一、前言
本文主要內容
1、bufferqueue 的四大流程；
2、簡要介紹四大流程的調用過程；
本文主要講解安卓圖形繪制中的buffer queue
頁面數據通過canvas/Open GL ES/vulkan繪制后，形成一幀buffer，交給buffer queue，buffer queue再把數據送到屏幕上顯示。所以buffer queue是圖形繪制中很核心的一環，它本身就是一個緩沖區池與隊列相結合的數據結構。
Buffer queue:圖像緩沖區，系統中絕大部分圖形數據都會緩沖于此。等待surfaceflinger組織進一步的合成和顯示
備注：本文摘錄的代碼只摘錄關鍵代碼和路徑，需讀者自行結合閱讀.由于本文的重點在bufferqueue，其它和bufferqueue相關的流程提到的時候會精簡列出，作為輔助理解。
二、Buffer queue
2.1、buffer queue的創建
bufferquque構建
//BufferQueue.cpp frameworks\native\libs\gui
void BufferQueue::createBufferQueue(sp<IGraphicBufferProducer>* outProducer,
sp<IGraphicBufferConsumer>* outConsumer,
bool consumerIsSurfaceFlinger) {
sp<BufferQueueCore> core(new BufferQueueCore());
sp<IGraphicBufferProducer> producer(new BufferQueueProducer(core, consumerIsSurfaceFlinger));
sp<IGraphicBufferConsumer> consumer(new BufferQueueConsumer(core));
*outProducer = producer;
*outConsumer = consumer;
}
構建調用流程
下面堆棧表示了底層surface創建bufferqueue的一個過程
04-20 11:45:54.739 674 707 E createBufferQueue: #00 pc 0004ca33 /system/lib/libgui.so (android::BufferQueue::createBufferQueue(android::sp<android::IGraphicBufferProducer>*, android::sp<android::IGraphicBufferConsumer>*, bool)+74)
04-20 11:45:54.739 674 707 E createBufferQueue: #01 pc 00068539 /system/lib/libsurfaceflinger.so (android::BufferQueueLayer::onFirstRef()+40)
04-20 11:45:54.739 674 707 E createBufferQueue: #02 pc 000d5ad1 /system/lib/libsurfaceflinger.so (android::surfaceflinger::DefaultFactory::createBufferQueueLayer(android::LayerCreationArgs const&)+52)
04-20 11:45:54.739 674 707 E createBufferQueue: #03 pc 000c11d5 /system/lib/libsurfaceflinger.so (android::SurfaceFlinger::createLayer(android::String8 const&, android::sp<android::Client> const&, unsigned int, unsigned int, int, unsigned int, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, android::sp<android::IBinder> const&, android::sp<android::Layer> const&, unsigned int*)+1056)
04-20 11:45:54.739 674 707 E createBufferQueue: #04 pc 0006e197 /system/lib/libsurfaceflinger.so (android::Client::createSurface(android::String8 const&, unsigned int, unsigned int, int, unsigned int, android::sp<android::IBinder> const&, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, unsigned int*)+122)
04-20 11:45:54.739 674 707 E createBufferQueue: #05 pc 0007b34f /system/lib/libgui.so (int android::SafeBnInterface<android::ISurfaceComposerClient>::MethodCaller<std::__1::tuple<android::String8 const&, unsigned int, unsigned int, int, unsigned int, android::sp<android::IGraphicBufferProducer> const&, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, unsigned int*> >::callHelper<android::SafeBnInterface<android::ISurfaceComposerClient>, int (android::ISurfaceComposerClient::*)(android::String8 const&, unsigned int, unsigned int, int, unsigned int, android::sp<android::IGraphicBufferProducer> const&, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, unsigned int*), std::__1::tuple<android::String8, unsigned int, unsigned int, int, unsigned int, android::sp<android::IGraphicBufferProducer>, android::LayerMetadata, android::sp<android::IBinder>, android::sp<android::IGraphicBufferProducer>, unsigned int>, 0u, 1u, 2u, 3u, 4u, 5u, 6u, 7u, 8u, 9u>(android
