Android에서 VSync 동작 원리 및 초기화 과정(5)

 이전 포스팅을 통해 안드로이드의 VSync( Vertical Synchronization, 수직동기화)가 초기화 되어가는 과정을 차근차근 살펴보았습니다. 이번 시간에는 C++ 단계에서 Java Framework 단계로 VSync가 동작하는 과정을 살펴보도록 하겠습니다. 본 포스팅의 이해를 위해 이전에 작성해 왔던 포스팅들을 참조해 주시길 바랍니다.

Android에서 VSync 동작 원리 및 초기화 과정(3)

http://elecs.tistory.com/134

SurfaceFlinger에서 DispSyncSource의 초기화 과정`

http://elecs.tistory.com/142

/frameworks/native/services/surfaceflinger/DispSync.cpp

class DispSyncThread: public Thread {
 
....
 
    void fireCallbackInvocations(const Vector<CallbackInvocation>& callbacks) {
        for (size_t i = 0; i < callbacks.size(); i++) {
            callbacks[i].mCallback->onDispSyncEvent(callbacks[i].mEventTime);
        }
    }
 
....
    
};

 DispSyncSource를 통해 설정하였던 Callback 함수인 onDispSyncEvent()를 호출합니다.

/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() {}
 
....
 
    virtual void setCallback(const sp<VSyncSource::Callback>& callback) {
        Mutex::Autolock lock(mMutex);
        mCallback = callback;
    }
 
private:
    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);
        }
    }
 
....
 
    DispSync* mDispSync;
    sp<VSyncSource::Callback> mCallback;
    Mutex mMutex;
};
 
 

등록된 Callback 함수 onVSyncEvent()를 통해 EventThread 가 실행됩니다.

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

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();
}

EventThread의 Condition 클래스를 통해 broadcast()가 호출되어 wait() 상태에 있던 동작이 다시 재개됩니다.

/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 {
 
    ....
 
        // 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;
            }
        }
 
        ....
 
        // 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) {
 
            ....
 
            } 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;
}

wait 상태에서 멈추어있던 waitForEvent() 함수가 broadcast 함수의 신호를 받고 다시 동작하며 waitForEvent() 함수의 실행을 종료시킵니다.

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

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);
 
    ....
 
    }
    return true;
}

waitForEvent() 함수가 동작되는 동안 수신된 이벤트에 대해 postEvent() 함수를 호출합니다.

/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);
}

DisplayEventReceiver 클래스 내에 있는 sendEvent() 함수를 호출합니다.

/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);
}

sendObjects() 함수를 통해 BitTube 클래스가 Pipe로 Looper와 연결한 File Descriptor에 값을 입력합니다.

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

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);
    ssize_t size = tube->write(vaddr, count*objSize);
 
    // should never happen because of SOCK_SEQPACKET
    LOG_ALWAYS_FATAL_IF((size >= 0) && (size % objSize),
            "BitTube::sendObjects(count=%d, size=%d), res=%d (partial events were sent!)",
            count, objSize, size);
 
    //ALOGE_IF(size<0, "error %d sending %d events", size, count);
    return size < 0 ? size : size / objSize;
}

write() 함수를 통해 Looper에 값을 입력합니다.

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

void BitTube::init(size_t rcvbuf, size_t sndbuf) {
    int sockets[2];
    if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sockets) == 0) {
        size_t size = DEFAULT_SOCKET_BUFFER_SIZE;
        setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf));
        setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf));
        // sine we don't use the "return channel", we keep it small...
        setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size));
        setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));
        fcntl(sockets[0], F_SETFL, O_NONBLOCK);
        fcntl(sockets[1], F_SETFL, O_NONBLOCK);
        mReceiveFd = sockets[0];
        mSendFd = sockets[1];
    } else {
        mReceiveFd = -errno;
        ALOGE("BitTube: pipe creation failed (%s)", strerror(-mReceiveFd));
    }
}
 
....
 
ssize_t BitTube::write(void const* vaddr, size_t size)
{
    ssize_t err, len;
    do {
        len = ::send(mSendFd, vaddr, size, MSG_DONTWAIT | MSG_NOSIGNAL);
        // cannot return less than size, since we're using SOCK_SEQPACKET
        err = len < 0 ? errno : 0;
    } while (err == EINTR);
    return err == 0 ? len : -err;
}

 File Descriptor를 통해 데이터를 전송합니다. 이는 이후 Looper에서 감지하여 이벤트를 처리하게 됩니다. Looper에 DisplayEventReceiver에서 wirte를 시도하였을 때 이를 읽어들이는 File Descriptor는 아래의 방법으로 등록됩니다.

/frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp

status_t NativeDisplayEventReceiver::initialize() {
    status_t result = mReceiver.initCheck();
    if (result) {
        ALOGW("Failed to initialize display event receiver, status=%d", result);
        return result;
    }
 
    int rc = mMessageQueue->getLooper()->addFd(mReceiver.getFd(), 0, ALOOPER_EVENT_INPUT,
            this, NULL);
    if (rc < 0) {
        return UNKNOWN_ERROR;
    }
    return OK;
}

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

int DisplayEventReceiver::getFd() const {
    if (mDataChannel == NULL)
        return NO_INIT;
 
    return mDataChannel->getFd();
}

/frameworks/native/libs/gui/BitTube.cpp

int BitTube::getFd() const
{
    return mReceiveFd;
}

위의 과정을 통해 Looper에 File Descriptor가 등록되며 이는 Looper 내에서 epoll_wait()함수에 의해 감지되어 이벤트를 등록합니다.

