Android에서 VSync 동작 원리 및 초기화 과정(2)

 지난 포스팅에서 Hardware에서 VSync를 담당하는 Thread가 생성되는 것을 확인하였습니다. 이번 포스팅에서는 지난 시간에 이어 VSync의 동작 원리를 좀 더 살펴보도록 하겠습니다.

/hardware/qcom/display/msm8960/libhwcomposer/hwc_vsync.cpp

static void *vsync_loop(void *param)
{
....
    do {
        if (LIKELY(!ctx->vstate.fakevsync)) {
            len = pread(fd_timestamp, vdata, MAX_DATA, 0);
            if (len < 0) {
                // If the read was just interrupted - it is not a fatal error
                // In either case, just continue.
                if (errno != EAGAIN &&
                    errno != EINTR  &&
                    errno != EBUSY) {
                    ALOGE ("FATAL:%s:not able to read file:%s, %s",
                           __FUNCTION__,
                           vsync_timestamp_fb0, strerror(errno));
                }
                continue;
            }
            // extract timestamp
            const char *str = vdata;
            if (!strncmp(str, "VSYNC=", strlen("VSYNC="))) {
                cur_timestamp = strtoull(str + strlen("VSYNC="), NULL, 0);
            }
        } else {
            usleep(16666);
            cur_timestamp = systemTime();
        }
        // send timestamp to HAL
        if(ctx->vstate.enable) {
            ALOGD_IF (logvsync, "%s: timestamp %llu sent to HWC for %s",
                      __FUNCTION__, cur_timestamp, "fb0");
            ctx->proc->vsync(ctx->proc, dpy, cur_timestamp);
        }
 
    } while (true);
    if(fd_timestamp >= 0)
        close (fd_timestamp);
 
    return NULL;
}

 VSync를 담당하는 thread는 초당 60번 VSync 신호를 보냅니다. 물론 임의로 설정된 시간 이후에 VSync가 동작하는 경우도 있습니다. VSync를 담당하는 thread는 HWComposer 클래스에 VSync 함수를 호출하게 됩니다. 해당 함수는 앞서 Callback 함수를 등록할 당시 설정되어 있습니다.

/frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

        if (mHwc->registerProcs) {
            mCBContext->hwc = this;
            mCBContext->procs.invalidate = &hook_invalidate;
            mCBContext->procs.vsync = &hook_vsync;
            if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))
                mCBContext->procs.hotplug = &hook_hotplug;
            else
                mCBContext->procs.hotplug = NULL;
            memset(mCBContext->procs.zero, 0, sizeof(mCBContext->procs.zero));
            mHwc->registerProcs(mHwc, &mCBContext->procs);
        }

Hardware에서 Framewokr를 호출하여 아래와 같이 함수가 실행됩니다.

/frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

void HWComposer::hook_vsync(const struct hwc_procs* procs, int disp,
        int64_t timestamp) {
    cb_context* ctx = reinterpret_cast<cb_context*>(
            const_cast<hwc_procs_t*>(procs));
    ctx->hwc->vsync(disp, timestamp);
}

다음으로 vsync 함수를 보도록 하겠습니다.

/frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

void HWComposer::vsync(int disp, int64_t timestamp) {
    if (uint32_t(disp) < HWC_NUM_PHYSICAL_DISPLAY_TYPES) {
        {
            Mutex::Autolock _l(mLock);
 
            // There have been reports of HWCs that signal several vsync events
            // with the same timestamp when turning the display off and on. This
            // is a bug in the HWC implementation, but filter the extra events
            // out here so they don't cause havoc downstream.
            if (timestamp == mLastHwVSync[disp]) {
                ALOGW("Ignoring duplicate VSYNC event from HWC (t=%lld)",
                        timestamp);
                return;
            }
 
            mLastHwVSync[disp] = timestamp;
        }
 
        char tag[16];
        snprintf(tag, sizeof(tag), "HW_VSYNC_%1u", disp);
        ATRACE_INT(tag, ++mVSyncCounts[disp] & 1);
 
        mEventHandler.onVSyncReceived(disp, timestamp);
    }
}

 여기서 mEventHandler는 SurfaceFlinger가 상속하고 있으며 실제로 SurfaceFlinger을 지칭합니다. mEventHandler에 의해 SurfaceFlinger가 실행됩니다.

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

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);
    }
}
 
....
 
void SurfaceFlinger::enableHardwareVsync() {
    Mutex::Autolock _l(mHWVsyncLock);
    if (!mPrimaryHWVsyncEnabled && mHWVsyncAvailable) {
        mPrimaryDispSync.beginResync();
        //eventControl(HWC_DISPLAY_PRIMARY, SurfaceFlinger::EVENT_VSYNC, true);
        mEventControlThread->setVsyncEnabled(true);
        mPrimaryHWVsyncEnabled = true;
    }
}

 SurfaceFlinger가 mEventControlThread를 호출하고 있는 모습입니다. 이어서 소스코드를 보도록 하겠습니다.

