SurfaceFlinger에서 EventThread의 초기화 과정

 SurfaceFlinger가 수직동기화(VSync)를 수행하기 위해 이를 담당하는 별도의 Thread를 만들어 줄 필요가 있습니다. EventThread 클래스가 바로 그러한 역할을 담당하고 있다고 보면 되겠습니다. 이번 포스팅에서 EventThread의 초기화 과정을 살펴보도록 하겠습니다.

/frameworks/native/services/surfaceflinger/EventThread.h

class VSyncSource : public virtual RefBase {
public:
    class Callback: public virtual RefBase {
    public:
        virtual ~Callback() {}
        virtual void onVSyncEvent(nsecs_t when) = 0;
    };
 
    virtual ~VSyncSource() {}
    virtual void setVSyncEnabled(bool enable) = 0;
    virtual void setCallback(const sp<Callback>& callback) = 0;
};
 
class EventThread : public Thread, private VSyncSource::Callback {
    class Connection : public BnDisplayEventConnection {
    public:
        Connection(const sp<EventThread>& eventThread);
        status_t postEvent(const DisplayEventReceiver::Event& event);
 
        // count >= 1 : continuous event. count is the vsync rate
        // count == 0 : one-shot event that has not fired
        // count ==-1 : one-shot event that fired this round / disabled
        int32_t count;
 
    private:
        virtual ~Connection();
        virtual void onFirstRef();
        virtual sp<BitTube> getDataChannel() const;
        virtual void setVsyncRate(uint32_t count);
        virtual void requestNextVsync();    // asynchronous
        sp<EventThread> const mEventThread;
        sp<BitTube> const mChannel;
    };
 
public:
 
    EventThread(const sp<VSyncSource>& src);
 
    sp<Connection> createEventConnection() const;
    status_t registerDisplayEventConnection(const sp<Connection>& connection);
 
    void setVsyncRate(uint32_t count, const sp<Connection>& connection);
    void requestNextVsync(const sp<Connection>& connection);
 
    // called before the screen is turned off from main thread
    void onScreenReleased();
 
    // called after the screen is turned on from main thread
    void onScreenAcquired();
 
    // called when receiving a hotplug event
    void onHotplugReceived(int type, bool connected);
 
    Vector< sp<EventThread::Connection> > waitForEvent(
            DisplayEventReceiver::Event* event);
 
    void dump(String8& result) const;
 
private:
    virtual bool        threadLoop();
    virtual void        onFirstRef();
 
    virtual void onVSyncEvent(nsecs_t timestamp);
 
    void removeDisplayEventConnection(const wp<Connection>& connection);
    void enableVSyncLocked();
    void disableVSyncLocked();
 
    // constants
    sp<VSyncSource> mVSyncSource;
    PowerHAL mPowerHAL;
 
    mutable Mutex mLock;
    mutable Condition mCondition;
 
    // protected by mLock
    SortedVector< wp<Connection> > mDisplayEventConnections;
    Vector< DisplayEventReceiver::Event > mPendingEvents;
    DisplayEventReceiver::Event mVSyncEvent[DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES];
    bool mUseSoftwareVSync;
    bool mVsyncEnabled;
 
    // for debugging
    bool mDebugVsyncEnabled;
};

/frameworks/native/include/gui/DisplayEventReceiver.h

class DisplayEventReceiver {
public:
    enum {
        DISPLAY_EVENT_VSYNC = 'vsyn',
        DISPLAY_EVENT_HOTPLUG = 'plug'
    };
 
    struct Event {
 
        struct Header {
            uint32_t type;
            uint32_t id;
            nsecs_t timestamp;
        };
 
        struct VSync {
            uint32_t count;
        };
 
        struct Hotplug {
            bool connected;
        };
 
        Header header;
        union {
            VSync vsync;
            Hotplug hotplug;
        };
    };
 
....
 
}

 전체적인 맥락을 파악하기 위해 EventThread의 전체 헤더 소스코드를 보았습니다. 이번에는 EventThread가 초기화 되는 과정을 살펴보도록 하겠습니다.

