Android4.2.2 SurfaceFlinger之图形缓存区申请与分配dequeueBuffer
本文均属自己阅读源码的点滴总结，转账请注明出处谢谢。
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Android源码版本Version：4.2.2; 硬件平台 全志A31
接着前面的BootAnimation的启动过程，可以看到内部会涉及很多OpenGL ES的相关操作，OpenGL ES通过之前创建的具备有SurfaceTexture等的Surface类，来操作远端的SF来完成相关的图像渲染。
这里主要涉及到ANativeWindow的2个核心回调函数，OpenGL ES在应用层的eglSwapBuffers就是调用了QueueBuffer和DequeueBuffer两个函数来完成的。
在介绍上面两个函数的实现时，有必要把BufferQueue这个类再提出来。他是由应用程序在客户端通过和服务端的Client交互，提交消息给SurfaceFlinger处理时创建的Layer对象时在SurfaceTextureLayer类构造中创建的：
BufferQueue中有一个成员变量BufferSlot mSlots[NUM_BUFFER_SLOTS];即一个BufferQueue实际上最大可以有32个Buffer，即一个应用程序申请的Surface在SF端的Layer可以有32个图像缓存区。而这32个图形缓存区都有上面的mSlots维护着，每个Buffer有以下几种可变的状态，由BufferState mBufferState维护：
分别是FREE,DEQUEUED,QUEUE,ACQUIRED这4个状态，分别是空闲，出列被填充数据，入列代表有了数据，最终将入列后有了图形数据的缓冲区进行渲染。
step1:先来看dequeueBuffer函数,可以理解为生产者，会用来申请Buffer并进行应用程序端的图像绘制。
int SurfaceTextureClient::dequeueBuffer(android_native_buffer_t** buffer,
int* fenceFd) {
ATRACE_CALL();
ALOGV("SurfaceTextureClient::dequeueBuffer");
Mutex::Autolock lock(mMutex);
int buf = -1;
int reqW = mReqWidth ? mReqWidth : mUserWidth;
int reqH = mReqHeight ? mReqHeight : mUserHeight;
sp fence;
status_t result = mSurfaceTexture->dequeueBuffer(&buf, fence, reqW, reqH,
mReqFormat, mReqUsage);//调用远程的BnSurfaceTexture来完成BufferQueue的操作
if (result < 0) {
ALOGV("dequeueBuffer: ISurfaceTexture::dequeueBuffer(%d, %d, %d, %d)"
"failed: %d", mReqWidth, mReqHeight, mReqFormat, mReqUsage,
result);
return result;
}
sp& gbuf(mSlots[buf].buffer);//对应到客户端的mSlots中，buf为SF侧分配到的id
if (result & ISurfaceTexture::RELEASE_ALL_BUFFERS) {
freeAllBuffers();
}
if ((result & ISurfaceTexture::BUFFER_NEEDS_REALLOCATION) || gbuf == 0) {
result = mSurfaceTexture->requestBuffer(buf, &gbuf);//返回的result是在重新分配后找到对应的缓冲区信息，需要进行request
if (result != NO_ERROR) {
ALOGE("dequeueBuffer: ISurfaceTexture::requestBuffer failed: %d",
result);
return result;
}
}
if (fence.get()) {
*fenceFd = fence->dup();
if (*fenceFd == -1) {
ALOGE("dequeueBuffer: error duping fence: %d", errno);
// dup() should never fail; something is badly wrong. Soldier on
// and hope for the best; the worst that should happen is some
// visible corruption that lasts until the next frame.