04-20 11:45:54.739 674 707 E createBufferQueue: #06 pc 00079e2f /system/lib/libgui.so (int android::SafeBnInterface<android::ISurfaceComposerClient>::callLocal<int (android::ISurfaceComposerClient::*)(android::String8 const&, unsigned int, unsigned int, int, unsigned int, android::sp<android::IBinder> const&, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, unsigned int*)>(android::Parcel const&, android::Parcel*, int (android::ISurfaceComposerClient::*)(android::String8 const&, unsigned int, unsigned int, int, unsigned int, android::sp<android::IBinder> const&, android::LayerMetadata, android::sp<android::IBinder>*, android::sp<android::IGraphicBufferProducer>*, unsigned int*))+158)
04-20 11:45:54.739 674 707 E createBufferQueue: #07 pc 00079d3f /system/lib/libgui.so (android::BnSurfaceComposerClient::onTransact(unsigned int, android::Parcel const&, android::Parcel*, unsigned int)+46)
BufferQueueCore:bufferqueue的實際實例，由他來銜接生產者和消費者
BufferQueueProducer:bufferqueue工廠模式的接口，生產者
BufferQueueConsumer:bufferqueue消費接口，消費者
BufferQueueCore 負責維護 BufferQueue 的基本數據結構，而 BufferQueueProducer 和 BufferQueueConsumer 則負責提供操作 BufferQueue 的基本接口。
2.2、buffer queue整體流程
2.2.1、流程概要
image.png
這張圖片清晰的表述的buffer queue的工作方式，兩大功能：生產buffer、消費buffer。
dequeueBuffer:向buffer queue請求一個緩沖區，并指定緩沖區的寬度、高度、像素格式和用法標志
queueBuffer:把緩沖區返回到隊列
acquireBuffer:獲取緩沖區內容
releaseBuffer:把數據返回到隊列
這里有兩次返回隊列，其實都是返回buffer queue的數據結構中。簡述兩個流程
1、dequeue、queue等于先申請緩沖區，并告訴producer申請成功，producer就把數據queue給buffer queue數據結構中
2、當consumer要的時候先去acquere獲取緩沖區內容，獲取后進行合成，合成完了需要釋放，就給回buffer queue來釋放
3、buffer queue操作數據時不會復制一份數據，只會操作數據本身。
通篇文章，我們就在理解BufferQueueProducer 和 BufferQueueConsumer如何操作上面這四個流程
requestBuffer
int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence, reqWidth, reqHeight,
reqFormat, reqUsage, &mBufferAge,
enableFrameTimestamps ? &frameTimestamps
: nullptr);
...
if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == nullptr) {
if (mReportRemovedBuffers && (gbuf != nullptr)) {
mRemovedBuffers.push_back(gbuf);
}
result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);
requestBuffer單獨提一下這個流程，當我們首次dequeueBuffer過后，surface會調用requestBuffer,requestBuffer的主要作用就是把GraphicBuffer傳給應用側，
這個動作只需要做一次，應用側拿到GraphicBuffer過后就可以把共享內存mmap到應用側的內存空間，然后才可以繪制圖像。
2.2.2、bufferqueue被調用流程
本小結主要目的是，在我們認識bufferqueu的是如何工作的同時，不脫離surfaceflinger整體框架。簡單提一下它是如果調用來的。以便我們更深入理解。
1、dequeueBuffer&&queueBuffer
當app創建window的時候，會創建surface來顯示。實則會創建底層的surface。底層surface創建時會hook住dequeueBuffer和queueBuffer方法。
surface持有mGraphicBufferProducer對象來創建bufferqueue
dequeueBuffer
方法位置：BufferQueueProducer::dequeueBuffer
04-20 09:48:38.909 6053 6263 E graphic dequeueBuffer: #00 pc 0006dea5 /system/lib/libgui.so (android::BpGraphicBufferProducer::dequeueBuffer(int*, android::sp<android::Fence>*, unsigned int, unsigned int, int, unsigned long long, unsigned long long*, android::FrameEventHistoryDelta*)+136)
04-20 09:48:38.909 6053 6263 E graphic dequeueBuffer: #01 pc 000842ed /system/lib/libgui.so (android::Surface::dequeueBuffer(ANativeWindowBuffer**, int*)+316)
04-20 09:48:38.909 6053 6263 E graphic dequeueBuffer: #02 pc 0015004b /system/lib/libhwui.so (android::uirenderer::renderthread::ReliableSurface::hook_dequeueBuffer(ANativeWindow*, int (*)(ANativeWindow*, ANativeWindowBuffer**, int*), void*, ANativeWindowBuffer**, int*)+42)
04-20 09:48:38.909 6053 6263 E graphic dequeueBuffer: #03 pc 00083181 /system/lib/libgui.so (android::Surface::hook_dequeueBuffer(ANativeWindow*, ANativeWindowBuffer**, int*)+60)
