roger-yu

题外话

我竟然已经写了这个系列的十一篇文章了,虽然内容很浅显,虽然内容很枯燥,虽然内容也许没营养,但我为自己的坚持点赞!


 

一、前言


前面的两篇文章,分别讲解了Producer的处理逻辑和queue buffer后通过FrameAvailableListener通知到Consumer的基本过程。

流程已经走到了BufferQueueConsumer::acquireBuffer中,所以这篇文章聚焦Consumer的一些处理逻辑。

还是把流程图贴上来

从流程图中看,这篇文章就是讲解右半部分的内容。

 

二、消费者-Consumer的相关逻辑


了解了 BufferQueueCore 和 BufferQueueProducer,接着看 BufferQueue 的最后一个元素:BufferQueueConsumer。

BufferQueueConsumer作为消费者的一个代表元素通过 acquireBuffer 来获取图像缓冲区,通过 releaseBuffer 来释放该缓冲区。

下面就分别看看 BufferQueueConsumer 中 acquireBuffer 和 releaseBuffer 两个操作的具体流程。

2.1 代码位置

/frameworks/native/libs/gui/BufferQueueConsumer.cpp

 

2.2 acquireBuffer的逻辑

先看 acquireBuffer 的过程,上源码

status_t BufferQueueConsumer::acquireBuffer(BufferItem* outBuffer,
        nsecs_t expectedPresent, uint64_t maxFrameNumber) {
    ATRACE_CALL();

    int numDroppedBuffers = 0;
    sp<IProducerListener> listener;
    {
        std::unique_lock<std::mutex> lock(mCore->mMutex);

        // Check that the consumer doesn't currently have the maximum number of
        // buffers acquired. We allow the max buffer count to be exceeded by one
        // buffer so that the consumer can successfully set up the newly acquired
        // buffer before releasing the old one.
        
        // 检查acquire的buffer的数量是否超出了限制
        int numAcquiredBuffers = 0;
        for (int s : mCore->mActiveBuffers) {
            if (mSlots[s].mBufferState.isAcquired()) {
                ++numAcquiredBuffers;
            }
        }
        const bool acquireNonDroppableBuffer = mCore->mAllowExtraAcquire &&
                numAcquiredBuffers == mCore->mMaxAcquiredBufferCount + 1;
        if (numAcquiredBuffers >= mCore->mMaxAcquiredBufferCount + 1 &&
            !acquireNonDroppableBuffer) {
            BQ_LOGE("acquireBuffer: max acquired buffer count reached: %d (max %d)",
                    numAcquiredBuffers, mCore->mMaxAcquiredBufferCount);
            return INVALID_OPERATION;
        }

        bool sharedBufferAvailable = mCore->mSharedBufferMode &&
                mCore->mAutoRefresh && mCore->mSharedBufferSlot !=
                BufferQueueCore::INVALID_BUFFER_SLOT;

        // In asynchronous mode the list is guaranteed to be one buffer deep,
        // while in synchronous mode we use the oldest buffer.
        // 检查BufferQueueCore中的mQueue队列是否为空
        if (mCore->mQueue.empty() && !sharedBufferAvailable) {
            return NO_BUFFER_AVAILABLE;
        }
        // 获取BufferQueueCore中的mQueue队列的迭代器
        BufferQueueCore::Fifo::iterator front(mCore->mQueue.begin());
        
        // If expectedPresent is specified, we may not want to return a buffer yet.
        // If it's specified and there's more than one buffer queued, we may want
        // to drop a buffer.
        // Skip this if we're in shared buffer mode and the queue is empty,
        // since in that case we'll just return the shared buffer.
        if (expectedPresent != 0 && !mCore->mQueue.empty()) {
            
            // expectedPresent表示期望这个buffer什么时候显示到屏幕上。
            // 如果buffer的期望显示时间小于expectedPresent,我们会acquire and return这个buffer
            // 如果我们不想显示它直到expectedPresent之后,可以返回PRESENT_LATER
            
            // The 'expectedPresent' argument indicates when the buffer is expected
            // to be presented on-screen. If the buffer's desired present time is
            // earlier (less) than expectedPresent -- meaning it will be displayed
            // on time or possibly late if we show it as soon as possible -- we
            // acquire and return it. If we don't want to display it until after the
            // expectedPresent time, we return PRESENT_LATER without acquiring it.
            //
            
            // 安全起见,如果expectedPresent超过了buffer的期望显示时间1秒,我们会推迟acquire
            // To be safe, we don't defer acquisition if expectedPresent is more
            // than one second in the future beyond the desired present time
            // (i.e., we'd be holding the buffer for a long time).
            //
            // NOTE: Code assumes monotonic time values from the system clock
            // are positive.
            