}
} else {
*fenceFd = -1;
}
*buffer = gbuf.get();//GraphicBuffer
return OK;
} 上面函数的核心在于status_t result = mSurfaceTexture->dequeueBuffer(&buf, fence, reqW, reqH,mReqFormat, mReqUsage);那么这个应用层即所谓的客户端侧的mSurfaceTexture是什么呢？这个其实就是应用程序侧在创建了SurfaceControl后获得的，即所谓的BufferQueue的Binder匿名代理（继承了BnSurfaceTexture而已）BpSurfaceTexture。从而这样建立起来的Binder通信机制，使得在ANativeWindow将最终的Buffer处理都扔回给了之前由SurfaceFlinger创建的Layer，SurfaceTexture对象中去了。
step2:现在回到BufferQueue去吧，看看是如何做调用的。
通过BpSurfaceTexture来到BnSurfaceTexture，如下：
因为BufferQueue继承了BnSurfaceTexture后，故调用BufferQueue的成员函数queueBuffer，其实也就是SurfaceFlinger再操纵着这些个Buffer。
case DEQUEUE_BUFFER: {//图像缓冲区申请
CHECK_INTERFACE(ISurfaceTexture, data, reply);
uint32_t w = data.readInt32();
uint32_t h = data.readInt32();
uint32_t format = data.readInt32();
uint32_t usage = data.readInt32();
int buf;
sp fence;
int result = dequeueBuffer(&buf, fence, w, h, format, usage);
bool hasFence = fence.get() && fence->isValid();
reply->writeInt32(buf);
reply->writeInt32(hasFence);
if (hasFence) {
reply->write(*fence.get());
}
reply->writeInt32(result);
return NO_ERROR;
} break;
status_t BufferQueue::dequeueBuffer(int *outBuf, sp& outFence,
uint32_t w, uint32_t h, uint32_t format, uint32_t usage) {
{
......
{ // Scope for the lock
Mutex::Autolock lock(mMutex);
if (format == 0) {
format = mDefaultBufferFormat;
}
// turn on usage bits the consumer requested
usage |= mConsumerUsageBits;
int found = -1;
int dequeuedCount = 0;
bool tryAgain = true;
while (tryAgain) {
if (mAbandoned) {
ST_LOGE("dequeueBuffer: SurfaceTexture has been abandoned!");
return NO_INIT;
}
const int maxBufferCount = getMaxBufferCountLocked();
// Free up any buffers that are in slots beyond the max buffer
// count.
for (int i = maxBufferCount; i < NUM_BUFFER_SLOTS; i++) {//寻找所有的slots
assert(mSlots[i].mBufferState == BufferSlot::FREE);
if (mSlots[i].mGraphicBuffer != NULL) {
freeBufferLocked(i);
returnFlags |= ISurfaceTexture::RELEASE_ALL_BUFFERS;
}
}
// look for a free buffer to give to the client
found = INVALID_BUFFER_SLOT;
dequeuedCount = 0;
for (int i = 0; i < maxBufferCount; i++) {
const int state = mSlots[i].mBufferState;
if (state == BufferSlot::DEQUEUED) {
dequeuedCount++;//统计已经出列的buffer个数
}
if (state == BufferSlot::FREE) {
/* We return the oldest of the free buffers to avoid
* stalling the producer if possible. This is because
* the consumer may still have pending reads of the
* buffers in flight.
*/
if ((found < 0) ||
mSlots[i].mFrameNumber < mSlots[found].mFrameNumber) {
found = i;//寻址free里面帧号最小的位置Slot
}
}
}
// clients are not allowed to dequeue more than one buffer
// if they didn't set a buffer count.
if (!mOverrideMaxBufferCount && dequeuedCount) {
ST_LOGE("dequeueBuffer: can't dequeue multiple buffers without "
"setting the buffer count");
return -EINVAL;
}
// See whether a buffer has been queued since the last
// setBufferCount so we know whether to perform the min undequeued
// buffers check below.
if (mBufferHasBeenQueued) {
// make sure the client is not trying to dequeue more buffers
// than allowed.
const int newUndequeuedCount = maxBufferCount - (dequeuedCount+1);
const int minUndequeuedCount = getMinUndequeuedBufferCountLocked();
if (newUndequeuedCount < minUndequeuedCount) {
ST_LOGE("dequeueBuffer: min undequeued buffer count (%d) "
"exceeded (dequeued=%d undequeudCount=%d)",
minUndequeuedCount, dequeuedCount,
newUndequeuedCount);
return -EBUSY;
}
}
// If no buffer is found, wait for a buffer to be released or for
// the max buffer count to change.
tryAgain = found == INVALID_BUFFER_SLOT;
if (tryAgain) {
for (int i = 0; i < maxBufferCount; i++) {
BQ_LOGD("#dequeueBuffer tryAgain buf:%d state:%d", i, mSlots[i].mBufferState);
}
mDequeueCondition.wait(mMutex);//等待有buffer的释放
}
}
if (found == INVALID_BUFFER_SLOT) {
// This should not happen.