/frameworks/native/services/surfaceflinger/EventControlThread.cpp

void EventControlThread::setVsyncEnabled(bool enabled) {
    Mutex::Autolock lock(mMutex);
    mVsyncEnabled = enabled;
    mCond.signal();
}
 
bool EventControlThread::threadLoop() {
    Mutex::Autolock lock(mMutex);
 
    bool vsyncEnabled = mVsyncEnabled;
 
    mFlinger->eventControl(HWC_DISPLAY_PRIMARY, SurfaceFlinger::EVENT_VSYNC,
            mVsyncEnabled);
 
    while (true) {
        status_t err = mCond.wait(mMutex);
        if (err != NO_ERROR) {
            ALOGE("error waiting for new events: %s (%d)",
                strerror(-err), err);
            return false;
        }
 
        if (vsyncEnabled != mVsyncEnabled) {
            mFlinger->eventControl(HWC_DISPLAY_PRIMARY,
                    SurfaceFlinger::EVENT_VSYNC, mVsyncEnabled);
            vsyncEnabled = mVsyncEnabled;
        }
    }
 
    return false;
}

 여기서 thread를 통해 SurfaceFlinger의 eventControl() 함수가 호출됨을 보실 수 있습니다.

/frameworks/native/services/surfaceflinger/SurfaceFlinger.h

    /* ------------------------------------------------------------------------
     * H/W composer
     */
 
    HWComposer& getHwComposer() const { return *mHwc; }

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

void SurfaceFlinger::eventControl(int disp, int event, int enabled) {
    ATRACE_CALL();
    getHwComposer().eventControl(disp, event, enabled);
}
 
 

 이전에 새로 생성하였던 HWComposer 클래스의 eventControl() 함수가 호출된다.

/frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp

void HWComposer::eventControl(int disp, int event, int enabled) {
    if (uint32_t(disp)>31 || !mAllocatedDisplayIDs.hasBit(disp)) {
        ALOGD("eventControl ignoring event %d on unallocated disp %d (en=%d)",
              event, disp, enabled);
        return;
    }
    if (event != EVENT_VSYNC) {
        ALOGW("eventControl got unexpected event %d (disp=%d en=%d)",
              event, disp, enabled);
        return;
    }
    status_t err = NO_ERROR;
    if (mHwc && !mDebugForceFakeVSync) {
        // NOTE: we use our own internal lock here because we have to call
        // into the HWC with the lock held, and we want to make sure
        // that even if HWC blocks (which it shouldn't), it won't
        // affect other threads.
        Mutex::Autolock _l(mEventControlLock);
        const int32_t eventBit = 1UL << event;
        const int32_t newValue = enabled ? eventBit : 0;
        const int32_t oldValue = mDisplayData[disp].events & eventBit;
        if (newValue != oldValue) {
            ATRACE_CALL();
            err = mHwc->eventControl(mHwc, disp, event, enabled);
            if (!err) {
                int32_t& events(mDisplayData[disp].events);
                events = (events & ~eventBit) | newValue;
            }
        }
        // error here should not happen -- not sure what we should
        // do if it does.
        ALOGE_IF(err, "eventControl(%d, %d) failed %s",
                event, enabled, strerror(-err));
    }
 
    if (err == NO_ERROR && mVSyncThread != NULL) {
        mVSyncThread->setEnabled(enabled);
    }
}

여기서 다시 hardware library 단계에서 함수가 호출되고 있는 것을 보실 수 있습니다.

/hardware/qcom/display/msm8960/libhwcomposer/hwc.cpp

static int hwc_eventControl(struct hwc_composer_device_1* dev, int dpy,
                             int event, int enable)
{
    int ret = 0;
    hwc_context_t* ctx = (hwc_context_t*)(dev);
    if(!ctx->dpyAttr[dpy].isActive) {
        ALOGE("Display is blanked - Cannot %s vsync",
              enable ? "enable" : "disable");
        return -EINVAL;
    }
 
    switch(event) {
        case HWC_EVENT_VSYNC:
            if (ctx->vstate.enable == enable)
                break;
            ret = hwc_vsync_control(ctx, dpy, enable);
            if(ret == 0)
                ctx->vstate.enable = !!enable;
            ALOGD_IF (VSYNC_DEBUG, "VSYNC state changed to %s",
                      (enable)?"ENABLED":"DISABLED");
            break;
        default:
            ret = -EINVAL;
    }
    return ret;
}

/hardware/qcom/display/msm8960/libhwcomposer/hwc_vsync.cpp

int hwc_vsync_control(hwc_context_t* ctx, int dpy, int enable)
{
    int ret = 0;
    if(!ctx->vstate.fakevsync &&
       ioctl(ctx->dpyAttr[dpy].fd, MSMFB_OVERLAY_VSYNC_CTRL,
             &enable) < 0) {
        ALOGE("%s: vsync control failed. Dpy=%d, enable=%d : %s",
              __FUNCTION__, dpy, enable, strerror(errno));
        ret = -errno;
    }
    return ret;
}

출저 : http://blog.csdn.net/fuyajun01/article/details/41411715