            // 检查是否需要丢弃一些帧,主要是判断timestamps & expectedPresent
            // Start by checking to see if we can drop frames. We skip this check if
            // the timestamps are being auto-generated by Surface. If the app isn't
            // generating timestamps explicitly, it probably doesn't want frames to
            // be discarded based on them.
            while (mCore->mQueue.size() > 1 && !mCore->mQueue[0].mIsAutoTimestamp) {
                const BufferItem& bufferItem(mCore->mQueue[1]);

                // If dropping entry[0] would leave us with a buffer that the
                // consumer is not yet ready for, don't drop it.
                if (maxFrameNumber && bufferItem.mFrameNumber > maxFrameNumber) {
                    break;
                }

                // If entry[1] is timely, drop entry[0] (and repeat). We apply an
                // additional criterion here: we only drop the earlier buffer if our
                // desiredPresent falls within +/- 1 second of the expected present.
                // Otherwise, bogus desiredPresent times (e.g., 0 or a small
                // relative timestamp), which normally mean "ignore the timestamp
                // and acquire immediately", would cause us to drop frames.
                //
                // We may want to add an additional criterion: don't drop the
                // earlier buffer if entry[1]'s fence hasn't signaled yet.
                nsecs_t desiredPresent = bufferItem.mTimestamp;
                
                // desiredPresent比expectedPresent小了1 second多,或desiredPresent大于expectedPresent
                if (desiredPresent < expectedPresent - MAX_REASONABLE_NSEC ||
                        desiredPresent > expectedPresent) {
                    // This buffer is set to display in the near future, or
                    // desiredPresent is garbage. Either way we don't want to drop
                    // the previous buffer just to get this on the screen sooner.
                    BQ_LOGV("acquireBuffer: nodrop desire=%" PRId64 " expect=%"
                            PRId64 " (%" PRId64 ") now=%" PRId64,
                            desiredPresent, expectedPresent,
                            desiredPresent - expectedPresent,
                            systemTime(CLOCK_MONOTONIC));
                    break;
                }

                BQ_LOGV("acquireBuffer: drop desire=%" PRId64 " expect=%" PRId64
                        " size=%zu",
                        desiredPresent, expectedPresent, mCore->mQueue.size());
                // 处理要drop的buffer
                if (!front->mIsStale) {
                    // Front buffer is still in mSlots, so mark the slot as free 
                    // 对应的BufferSlot设置为FREE状态
                    mSlots[front->mSlot].mBufferState.freeQueued();

                    // After leaving shared buffer mode, the shared buffer will
                    // still be around. Mark it as no longer shared if this
                    // operation causes it to be free.
                    if (!mCore->mSharedBufferMode &&
                            mSlots[front->mSlot].mBufferState.isFree()) {
                        mSlots[front->mSlot].mBufferState.mShared = false;
                    }
                    
                    // mActiveBuffers :绑定了GraphicBuffer且状态为非FREE的BufferSlot集合;
                    // mFreeBuffers :绑定了GraphicBuffer且状态为FREE的BufferSlot集合;
                    
                    // Don't put the shared buffer on the free list
                    if (!mSlots[front->mSlot].mBufferState.isShared()) {
                        mCore->mActiveBuffers.erase(front->mSlot); // 从mActiveBuffers删除
                        mCore->mFreeBuffers.push_back(front->mSlot);// 添加进mFreeBuffers
                    }

                    if (mCore->mBufferReleasedCbEnabled) {
                        listener = mCore->mConnectedProducerListener; // 设置生产者的监听器
                    }
                    ++numDroppedBuffers; // 计数加1,记录drop了几个buffer
                }

                mCore->mQueue.erase(front);// 从mQueue中删除
                front = mCore->mQueue.begin();// 重置front,进入下一次while循环
            }