ST_LOGE("dequeueBuffer: no available buffer slots");
return -EBUSY;
}
const int buf = found;
*outBuf = found;//找到可以使用的buffer，记录到outBuf里面
ATRACE_BUFFER_INDEX(buf);
const bool useDefaultSize = !w && !h;
if (useDefaultSize) {
// use the default size
w = mDefaultWidth;
h = mDefaultHeight;
}
// buffer is now in DEQUEUED (but can also be current at the same time,
// if we're in synchronous mode)
mSlots[buf].mBufferState = BufferSlot::DEQUEUED;
const sp& buffer(mSlots[buf].mGraphicBuffer);
if ((buffer == NULL) ||
(uint32_t(buffer->width) != w) ||
(uint32_t(buffer->height) != h) ||
(uint32_t(buffer->format) != format) ||
((uint32_t(buffer->usage) & usage) != usage))//当前找到的slots中的buffer没有缓冲区或者相关属性不匹配则从新申请
{
mSlots[buf].mAcquireCalled = false;
mSlots[buf].mGraphicBuffer = NULL;
mSlots[buf].mRequestBufferCalled = false;
mSlots[buf].mEglFence = EGL_NO_SYNC_KHR;
mSlots[buf].mFence.clear();
mSlots[buf].mEglDisplay = EGL_NO_DISPLAY;
returnFlags |= ISurfaceTexture::BUFFER_NEEDS_REALLOCATION;//重新申请
}
dpy = mSlots[buf].mEglDisplay;
eglFence = mSlots[buf].mEglFence;
outFence = mSlots[buf].mFence;
mSlots[buf].mEglFence = EGL_NO_SYNC_KHR;
mSlots[buf].mFence.clear();
} // end lock scope
if (returnFlags & ISurfaceTexture::BUFFER_NEEDS_REALLOCATION) {
status_t error;
mGraphicBufferAlloc->acquireBufferReferenceSlot(*outBuf);
sp graphicBuffer(
mGraphicBufferAlloc->createGraphicBuffer(
w, h, format, usage, &error));//重新请求SF端申请一个buffer
if (graphicBuffer == 0) {
ST_LOGE("dequeueBuffer: SurfaceComposer::createGraphicBuffer "
"failed");
return error;
}
{ // Scope for the lock
Mutex::Autolock lock(mMutex);
if (mAbandoned) {
ST_LOGE("dequeueBuffer: SurfaceTexture has been abandoned!");
return NO_INIT;
}
mSlots[*outBuf].mGraphicBuffer = graphicBuffer;//申请的buffer加入到当前的mSlots的成员mGraphicBuffer中
}
}
if (eglFence != EGL_NO_SYNC_KHR) {
EGLint result = eglClientWaitSyncKHR(dpy, eglFence, 0, 1000000000);
// If something goes wrong, log the error, but return the buffer without
// synchronizing access to it. It's too late at this point to abort the
// dequeue operation.
if (result == EGL_FALSE) {
ST_LOGE("dequeueBuffer: error waiting for fence: %#x", eglGetError());
} else if (result == EGL_TIMEOUT_EXPIRED_KHR) {
ST_LOGE("dequeueBuffer: timeout waiting for fence");
}
eglDestroySyncKHR(dpy, eglFence);
}
BQ_LOGD("#dequeueBuffer: returning slot=%d buf=%p flags=%#x %p", *outBuf,
mSlots[*outBuf].mGraphicBuffer->handle, returnFlags, this);
return returnFlags;
} DeQueueuffer的内容比较多，我们分以下几个Step来进行分析。
step1:清空超过最大需求的Buffer
// Free up any buffers that are in slots beyond the max buffer
// count.