/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

void SurfaceFlinger::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);
    mEventQueue.setEventThread(mSFEventThread);
 
    mEventControlThread = new EventControlThread(this);
    mEventControlThread->run("EventControl", PRIORITY_URGENT_DISPLAY);
 
....
 
}

 SurfaceFlinger에서 EventThread가 초기화 되는 부분입니다. 여기서 mEventThread 클래스 변수가 초기화 되는 과정을 살펴보도록 하겠습니다.

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

            vsyncPhaseOffsetNs, true);

 수직동기화의 Source를 설정하는 부분입니다. 해당 부분은 다음과 같이 구성되어 있습니다.

/frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

class DispSyncSource : public VSyncSource, private DispSync::Callback {
public:
    DispSyncSource(DispSync* dispSync, nsecs_t phaseOffset, bool traceVsync) :
            mValue(0),
            mPhaseOffset(phaseOffset),
            mTraceVsync(traceVsync),
            mDispSync(dispSync) {}
 
    virtual ~DispSyncSource() {}
 
....
 
private:
 
....
 
    int mValue;
 
    const nsecs_t mPhaseOffset;
    const bool mTraceVsync;
 
    DispSync* mDispSync;
    sp<VSyncSource::Callback> mCallback;
    Mutex mMutex;
};

mEventThread = new EventThread(vsyncSrc);

 이전 줄에서 설정하였던 수직동기화 정보를 기반으로 EventThread 생성자를 실행합니다.

/frameworks/native/services/surfaceflinger/EventThread.cpp

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() {
    run("EventThread", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);
}
 
....
 
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 report this event
        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;
}

 위의 소스코드에서 waitForEvent() 소스코드를 통해 EventThread의 연결과정이 드러나고 있습니다. 해당 부분을 살펴보면 다음과 같습니다.

/frameworks/native/services/surfaceflinger/EventThread.cpp

// 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< 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();
        }
 
        // 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);
            }
        }
    } 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;
}

 위의 waitForEvent() 함수의 실행이 종료되면 이후에 Connection 클래스가 생성됩니다.

/frameworks/native/services/surfaceflinger/EventThread.cpp

EventThread::Connection::Connection(
        const sp<EventThread>& eventThread)
    : count(-1), mEventThread(eventThread), mChannel(new BitTube())
{
}
 
EventThread::Connection::~Connection() {
    // do nothing here -- clean-up will happen automatically
    // when the main thread wakes up
}
 
void EventThread::Connection::onFirstRef() {
    // NOTE: mEventThread doesn't hold a strong reference on us
    mEventThread->registerDisplayEventConnection(this);
}

 위의 Connection 클래스가 생성되면서 mChannel을 통해 File Descriptor을 통해 프로세스간 통신을 수행하는 BitTube 클래스가 새로 생성됩니다. BitTube에 대한 자세한 내용은 이전의 포스팅을 참조해주시기 바랍니다.

안드로이드 프레임워크 프로그래밍(24) [BitTube 클래스]

http://elecs.tistory.com/138

 Connection 클래스의 onFirstRef()함수에 의해 EventThread 클래스의 registerDisplayEventConnection()함수가 호출됩니다.

/frameworks/native/services/surfaceflinger/EventThread.cpp

status_t EventThread::registerDisplayEventConnection(
        const sp<EventThread::Connection>& connection) {
    Mutex::Autolock _l(mLock);
    mDisplayEventConnections.add(connection);
    mCondition.broadcast();
    return NO_ERROR;
}
 

 EventThread에 EventThread의 Connection 클래스를 벡터에 등록한 후 멈추어 있던 Thread들을 모두 깨웁니다. 위의 과정을 통해 생성된 Connection 클래스는 이후 postEvent() 함수를 수행하게 됩니다.

/frameworks/native/services/surfaceflinger/EventThread.cpp

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);
}

/frameworks/native/libs/gui/DisplayEventReceiver.cpp

ssize_t DisplayEventReceiver::sendEvents(const sp<BitTube>& dataChannel,
        Event const* events, size_t count)
{
    return BitTube::sendObjects(dataChannel, events, count);
}