            // See if the front buffer is ready to be acquired
            nsecs_t desiredPresent = front->mTimestamp;
            bool bufferIsDue = desiredPresent <= expectedPresent ||
                    desiredPresent > expectedPresent + MAX_REASONABLE_NSEC;
            bool consumerIsReady = maxFrameNumber > 0 ?
                    front->mFrameNumber <= maxFrameNumber : true;
            if (!bufferIsDue || !consumerIsReady) {
                BQ_LOGV("acquireBuffer: defer desire=%" PRId64 " expect=%" PRId64
                        " (%" PRId64 ") now=%" PRId64 " frame=%" PRIu64
                        " consumer=%" PRIu64,
                        desiredPresent, expectedPresent,
                        desiredPresent - expectedPresent,
                        systemTime(CLOCK_MONOTONIC),
                        front->mFrameNumber, maxFrameNumber);
                ATRACE_NAME("PRESENT_LATER");
                return PRESENT_LATER;
            }

            BQ_LOGV("acquireBuffer: accept desire=%" PRId64 " expect=%" PRId64 " "
                    "(%" PRId64 ") now=%" PRId64, desiredPresent, expectedPresent,
                    desiredPresent - expectedPresent,
                    systemTime(CLOCK_MONOTONIC));
        }
        // 走到这里就说明:该丢弃的已经都丢弃了,余下的就可以拿去显示了。
        int slot = BufferQueueCore::INVALID_BUFFER_SLOT;

        if (sharedBufferAvailable && mCore->mQueue.empty()) {
            // make sure the buffer has finished allocating before acquiring it
            // 共享Buffer模式下处理
            mCore->waitWhileAllocatingLocked(lock);

            slot = mCore->mSharedBufferSlot;

            // Recreate the BufferItem for the shared buffer from the data that
            // was cached when it was last queued.
            outBuffer->mGraphicBuffer = mSlots[slot].mGraphicBuffer;
            outBuffer->mFence = Fence::NO_FENCE;
            outBuffer->mFenceTime = FenceTime::NO_FENCE;
            outBuffer->mCrop = mCore->mSharedBufferCache.crop;
            outBuffer->mTransform = mCore->mSharedBufferCache.transform &
                    ~static_cast<uint32_t>(
                    NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY);
            outBuffer->mScalingMode = mCore->mSharedBufferCache.scalingMode;
            outBuffer->mDataSpace = mCore->mSharedBufferCache.dataspace;
            outBuffer->mFrameNumber = mCore->mFrameCounter;
            outBuffer->mSlot = slot;
            outBuffer->mAcquireCalled = mSlots[slot].mAcquireCalled;
            outBuffer->mTransformToDisplayInverse =
                    (mCore->mSharedBufferCache.transform &
                    NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY) != 0;
            outBuffer->mSurfaceDamage = Region::INVALID_REGION;
            outBuffer->mQueuedBuffer = false;
            outBuffer->mIsStale = false;
            outBuffer->mAutoRefresh = mCore->mSharedBufferMode &&
                    mCore->mAutoRefresh;
        } else if (acquireNonDroppableBuffer && front->mIsDroppable) {
            BQ_LOGV("acquireBuffer: front buffer is not droppable");
            return NO_BUFFER_AVAILABLE;
        } else {
            // 从front获取对应的slot index
            slot = front->mSlot;
            *outBuffer = *front;
        }

        ATRACE_BUFFER_INDEX(slot);

        BQ_LOGV("acquireBuffer: acquiring { slot=%d/%" PRIu64 " buffer=%p }",
                slot, outBuffer->mFrameNumber, outBuffer->mGraphicBuffer->handle);

        if (!outBuffer->mIsStale) {
            mSlots[slot].mAcquireCalled = true;
            // Don't decrease the queue count if the BufferItem wasn't
            // previously in the queue. This happens in shared buffer mode when
            // the queue is empty and the BufferItem is created above.
            if (mCore->mQueue.empty()) {
                mSlots[slot].mBufferState.acquireNotInQueue();
            } else {
                // 将BufferState状态改为acquire
                mSlots[slot].mBufferState.acquire();
            }
            mSlots[slot].mFence = Fence::NO_FENCE;
        }

        // If the buffer has previously been acquired by the consumer, set
        // mGraphicBuffer to NULL to avoid unnecessarily remapping this buffer
        // on the consumer side
        if (outBuffer->mAcquireCalled) {
            outBuffer->mGraphicBuffer = nullptr;
        }
        //将该Buffer从mQueue中移除
        mCore->mQueue.erase(front);