//接上面，binder調用到BufferQueueProducer
04-20 09:48:38.946 668 697 E dequeueBuffer: #00 pc 00052653 /system/lib/libgui.so (android::BufferQueueProducer::dequeueBuffer(int*, android::sp<android::Fence>*, unsigned int, unsigned int, int, unsigned long long, unsigned long long*, android::FrameEventHistoryDelta*)+250)
04-20 09:48:38.946 668 697 E dequeueBuffer: #01 pc 0006d117 /system/lib/libgui.so (android::BnGraphicBufferProducer::onTransact(unsigned int, android::Parcel const&, android::Parcel*, unsigned int)+374)
queueBuffer
方法位置：BufferQueueProducer::queueBuffer
04-20 09:48:38.987 6053 6263 E queueBuffer: #00 pc 0006e4e1 /system/lib/libgui.so (android::BpGraphicBufferProducer::queueBuffer(int, android::IGraphicBufferProducer::QueueBufferInput const&, android::IGraphicBufferProducer::QueueBufferOutput*)+128)
04-20 09:48:38.987 6053 6263 E queueBuffer: #01 pc 00084d83 /system/lib/libgui.so (android::Surface::queueBuffer(ANativeWindowBuffer*, int)+722)
04-20 09:48:38.987 6053 6263 E queueBuffer: #02 pc 00083259 /system/lib/libgui.so (android::Surface::hook_queueBuffer(ANativeWindow*, ANativeWindowBuffer*, int)+60)
//接上面，binder調用到BufferQueueProducer
04-20 09:48:39.017 668 697 E queueBuffer: #00 pc 00053877 /system/lib/libgui.so (android::BufferQueueProducer::queueBuffer(int, android::IGraphicBufferProducer::QueueBufferInput const&, android::IGraphicBufferProducer::QueueBufferOutput*)+266)
04-20 09:48:39.017 668 697 E queueBuffer: #01 pc 0006d345 /system/lib/libgui.so (android::BnGraphicBufferProducer::onTransact(unsigned int, android::Parcel const&, android::Parcel*, unsigned int)+932)
2、acquireBuffer&&releaseBuffer
1、我們知道系統的刷新由Vsync信號控制，一個Vsync信號來時。surfaceflinger代碼會走INVALIDATE/REFRESH,最終都是走refresh
2、下面接收vsync的MessageQueue隊列，實際就是surfaceflinger初始化時創建的mEventQueue。
3、EventThread通過Connection通知VSYNC信號到達時可以觸發回調通知MessageQueue
4、vsync這個流程對應bufferqueue流程中的acquireBuffer、releaseBuffer，調用流程堆棧下面已列出。
//SurfaceFlinger.cpp
void SurfaceFlinger::onMessageReceived(int32_t what, nsecs_t expectedVSyncTime) {
ATRACE_CALL();
switch (what) {
case MessageQueue::INVALIDATE: {
onMessageInvalidate(expectedVSyncTime);
break;
}
case MessageQueue::REFRESH: {
onMessageRefresh();
break;
}
}
}
acquireBuffer
方法位置：BufferQueueConsumer::acquireBuffer
04-19 19:33:38.926 666 666 E acquireBuffer: #00 pc 0004d34f /system/lib/libgui.so (android::BufferQueueConsumer::acquireBuffer(android::BufferItem*, long long, unsigned long long)+74)
04-19 19:33:38.926 666 666 E acquireBuffer: #01 pc 000645cf /system/lib/libgui.so (android::ConsumerBase::acquireBufferLocked(android::BufferItem*, long long, unsigned long long)+62)
04-19 19:33:38.926 666 666 E acquireBuffer: #02 pc 0007a7a1 /system/lib/libsurfaceflinger.so (android::FramebufferSurface::advanceFrame(bool)+112)
04-19 19:33:38.926 666 666 E acquireBuffer: #03 pc 000edf1f /system/lib/libsurfaceflinger.so (android::compositionengine::impl::RenderSurface::queueBuffer(android::base::unique_fd_impl<android::base::DefaultCloser>, bool)+358)
04-19 19:33:38.926 666 666 E acquireBuffer: #04 pc 000e46e7 /system/lib/libsurfaceflinger.so (android::compositionengine::impl::Output::finishFrame(android::compositionengine::CompositionRefreshArgs const&)+454)
04-19 19:33:38.926 666 666 E acquireBuffer: #05 pc 000de3e5 /system/lib/libsurfaceflinger.so (android::compositionengine::impl::Display::finishFrame(android::compositionengine::CompositionRefreshArgs const&)+72)
