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Android P 图形显示系统（十二） BufferQueue（三）

作者：互联网

文章目录

Buffer状态

对于生产者这边，BufferQueue的流程基本讲完了。简单说来，首先提需求，告诉BufferQueue需要什么样的Buffer，大小，格式，usage等等；然后dequeue Buffer出来，往Buffer里面绘制显示数据；绘制完成后，queue到BufferQueue里面，并通知消费者进行消费。如此不断的的dequeue，绘制，queue。

消费者这边的流程，我们还没有讲到。对于消费者来说，收到通知后，将从BufferQueue里面取queue过来的Buffer进行合成，合成完的Buffer再释放掉，这里的释放，是概念上的，并没有真正释放内存，只是让其返回队列，可以被再次dequeue。消费者这边也是不断的接通知，取buffer合成，然后释放，不断循环。
此图是Android官网对BufferQueue通信过程的描述，这很好的描述这个过程。
BufferQueue数据流

在Android 6.0及之前的版本，在这些通信过程中，都将Buffer的状态标记为具体的状态。这四个过程Buffer分别对应不同的四个状态：

目前，Buffer的状态，都是通过各个状态的Buffer的量来表示状态，对应的关系如下：

Buffer状态 mShared mDequeueCount mQueueCount mAcquireCount
FREE false 0 0 0
DEQUEUED false 1 0 0
QUEUED false 0 1 0
ACQUIRED false 0 0 1
SHARED true any any any

Buffer的状态在代码中用BufferState描述，BufferState的定义如下：

* frameworks/native/libs/gui/include/gui/BufferSlot.h

struct BufferState {

    BufferState()
    : mDequeueCount(0),
      mQueueCount(0),
      mAcquireCount(0),
      mShared(false) {
    }

    uint32_t mDequeueCount;
    uint32_t mQueueCount;
    uint32_t mAcquireCount;
    bool mShared;

    ... ...
};

前面讲解dequeueBuffer和queueBuffer流程时，BufferQueue有很多个队列，我们再来看一下BufferQueue中，几个队列间的关系。

BufferQueueCore中的定义如下：

* frameworks/native/libs/gui/include/gui/BufferQueueCore.h

class BufferQueueCore : public virtual RefBase {

    ... ...

    typedef Vector<BufferItem> Fifo;
    
    ... ...

    // mSlots is an array of buffer slots that must be mirrored on the producer
    // side. This allows buffer ownership to be transferred between the producer
    // and consumer without sending a GraphicBuffer over Binder. The entire
    // array is initialized to NULL at construction time, and buffers are
    // allocated for a slot when requestBuffer is called with that slot's index.
    BufferQueueDefs::SlotsType mSlots;

    // mQueue is a FIFO of queued buffers used in synchronous mode.
    Fifo mQueue;

    // mFreeSlots contains all of the slots which are FREE and do not currently
    // have a buffer attached.
    std::set<int> mFreeSlots;

    // mFreeBuffers contains all of the slots which are FREE and currently have
    // a buffer attached.
    std::list<int> mFreeBuffers;

    // mUnusedSlots contains all slots that are currently unused. They should be
    // free and not have a buffer attached.
    std::list<int> mUnusedSlots;

    // mActiveBuffers contains all slots which have a non-FREE buffer attached.
    std::set<int> mActiveBuffers;

我们从数学的角度来看看他们之间的关系：
mSlots的数组大小为NUM_BUFFER_SLOTS，但是其中，真正用起来的也只有MaxBufferCount个，其他的都不会被用到。所以，我们可以这么理解，mSlots是BufferQueue中实际流转起来的Buffer。

mSlots = mFreeBuffers + mActiveBuffers

对于整体而言：

NUM_BUFFER_SLOTS = mUnusedSlots + mFreeSlots + mFreeBuffers + mActiveBuffers

mSlots是BufferSlot的集合，BufferSlot定义如下：

struct BufferSlot {

    BufferSlot()
    : mGraphicBuffer(nullptr),
      mEglDisplay(EGL_NO_DISPLAY),
      mBufferState(),
      mRequestBufferCalled(false),
      mFrameNumber(0),
      mEglFence(EGL_NO_SYNC_KHR),
      mFence(Fence::NO_FENCE),
      mAcquireCalled(false),
      mNeedsReallocation(false) {
    }