/system/core/libutils/Looper.cpp

int Looper::pollInner(int timeoutMillis) {
 
....
 
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
 
....
 
    for (int i = 0; i < eventCount; i++) {
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeReadPipeFd) {
            if (epollEvents & EPOLLIN) {
                awoken();
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
            }
        } else {
            ssize_t requestIndex = mRequests.indexOfKey(fd);
            if (requestIndex >= 0) {
                int events = 0;
                if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
                if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
                if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
                if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
                pushResponse(events, mRequests.valueAt(requestIndex));
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
                        "no longer registered.", epollEvents, fd);
            }
        }
    }
 
....
 
}

epoll_wait()에 의해 감지된 File Descriptor는 이후 등록된 이벤트와 함께 pushResponse() 함수를 통해 등록됩니다.

/system/core/libutils/Looper.cpp

void Looper::pushResponse(int events, const Request& request) {
    Response response;
    response.events = events;
    response.request = request;
    mResponses.push(response);
}

pushResponse() 함수에 의해 등록된 이벤트는 이후 가지고 있는 Callback 함수를 수행합니다.

/system/core/libutils/Looper.cpp

int Looper::pollInner(int timeoutMillis) {
 
....
 
    // Invoke all response callbacks.
    for (size_t i = 0; i < mResponses.size(); i++) {
        Response& response = mResponses.editItemAt(i);
        if (response.request.ident == ALOOPER_POLL_CALLBACK) {
            int fd = response.request.fd;
            int events = response.events;
            void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
            ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
                    this, response.request.callback.get(), fd, events, data);
#endif
            int callbackResult = response.request.callback->handleEvent(fd, events, data);
            if (callbackResult == 0) {
                removeFd(fd);
            }
            // Clear the callback reference in the response structure promptly because we
            // will not clear the response vector itself until the next poll.
            response.request.callback.clear();
            result = ALOOPER_POLL_CALLBACK;
        }
    }
    return result;
}

DisplayEventReceiver 클래스를 통해 등록하였던 Callback 함수인 handleEvent() 함수를 실행합니다.

/frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp

int NativeDisplayEventReceiver::handleEvent(int receiveFd, int events, void* data) {
    if (events & (ALOOPER_EVENT_ERROR | ALOOPER_EVENT_HANGUP)) {
        ALOGE("Display event receiver pipe was closed or an error occurred.  "
                "events=0x%x", events);
        return 0; // remove the callback
    }
 
    if (!(events & ALOOPER_EVENT_INPUT)) {
        ALOGW("Received spurious callback for unhandled poll event.  "
                "events=0x%x", events);
        return 1; // keep the callback
    }
 
    // Drain all pending events, keep the last vsync.
    nsecs_t vsyncTimestamp;
    int32_t vsyncDisplayId;
    uint32_t vsyncCount;
    if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
        ALOGV("receiver %p ~ Vsync pulse: timestamp=%lld, id=%d, count=%d",
                this, vsyncTimestamp, vsyncDisplayId, vsyncCount);
        mWaitingForVsync = false;
        dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount);
    }
 
    return 1; // keep the callback
}

Callback 함수로서 handleEvent()함수가 실행되며 이는 Java Framework에 있는 함수를 호출합니다.

/frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp

void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, int32_t id, uint32_t count) {
    JNIEnv* env = AndroidRuntime::getJNIEnv();
 
    ALOGV("receiver %p ~ Invoking vsync handler.", this);
    env->CallVoidMethod(mReceiverObjGlobal,
            gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, id, count);
    ALOGV("receiver %p ~ Returned from vsync handler.", this);
 
    mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}
 
....
 
int register_android_view_DisplayEventReceiver(JNIEnv* env) {
    int res = jniRegisterNativeMethods(env, "android/view/DisplayEventReceiver",
            gMethods, NELEM(gMethods));
    LOG_FATAL_IF(res < 0, "Unable to register native methods.");
 
    FIND_CLASS(gDisplayEventReceiverClassInfo.clazz, "android/view/DisplayEventReceiver");
 
    GET_METHOD_ID(gDisplayEventReceiverClassInfo.dispatchVsync,
            gDisplayEventReceiverClassInfo.clazz,
            "dispatchVsync", "(JII)V");
    GET_METHOD_ID(gDisplayEventReceiverClassInfo.dispatchHotplug,
            gDisplayEventReceiverClassInfo.clazz,
            "dispatchHotplug", "(JIZ)V");
    return 0;
}

/frameworks/base/core/java/android/view/DisplayEventReceiver.java

    /**
     * Called when a vertical sync pulse is received.
     * The recipient should render a frame and then call {@link #scheduleVsync}
     * to schedule the next vertical sync pulse.
     *
     * @param timestampNanos The timestamp of the pulse, in the {@link System#nanoTime()}
     * timebase.
     * @param builtInDisplayId The surface flinger built-in display id such as
     * {@link SurfaceControl#BUILT_IN_DISPLAY_ID_MAIN}.
     * @param frame The frame number.  Increases by one for each vertical sync interval.
     */
    public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
    }
 
....
 
    // Called from native code.
    @SuppressWarnings("unused")
    private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
        onVsync(timestampNanos, builtInDisplayId, frame);
    }