver方法
    mLooper->addFd(mEventTube.getFd(), 0, Looper::EVENT_INPUT, MessageQueue::cb_eventReceiver,
                   this);
}

3.3 EventThread.createEventConnection

sp EventThread::createEventConnection() const {
    return new Connection(const_cast(this));
}

EventThread::Connection::Connection(EventThread* eventThread)
      // 注意这里的count是被赋值为-1，也就是不接收Vsync事件
      : count(-1), mEventThread(eventThread), mChannel(gui::BitTube::DefaultSize) {}

3.3.1 EventThread.Connection.onFirstRef

void EventThread::Connection::onFirstRef() {
    // NOTE: mEventThread doesn't hold a strong reference on us
    mEventThread->registerDisplayEventConnection(this);
}

在Connection对象生成之后，就被注册到其对应的EventThread中

3.3.2 EventThread.registerDisplayEventConnection

status_t EventThread::registerDisplayEventConnection(
        const sp& connection) {
    std::lock_guard lock(mMutex);
    // 添加到mDisplayEventConnections集合中
    mDisplayEventConnections.add(connection);
    // waitForEventLocked 可以继续执行了。
    mCondition.notify_all();
    return NO_ERROR;
}

注册connection后，waitForEventLocked便可以开始继续执行了。但是由于只有这一个connection，而且这个connection.count 还是 -1，所以最后还是会在waitForEventLocked中一直等待Vsync事件

3.4 EventThread.Connection.stealReceiveChannel

status_t EventThread::Connection::stealReceiveChannel(gui::BitTube* outChannel) {
    // 将Connection的mChannel复制
    outChannel->setReceiveFd(mChannel.moveReceiveFd());
    return NO_ERROR;
}

3.5 SF EventThread connection开始监听Vsync信号

SurfaceFlinger::initializeDisplays
    SurfaceFlinger::onInitializeDisplays
        SurfaceFlinger::setTransactionState
            SurfaceFlinger::setTransactionFlags
                SurfaceFlinger::signalTransaction
                    MessageQueue::invalidate
                        EventThread::Connection::requestNextVsync
                            EventThread::requestNextVsync

也就是说当显示屏准备完毕，sfEventThread就可以开始监听Vsync信号了

void EventThread::requestNextVsync(const sp& connection) {
    std::lock_guard lock(mMutex);

    if (mResyncWithRateLimitCallback) {
        mResyncWithRateLimitCallback();
    }

    if (connection->count < 0) {
        connection->count = 0; // 只接受一次Vsync信号
        mCondition.notify_all();
    }
}

3.6 MessageQueue.cb_eventReceiver 处理Vsync信号

当SF EventThread收到Vsync信号时，最终会通过BitTube发送给对应connection的receiver[2.5]。

int MessageQueue::cb_eventReceiver(int fd, int events, void* data) {
    MessageQueue* queue = reinterpret_cast(data);
    return queue->eventReceiver(fd, events);
}

int MessageQueue::eventReceiver(int /*fd*/, int /*events*/) {
    ssize_t n;
    DisplayEventReceiver::Event buffer[8];
    while ((n = DisplayEventReceiver::getEvents(&mEventTube, buffer, 8)) > 0) {
        for (int i = 0; i < n; i++) {
            if (buffer[i].header.type == DisplayEventReceiver::DISPLAY_EVENT_VSYNC) {
                mHandler->dispatchInvalidate();
                break;
            }
        }
    }
    return 1;
}

3.6.1 MessageQueue.Handler.dispatchInvalidate

void MessageQueue::Handler::dispatchInvalidate() {
    if ((android_atomic_or(eventMaskInvalidate, &mEventMask) & eventMaskInvalidate) == 0) {
        mQueue.mLooper->sendMessage(this, Message(MessageQueue::INVALIDATE));
    }
}

void MessageQueue::Handler::handleMessage(const Message& message) {
    switch (message.what) {
        case INVALIDATE:
            android_atomic_and(~eventMaskInvalidate, &mEventMask);
            mQueue.mFlinger->onMessageReceived(message.what);
            break;
        case REFRESH:
            android_atomic_and(~eventMaskRefresh, &mEventMask);
            mQueue.mFlinger->onMessageReceived(message.what);
            break;
    }
}

转到SufaceFlinger主线程中处理了。

四. 开启硬件Vsync

4.1 EventThread.enableVSyncLocked

当显示屏准备完毕后，SF EventThread请求一次Vsync信号。那么走到waitForEventLocked中，会调用enableVSyncLocked方法开启硬件Vsync信号。

void EventThread::enableVSyncLocked() {
    if (!mUseSoftwareVSync) {
        // mUseSoftwareVSync也就是显示屏power状态
        // 灭屏为true，亮屏置为false
        if (!mVsyncEnabled) {
            mVsyncEnabled = true;
            mVSyncSource->setCallback(this);
            mVSyncSource->setVSyncEnabled(true);
        }
    }
    mDebugVsyncEnabled = true;
}

4.2 DispSyncSource.setVSyncEnabled

void DispSyncSource::setVSyncEnabled(bool enable) {
    std::lock_guard lock(mVsyncMutex);
    if (enable) {
        status_t err = mDispSync->addEventListener(mName, mPhaseOffset,
                                                   static_cast(this),
                                                   mLastCallbackTime);
        if (err != NO_ERROR) {
            ALOGE("error registering vsync callback: %s (%d)", strerror(-err), err);
        }
        // ATRACE_INT(mVsyncOnLabel.c_str(), 1);
    } else {
        status_t err = mDispSync->removeEventListener(static_cast(this),
                                                      &mLastCallbackTime);
        if (err != NO_ERROR) {
            ALOGE("error unregistering vsync callback: %s (%d)", strerror(-err), err);
        }
        // ATRACE_INT(mVsyncOnLabel.c_str(), 0);
    }
    mEnabled = enable;
}

开启Vsync信号就是添加EventListener，关闭就是将这个listener移除

4.3 DispSync.addEventListener

status_t DispSync::addEventListener(const char* name, nsecs_t phase, Callback* callback) {
    Mutex::Autolock lock(mMutex);
    return mThread->addEventListener(name, phase, callback);
}

接下来就是到了DispSync部分了。

大致流程如下：

图片转自：https://www.jianshu.com/p/d3e4b1805c92 强烈推荐仔细阅读