        // We might have freed a slot while dropping old buffers, or the producer
        // may be blocked waiting for the number of buffers in the queue to
        // decrease.
        mCore->mDequeueCondition.notify_all();

        ATRACE_INT(mCore->mConsumerName.string(),
                static_cast<int32_t>(mCore->mQueue.size()));
#ifndef NO_BINDER
        mCore->mOccupancyTracker.registerOccupancyChange(mCore->mQueue.size());
#endif
        VALIDATE_CONSISTENCY();
    }
    // 回调,通知生产者
    if (listener != nullptr) {
        for (int i = 0; i < numDroppedBuffers; ++i) {
            listener->onBufferReleased();
        }
    }

    return NO_ERROR;
}

 


acquireBuffer 函数中的逻辑也非常的清晰,源码中也做了详细注释。

主要就是这几件事情:

  1. 判断 BufferQueueCore 中的 mQueue 是否为空,mQueue 就是前面 BufferQueueProducer 调用 queueBuffer 函数时,将缓冲区入队的容器;
  2. 取出对应的 BufferSlot(会有一些判断规则,舍弃一些buffer);
  3. 将 BufferState 改为 acquire 状态;
  4. 将该 Buffer 从 mQueue 中移除;


2.3 消费者acquire拿到buffer后又是怎样通知release buffer呢?

要回答这个问题,我们需要在回到调用acquireBuffer的地方,即 BLASTBufferQueue::processNextBufferLocked 函数中,先看其代码:

void BLASTBufferQueue::processNextBufferLocked(bool useNextTransaction) {
    ......
    SurfaceComposerClient::Transaction localTransaction;
    bool applyTransaction = true;
    SurfaceComposerClient::Transaction* t = &localTransaction;

    // acquireBuffer获取要处理的buffer
    BufferItem bufferItem;
    status_t status =
            mBufferItemConsumer->acquireBuffer(&bufferItem, 0 /* expectedPresent */, false);
    ......
    // 拿到了实际的GraphicBuffer了
    auto buffer = bufferItem.mGraphicBuffer;
    mNumFrameAvailable--;
    
    // 某些情况下,直接releaseBuffer而无需送SurfaceFlinger合成显示

    mLastAcquiredFrameNumber = bufferItem.mFrameNumber;
    ReleaseCallbackId releaseCallbackId(buffer->getId(), mLastAcquiredFrameNumber);
    mSubmitted[releaseCallbackId] = bufferItem;

    ....

    // Ensure BLASTBufferQueue stays alive until we receive the transaction complete callback.
    incStrong((void*)transactionCallbackThunk);

    // release buffer的回到函数
    auto releaseBufferCallback =
            std::bind(releaseBufferCallbackThunk, wp<BLASTBufferQueue>(this) /* callbackContext */,
                      std::placeholders::_1, std::placeholders::_2, std::placeholders::_3,
                      std::placeholders::_4);
    t->setBuffer(mSurfaceControl, buffer, releaseCallbackId, releaseBufferCallback);

    ......


    if (applyTransaction) {
        t->setApplyToken(mApplyToken).apply();
    }
}

上述代码,做了比较多的简化,只保留我认为比较重要的部分。

  1. 调用acquireBuffer获取一个BufferItem;
  2. 取出GraphicBuffer -- auto buffer = bufferItem.mGraphicBuffer;
  3. 通过事务Transaction来向SurfaceFlinger提交Buffer与图层的属性;

t->setBuffer(mSurfaceControl, buffer, releaseCallbackId, releaseBufferCallback);

对于setBuffer,就是设置传递给SF的buffer,并指定了一个releaseBufferCallback,暂时可以理解为SF消费完这个buffer,就会通过这个callback通知来释放这个buffer。


本文作者@二的次方  2022-03-23 发布于博客园


在acquireBuffer中加入log,打印调用堆栈信息,如下:

11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#00 pc 000580ff  /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, android::sp<android::Fence> const&, void*, void*)+130)
11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#01 pc 00059117  /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, void*, void*, android::sp<android::Fence> const&)+30)
11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#02 pc 00076d27  /system/lib/libgui.so (android::ConsumerBase::releaseBufferLocked(int, android::sp<android::GraphicBuffer>, void*, void*)+134)
11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#03 pc 0007580d  /system/lib/libgui.so (android::BufferItemConsumer::releaseBuffer(android::BufferItem const&, android::sp<android::Fence> const&)+140)
11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#04 pc 0006c467  /system/lib/libgui.so (android::BLASTBufferQueue::releaseBufferCallback(android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)+1362)
11-13 01:23:59.275  3016  3030 E BufferQueueConsumer: stackdump:#05 pc 0006d827  /system/lib/libgui.so (android::releaseBufferCallbackThunk(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)+62)
11-13 01:23:59.276  3016  3030 E BufferQueueConsumer: stackdump:#06 pc 0007039b  /system/lib/libgui.so (std::__1::__function::__func<std::__1::__bind<void (&)(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int), android::wp<android::BLASTBufferQueue>, std::__1::placeholders::__ph<1> const&, std::__1::placeholders::__ph<2> const&, std::__1::placeholders::__ph<3> const&, std::__1::placeholders::__ph<4> const&>, std::__1::allocator<std::__1::__bind<void (&)(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int), android::wp<android::BLASTBufferQueue>, std::__1::placeholders::__ph<1> const&, std::__1::placeholders::__ph<2> const&, std::__1::placeholders::__ph<3> const&, std::__1::placeholders::__ph<4> const&> >, void (android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)>::operator()(android::ReleaseCallbackId const&, android::sp<android::Fence> const&,
11-13 01:23:59.276  3016  3030 E BufferQueueConsumer: stackdump:#07 pc 000a7d47  /system/lib/libgui.so (android::TransactionCompletedListener::onTransactionCompleted(android::ListenerStats)+3382)
11-13 01:23:59.276  3016  3030 E BufferQueueConsumer: stackdump:#08 pc 000925a5  /system/lib/libgui.so (int android::SafeBnInterface<android::ITransactionCompletedListener>::callLocalAsync<void (android::ITransactionCompletedListener::*)(android::ListenerStats)>(android::Parcel const&, android::Parcel*, void (android::ITransactionCompletedListener::*)(android::ListenerStats))+204)
11-13 01:23:59.276  3016  3030 E BufferQueueConsumer: stackdump:#09 pc 00028ddb  /system/lib/libbinder.so (android::BBinder::transact(unsigned int, android::Parcel const&, android::Parcel*, unsigned int)+162)

 

看上面的调用栈,是不是一目了然,从遥远的Binder来的神秘信息触发了这一些列的事件:

>>> releaseBufferCallbackThunk

>>> BLASTBufferQueue::releaseBufferCallback

>>> BufferItemConsumer::releaseBuffer

>>> ConsumerBase::releaseBufferLocked

>>> BufferQueueConsumer::releaseBuffer

 

2.4 releaseBuffer的逻辑

老规矩,直接看代码,是不是很简单啊!

status_t BufferQueueConsumer::releaseBuffer(int slot, uint64_t frameNumber,
        const sp<Fence>& releaseFence, EGLDisplay eglDisplay,
        EGLSyncKHR eglFence) {
    ATRACE_CALL();
    ATRACE_BUFFER_INDEX(slot);

    if (slot < 0 || slot >= BufferQueueDefs::NUM_BUFFER_SLOTS ||
            releaseFence == nullptr) {
        BQ_LOGE("releaseBuffer: slot %d out of range or fence %p NULL", slot,
                releaseFence.get());
        return BAD_VALUE;
    }

    sp<IProducerListener> listener;
    { // Autolock scope
        std::lock_guard<std::mutex> lock(mCore->mMutex);

        // If the frame number has changed because the buffer has been reallocated,
        // we can ignore this releaseBuffer for the old buffer.
        // Ignore this for the shared buffer where the frame number can easily
        // get out of sync due to the buffer being queued and acquired at the
        // same time.
        if (frameNumber != mSlots[slot].mFrameNumber &&
                !mSlots[slot].mBufferState.isShared()) {
            return STALE_BUFFER_SLOT;
        }

        if (!mSlots[slot].mBufferState.isAcquired()) {
            BQ_LOGE("releaseBuffer: attempted to release buffer slot %d "
                    "but its state was %s", slot,
                    mSlots[slot].mBufferState.string());
            return BAD_VALUE;
        }

        mSlots[slot].mEglDisplay = eglDisplay;
        mSlots[slot].mEglFence = eglFence;
        mSlots[slot].mFence = releaseFence;
        mSlots[slot].mBufferState.release();//置为FREE状态