    // Buffer序号对应的Buffer
    sp<GraphicBuffer> mGraphicBuffer;

    // 创建EGLSyncKHR对象用
    EGLDisplay mEglDisplay;

    // Buffer序号当前的状态
    BufferState mBufferState;

    // mRequestBufferCalled 表示Producer确实已经调用requestBuffer
    bool mRequestBufferCalled;

    // mFrameNumber 表示该Buffer序号已经被queue的次数.  主要用于dequeueBuffer时，遵从LRU，这很有用，因为buffer 变FREE时，可能release Fence还没有发信号出来。
    uint64_t mFrameNumber;

    // 现在已经被mFence替换了，基本不用
    EGLSyncKHR mEglFence;

    // mFence 是同步的一种方式，上一个owner使用完Buffer后，需要发信号出来，下一个owner才可以使用。
    sp<Fence> mFence;

    // 表示Buffer已经被Consumer取走
    bool mAcquireCalled;

    // 表示Buffer需要重新分配，需要设置BUFFER_NEEDS_REALLOCATION 通知Producer，不要用原来的缓存的Buffer
    bool mNeedsReallocation;
};

看完Buffer的状态后，再回头去看看前面介绍的dequeueBuffer和queueBuffer，是不是就很好理解了。

我们再来看看BufferQueue的工作模式，BufferQueue可以工作在几个模式：

            if (mDequeueTimeout >= 0) {
                status_t result = mCore->mDequeueCondition.waitRelative(
                        mCore->mMutex, mDequeueTimeout);
                if (result == TIMED_OUT) {
                    return result;
                }
            } else {
                mCore->mDequeueCondition.wait(mCore->mMutex);
            }

        if (tryAgain) {
            if ((mCore->mDequeueBufferCannotBlock || mCore->mAsyncMode) &&
                    (acquiredCount <= mCore->mMaxAcquiredBufferCount)) {
                return WOULD_BLOCK;
            }

mAsyncMode是通过BufferQueueProducer的setAsyncMode函数设置的，从Producer调用过来，受Producer控制。

mDequeueBufferCannotBlock则是在Producer 连接到BufferQueue时，根据条件判断的，具体逻辑如下：

status_t BufferQueueProducer::connect(const sp<IProducerListener>& listener,
        int api, bool producerControlledByApp, QueueBufferOutput *output) {
    ... ...

    if (mDequeueTimeout < 0) {
        mCore->mDequeueBufferCannotBlock =
                mCore->mConsumerControlledByApp && producerControlledByApp;
    }

    mCore->mAllowAllocation = true;
    VALIDATE_CONSISTENCY();
    return status;
}

现在，再回头去看看前面介绍的dequeueBuffer和queueBuffer，是不是就更好理解了。

acquireBuffer流程

Buffer queue到BufferQueue中后，将通知消费者去消费。消费时，通过acquireBuffer来获取Buffer，我们且不管acquireBuffer是什么地方调的，我们先来看BufferQueue中acquireBuffer的处理流程。

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

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

    int numDroppedBuffers = 0;
    sp<IProducerListener> listener;
    {
        Mutex::Autolock lock(mCore->mMutex);

        int numAcquiredBuffers = 0;
        for (int s : mCore->mActiveBuffers) {
            if (mSlots[s].mBufferState.isAcquired()) {
                ++numAcquiredBuffers;
            }
        }
        if (numAcquiredBuffers >= mCore->mMaxAcquiredBufferCount + 1) {
            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.
        if (mCore->mQueue.empty() && !sharedBufferAvailable) {
            return NO_BUFFER_AVAILABLE;
        }

如果有Buffer或是共享Buffer模式，继续～

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

        BufferQueueCore::Fifo::iterator front(mCore->mQueue.begin());

        if (expectedPresent != 0 && !mCore->mQueue.empty()) {
            const int MAX_REASONABLE_NSEC = 1000000000ULL; // 1 second

            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 (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());

                if (!front->mIsStale) {
                    // Front buffer is still in mSlots, so mark the slot as free
                    mSlots[front->mSlot].mBufferState.freeQueued();

                    if (!mCore->mSharedBufferMode &&
                            mSlots[front->mSlot].mBufferState.isFree()) {
                        mSlots[front->mSlot].mBufferState.mShared = false;
                    }