04-19 19:33:38.926 666 666 E acquireBuffer: #06 pc 000e3011 /system/lib/libsurfaceflinger.so (android::compositionengine::impl::Output::present(android::compositionengine::CompositionRefreshArgs const&)+92)
04-19 19:33:38.926 666 666 E acquireBuffer: #07 pc 000dcfa1 /system/lib/libsurfaceflinger.so (android::compositionengine::impl::CompositionEngine::present(android::compositionengine::CompositionRefreshArgs&)+144)
04-19 19:33:38.926 666 666 E acquireBuffer: #08 pc 000baf81 /system/lib/libsurfaceflinger.so (android::SurfaceFlinger::onMessageRefresh()+1280)
04-19 19:33:38.926 666 666 E acquireBuffer: #09 pc 000b8b1d /system/lib/libsurfaceflinger.so (android::SurfaceFlinger::onMessageReceived(int, long long)+52)
releaseBuffer
方法位置：BufferQueueConsumer::releaseBuffer
04-19 19:33:38.996 666 666 E releaseBuffer: #00 pc 0004e429 /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, android::sp<android::Fence> const&, void*, void*)+72)
04-19 19:33:38.996 666 666 E releaseBuffer: #01 pc 0004f3af /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, void*, void*, android::sp<android::Fence> const&)+36)
04-19 19:33:38.996 666 666 E releaseBuffer: #02 pc 0006498d /system/lib/libgui.so (android::ConsumerBase::releaseBufferLocked(int, android::sp<android::GraphicBuffer>, void*, void*)+140)
04-19 19:33:38.996 666 666 E releaseBuffer: #03 pc 00066cab /system/lib/libsurfaceflinger.so (android::BufferLayerConsumer::releasePendingBuffer()+78)
04-19 19:33:38.996 666 666 E releaseBuffer: #04 pc 00067b1d /system/lib/libsurfaceflinger.so (android::BufferQueueLayer::releasePendingBuffer(long long)+28)
04-19 19:33:38.996 666 666 E releaseBuffer: #05 pc 000bb0bb /system/lib/libsurfaceflinger.so (android::SurfaceFlinger::onMessageRefresh()+1594)
04-19 19:33:38.996 666 666 E releaseBuffer: #06 pc 000b8b1d /system/lib/libsurfaceflinger.so (android::SurfaceFlinger::onMessageReceived(int, long long)+52)
2.2.3、認識BufferQueueCore
BufferQueueCore是bufferqueue實現的具體類。BufferQueueCore管理了幾個數據結構。操作這幾個數據結構，來實現bufferqueue的dequebuffer等方法。
以dequeueBuffer過程來講，它就是向bufferqueue申請一個GraphicBuffer,用GraphicBuffer來繪制圖像。
BufferQueueCore中六個對象比較重要：mQueue、mSlot、FreeSlots、FreeBuffers、ActiveBuffers、UnusedBuffers 下面我會一一介紹
namespace android {
class IConsumerListener;
class IProducerListener;
class BufferQueueCore : public virtual RefBase {
private:
BufferQueueDefs::SlotsType mSlots;
Fifo mQueue;
std::set<int> mFreeSlots;
std::list<int> mFreeBuffers;
std::list<int> mUnusedSlots;
std::set<int> mActiveBuffers;
1、mQueue&&mSlot
我們先認識這兩個最核心的數據結構。
class BufferItem {
public:
enum { INVALID_BUFFER_SLOT = -1 };
BufferItem() : mGraphicBuffer(nullptr), mFence(Fence::NO_FENCE) {}
~BufferItem() {}
sp<GraphicBuffer> mGraphicBuffer;
sp<Fence> mFence;
Rect mCrop;
typedef Vector<BufferItem> Fifo;
Fifo mQueue;
namespace BufferQueueDefs {
typedef BufferSlot SlotsType[NUM_BUFFER_SLOTS];
}
BufferQueueDefs::SlotsType mSlots;
我們平時說的bufferqueue隊列其實就是說的BufferItem這個mQueue數據容器。BufferItem擁有GraphicBuffer對象
而mSlot是一個BufferSlot大小為NUM_BUFFER_SLOTS(等于64)的數組，BufferSlot主要用來綁定GraphicBuffer,BufferSlot和GraphicBuffer一一對應。
BufferSlot中有一個BufferState對象，它專門用來表示GraphicBuffer的狀態
struct BufferSlot {
BufferState mBufferState;
// BufferState tracks the states in which a buffer slot can be.