        // After leaving shared buffer mode, the shared buffer will
        // still be around. Mark it as no longer shared if this
        // operation causes it to be free.
        if (!mCore->mSharedBufferMode && mSlots[slot].mBufferState.isFree()) {
            mSlots[slot].mBufferState.mShared = false;
        }
        // Don't put the shared buffer on the free list.
        if (!mSlots[slot].mBufferState.isShared()) {
            mCore->mActiveBuffers.erase(slot);// 从mActiveBuffers中删除
            mCore->mFreeBuffers.push_back(slot);//加入到mFreeBuffers中
        }

        if (mCore->mBufferReleasedCbEnabled) {
            listener = mCore->mConnectedProducerListener; / 设置listener
        }
        BQ_LOGV("releaseBuffer: releasing slot %d", slot);
        // 唤醒等待的线程
        mCore->mDequeueCondition.notify_all();
        VALIDATE_CONSISTENCY();
    } // Autolock scope

    // Call back without lock held
    if (listener != nullptr) {
        listener->onBufferReleased(); //通知producer
    }

    return NO_ERROR;
}

releaseBuffer方法的流程相对简单:

  • slot就是需要释放的BufferSlot的序号;

  • Buffer的FrameNumber变了,可能Buffer已经重新分配,这个是不用管;

  • 只能释放acquire状态的buffer序号,释放后是Buffer放会mFreeBuffers中;

  • releaseFence,从consumer那边传过来,producer可以dequeue mFreeBuffers中的buffer,但是只有releaseFence发信号出来后,consumer才真正用完,producer才可以写;

  • 最后通过listener通知producer。

 

2.5 ProducerListener是怎样工作的?

在前面的讲解中,有几处都有出现 listener->onBufferReleased() ,意思是通知producer有buffer释放了。这个listener是在哪里设置的?onBufferReleased又做了哪些工作呢?接下来分析

定义:/frameworks/native/libs/gui/include/gui/IProducerListener.h

首先我们看到这张类图:

1. 在BufferQueueCore中有成员 sp<IProducerListener> mConnectedProducerListener,它就是用来处理onBufferReleased事件的;

2. mConnectedProducerListener是在哪里被设置的呢?答案是 BufferQueueProducer::connect;

3. 根据调用栈来追踪:

11-13 01:20:19.388  2955  3013 E BufferQueueProducer: stackdump:#00 pc 0005e667  /system/lib/libgui.so (android::BufferQueueProducer::connect(android::sp<android::IProducerListener> const&, int, bool, android::IGraphicBufferProducer::QueueBufferOutput*)+1018)
11-13 01:20:19.388  2955  3013 E BufferQueueProducer: stackdump:#01 pc 0006ee41  /system/lib/libgui.so (android::BBQBufferQueueProducer::connect(android::sp<android::IProducerListener> const&, int, bool, android::IGraphicBufferProducer::QueueBufferOutput*)+176)
11-13 01:20:19.388  2955  3013 E BufferQueueProducer: stackdump:#02 pc 000a268b  /system/lib/libgui.so (android::Surface::connect(int, android::sp<android::IProducerListener> const&, bool)+138)
11-13 01:20:19.388  2955  3013 E BufferQueueProducer: stackdump:#03 pc 0009dd61  /system/lib/libgui.so (android::Surface::hook_perform(ANativeWindow*, int, ...)+128)

4. 可以肯定,是执行 native_window_api_connect() 一路走下来的。沿着这条路,看看listener是哪里产生的呢?

看起来是这个位置:

int Surface::connect(int api) {
    static sp<IProducerListener> listener = new StubProducerListener();
    return connect(api, listener);
}

5. 不过,奇怪的事情发生了,StubProducerListener看起来没有任何操作,乖乖(这一点还是很奇怪的

6. BLASTBufferQueue中还有使用AsyncProducerListener做一层封装,实现异步处理;

class StubProducerListener : public BnProducerListener {
public:
    virtual ~StubProducerListener();
    virtual void onBufferReleased() {}
    virtual bool needsReleaseNotify() { return false; }
};

三、小结


BufferQueue的运作流程到这里就算讲完了。生产者做了什么事情?消费者做了什么事情?图形缓存是怎样流转的?状态是怎样变化的?在几篇文章中基本上都有做了或简单或详细的介绍。通过BufferQueue的几篇文章,帮助自己建立起基本的逻辑框架,为我们后续研究和分析问题奠定基础。

 

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