                    // Don't put the shared buffer on the free list
                    if (!mSlots[front->mSlot].mBufferState.isShared()) {
                        mCore->mActiveBuffers.erase(front->mSlot);
                        mCore->mFreeBuffers.push_back(front->mSlot);
                    }

                    listener = mCore->mConnectedProducerListener;
                    ++numDroppedBuffers;
                }

                mCore->mQueue.erase(front);
                front = mCore->mQueue.begin();
            }

            bool bufferIsDue = desiredPresent <= expectedPresent ||
                    desiredPresent > expectedPresent + MAX_REASONABLE_NSEC;
            bool consumerIsReady = maxFrameNumber > 0 ?
                    front->mFrameNumber <= maxFrameNumber : true;
            if (!bufferIsDue || !consumerIsReady) {

                return PRESENT_LATER;
            }

        }

这里主要做了一些几件事：

该丢弃的丢弃了，余下的就可以用来去显示了。

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

        int slot = BufferQueueCore::INVALID_BUFFER_SLOT;

        if (sharedBufferAvailable && mCore->mQueue.empty()) {
            // make sure the buffer has finished allocating before acquiring it
            mCore->waitWhileAllocatingLocked();

            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 {
            slot = front->mSlot;
            *outBuffer = *front;
        }

如果是共享Buffer模式，即使mQueue为空，也会把共享的Buffer返回去。其他情况下就返回，mQueue的第一个Buffer。

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

        ATRACE_BUFFER_INDEX(slot);

        if (!outBuffer->mIsStale) {
            mSlots[slot].mAcquireCalled = true;

            if (mCore->mQueue.empty()) {
                mSlots[slot].mBufferState.acquireNotInQueue();
            } else {
                mSlots[slot].mBufferState.acquire();
            }
            mSlots[slot].mFence = Fence::NO_FENCE;
        }

        if (outBuffer->mAcquireCalled) {
            outBuffer->mGraphicBuffer = NULL;
        }

        mCore->mQueue.erase(front);

        mCore->mDequeueCondition.broadcast();

        ATRACE_INT(mCore->mConsumerName.string(),
                static_cast<int32_t>(mCore->mQueue.size()));
        mCore->mOccupancyTracker.registerOccupancyChange(mCore->mQueue.size());

        VALIDATE_CONSISTENCY();
    }

    if (listener != NULL) {
        for (int i = 0; i < numDroppedBuffers; ++i) {
            listener->onBufferReleased();
        }
    }

    return NO_ERROR;
}

acquire到Buffer后，修改mSlots中对应Buffer序号的mBufferState状态。acquire的Buffer，需要从mQueue中 删掉。留意这里的ATRACE_INT，这个在systrace分析时，非常有用。如果Buffer被丢弃了，可以通过Producer的监听者，去通知Producer Buffer已经被release掉了。

releaseBuffer流程分析

Consumer具体怎么消费的，我们暂时不管，我们先来看消费完成后，releaseBuffer的流程。

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

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 == NULL) {
        BQ_LOGE("releaseBuffer: slot %d out of range or fence %p NULL", slot,
                releaseFence.get());
        return BAD_VALUE;
    }

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

        // FrameNumber已经变，buffer已经被重新分配
        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();

        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);
            mCore->mFreeBuffers.push_back(slot);
        }

        listener = mCore->mConnectedProducerListener;
        BQ_LOGV("releaseBuffer: releasing slot %d", slot);

        mCore->mDequeueCondition.broadcast();
        VALIDATE_CONSISTENCY();
    } // Autolock scope

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

    return NO_ERROR;
}

就这么多～～

小结

本章主要通过测试应用，讲解ANativeWindow，Surface间的关系，Surface和Producer，Consumer间的关系；P应用怎么使用BufferQueue。讲解了BufferQueue相关的几个流程，dequeueBuffer，queueBuffer，acquireBuffer，releaseBuffer；以及Buffer的状态，DEQUEUED，QUEUED，ACQUIRED，FREE迁移。

标签：slot,BufferQueue,Buffer,mSlots,mCore,Android,mQueue,图形
来源： https://blog.csdn.net/chaojiangluo/article/details/100923125