struct BufferState {
// All slots are initially FREE (not dequeued, queued, acquired, or shared).
BufferState()
: mDequeueCount(0),
mQueueCount(0),
mAcquireCount(0),
mShared(false) {
}
uint32_t mDequeueCount;
uint32_t mQueueCount;
uint32_t mAcquireCount;
bool mShared;
// A buffer can be in one of five states, represented as below:
//
// | 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 |
2、FreeSlots、FreeBuffers、ActiveBuffers、UnusedBuffers
知道了mQueue&&mSlot作用后，我們再來認識他們四個容易了,首先成立這樣一個等式
mSlots = mFreeSlots + mFreeBuffers + mActiveBuffers + mUnusedSlots
mSlots是所有BufferSlot，而其它四個BufferSlot，則表示不同的狀態的BufferSlot。
mFreeSlots：BufferSlot狀態為FREE，且沒有GraphicBuffer與之相綁定的slot集合
mFreeBuffers：BufferSlot狀態為FREE，且有GraphicBuffer與之相綁定的slot集合
mActiveBuffers：BufferSlot狀態不為FREE（即DEQUEUED、QUEUED、ACQUIRED、SHARED）的slot集合。既然狀態不是FREE，那么該BufferSlot必然有一個GraphicBuffer與之相綁定
mUnusedSlots：未參與使用的slot集合，由 mMaxBufferCount 決定
2.2.4、BufferQueueProducer生產者
生產者這邊，在bufferqueue流程中，主要負責dequeueBuffer、queueBuffer等流程
class BufferQueueProducer : public BnGraphicBufferProducer {
public:
friend class BufferQueue; // Needed to access binderDied
explicit BufferQueueProducer(const sp<BufferQueueCore>& core,
bool consumerIsSurfaceFlinger = false);
~BufferQueueProducer() override;
virtual status_t requestBuffer(int slot, sp<GraphicBuffer>* buf);
virtual status_t dequeueBuffer(int* outSlot, sp<Fence>* outFence, uint32_t width,
uint32_t height, PixelFormat format, uint64_t usage,
uint64_t* outBufferAge,
FrameEventHistoryDelta* outTimestamps) override;
virtual status_t queueBuffer(int slot,
const QueueBufferInput& input, QueueBufferOutput* output);
...//等方法
1、dequeueBuffer
status_t BufferQueueProducer::dequeueBuffer(int* outSlot, sp<android::Fence>* outFence,
uint32_t width, uint32_t height, PixelFormat format,
uint64_t usage, uint64_t* outBufferAge,
FrameEventHistoryDelta* outTimestamps) {
// 獲取下一個buffer slot,如果有freeSlot直接拿來用
while (found == BufferItem::INVALID_BUFFER_SLOT) {
status_t status = waitForFreeSlotThenRelock(FreeSlotCaller::Dequeue, lock, &found);
if (status != NO_ERROR) {
return status;
}
...
// IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION 和IGraphicBufferProducer::RELEASE_ALL_BUFFERS標記會走 requestBuffer流程來獲取graphicBuffer對應的slot
if ((buffer == nullptr) ||
buffer->needsReallocation(width, height, format, BQ_LAYER_COUNT, usage))
{
mSlots[found].mAcquireCalled = false;
mSlots[found].mGraphicBuffer = nullptr;
mSlots[found].mRequestBufferCalled = false;
mSlots[found].mEglDisplay = EGL_NO_DISPLAY;
mSlots[found].mEglFence = EGL_NO_SYNC_KHR;
mSlots[found].mFence = Fence::NO_FENCE;
mCore->mBufferAge = 0;
mCore->mIsAllocating = true;
returnFlags |= BUFFER_NEEDS_REALLOCATION;
}
上面摘抄了dequebuffer流程主要的兩件事情
1、獲取下一個buffer slot,如果有freeSlot直接拿來用
2、IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION 和IGraphicBufferProducer::RELEASE_ALL_BUFFERS標記會走 requestBuffer流程來獲取graphicBuffer對應的slot
2、queueBuffer
queueBuffer將已填充的緩沖區返回給BufferQueue，即把bufferslot封裝成BufferItem返回到BufferItem隊列mQueue。
status_t BufferQueueProducer::queueBuffer(int slot,
const QueueBufferInput &input, QueueBufferOutput *output) {
// 構建bufferitem并賦值
BufferItem item;
item.mAcquireCalled = mSlots[slot].mAcquireCalled;
item.mGraphicBuffer = mSlots[slot].mGraphicBuffer;
...
item.mSlot = slot;
item.mFence = acquireFence;
item.mFenceTime = acquireFenceTime;
//bufferItem 入隊
if (mCore->mQueue.empty()) {
// When the queue is empty, we can ignore mDequeueBufferCannotBlock
// and simply queue this buffer
mCore->mQueue.push_back(item);
frameAvailableListener = mCore->mConsumerListener;
} else {
// When the queue is not empty, we need to look at the last buffer
// in the queue to see if we need to replace it
const BufferItem& last = mCore->mQueue.itemAt(
mCore->mQueue.size() - 1);
if (last.mIsDroppable) {
if (!last.mIsStale) {
mSlots[last.mSlot].mBufferState.freeQueued();
queueBuffer比較簡單，主要兩步
1、構建bufferItem，賦值等
2、把bufferItem返回到mQueue隊列中
2.2.4、BufferQueueConsumer消費者
1、acquireBuffer
acquireBuffer方法從緩沖隊列中嘗試取下一個掛起的BufferItem有這些情況：
隊列為空直接返回；如果緩沖已被獲取，返回之前獲取的緩沖項；如果expectedPresent標記為非0，緩沖會即將顯示；如果緩沖時間戳在將來，不會被獲取；
status_t BufferQueueConsumer::acquireBuffer(BufferItem* outBuffer,
nsecs_t expectedPresent, uint64_t maxFrameNumber) {
// 新舊緩沖區顯示邏輯，決定顯示的buffer
if (expectedPresent != 0 && !mCore->mQueue.empty()) {
while (mCore->mQueue.size() > 1 && !mCore->mQueue[0].mIsAutoTimestamp) {
const BufferItem& bufferItem(mCore->mQueue[1]);
...
此段邏輯比較簡單，就是拿緩存，邏輯主要是怎么拿，最后回調
2、releaseBuffer
releaseBuffer將bufferslot返回到bufferqueue隊列。可以在bufferqueue任在訪問時進行，當buffer不再可用，fence機制會發出信號。
如果releaseBuffer收到STALE_BUFFER_SLOT信號，那么consumer必須直接放棄所有引用。直接釋放。
代碼就是release為主
status_t BufferQueueConsumer::releaseBuffer(int slot, uint64_t frameNumber,
const sp<Fence>& releaseFence, EGLDisplay eglDisplay,
EGLSyncKHR eglFence) {
sp<IProducerListener> listener;
{ // Autolock scope
std::lock_guard<std::mutex> lock(mCore->mMutex);
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();
三、bufferqueue總結
bufferqueue是整個安卓圖形處理的核心，把數據生產者和數據消費者連接起來。
生產者dequeuebuffer申請一個緩沖區，并指定寬高等。填充緩沖區數據后用queueBuffer返回到隊列。隨后aquirebuffer獲取緩沖區，消費者處理完后。則返回到隊列release