for (int i = maxBufferCount; i < NUM_BUFFER_SLOTS; i++) {//寻找所有的slots
assert(mSlots[i].mBufferState == BufferSlot::FREE);
if (mSlots[i].mGraphicBuffer != NULL) {
freeBufferLocked(i);
returnFlags |= ISurfaceTexture::RELEASE_ALL_BUFFERS;
}
}
这里假设maxBufferCount = 2，对其余的Buffer进行图形缓存区的清除。
step2:查找一个合格的Free了的Buffer
found = INVALID_BUFFER_SLOT;
dequeuedCount = 0;
for (int i = 0; i < maxBufferCount; i++) {
const int state = mSlots[i].mBufferState;
if (state == BufferSlot::DEQUEUED) {
dequeuedCount++;//统计已经出列的buffer个数
}
if (state == BufferSlot::FREE) {
/* We return the oldest of the free buffers to avoid
* stalling the producer if possible. This is because
* the consumer may still have pending reads of the
* buffers in flight.
*/
if ((found < 0) ||
mSlots[i].mFrameNumber < mSlots[found].mFrameNumber) {
found = i;//寻址free里面帧号最小的位置Slot
}
}
}这里可以看到只执行2次循环，因为只需要2个图形缓存区而已。这里找到FREE的BufferSlot后，还需要查看当前的buffer所属于的帧号，这里found最终被定义为帧号小的BufferSlot.
step3:找到对应的BufferSlot的索index后，赋值给返回的outbuf参数，并切换当前状态从FREE到DEQUEUED。
step4: 实际图形缓存区的分配和申请，是实际DEqueuebuffer的重点所在
mSlots[buf].mBufferState = BufferSlot::DEQUEUED;
const sp& buffer(mSlots[buf].mGraphicBuffer);
if ((buffer == NULL) ||
(uint32_t(buffer->width) != w) ||
(uint32_t(buffer->height) != h) ||
(uint32_t(buffer->format) != format) ||
((uint32_t(buffer->usage) & usage) != usage))//当前找到的slots中的buffer没有缓冲区或者相关属性不匹配则从新申请
{
mSlots[buf].mAcquireCalled = false;
mSlots[buf].mGraphicBuffer = NULL;
mSlots[buf].mRequestBufferCalled = false;
mSlots[buf].mEglFence = EGL_NO_SYNC_KHR;
mSlots[buf].mFence.clear();
mSlots[buf].mEglDisplay = EGL_NO_DISPLAY;
returnFlags |= ISurfaceTexture::BUFFER_NEEDS_REALLOCATION;//重新申请
}
dpy = mSlots[buf].mEglDisplay;
eglFence = mSlots[buf].mEglFence;
outFence = mSlots[buf].mFence;
mSlots[buf].mEglFence = EGL_NO_SYNC_KHR;
mSlots[buf].mFence.clear();
} // end lock scope
if (returnFlags & ISurfaceTexture::BUFFER_NEEDS_REALLOCATION) {
status_t error;
mGraphicBufferAlloc->acquireBufferReferenceSlot(*outBuf);
sp graphicBuffer(
mGraphicBufferAlloc->createGraphicBuffer(
w, h, format, usage, &error));//重新请求SF端申请一个buffer
if (graphicBuffer == 0) {
ST_LOGE("dequeueBuffer: SurfaceComposer::createGraphicBuffer "
"failed");
return error;
}
{ // Scope for the lock
Mutex::Autolock lock(mMutex);
if (mAbandoned) {
ST_LOGE("dequeueBuffer: SurfaceTexture has been abandoned!");
return NO_INIT;
}
mSlots[*outBuf].mGraphicBuffer = graphicBuffer;//申请的buffer加入到当前的mSlots的成员mGraphicBuffer中
}
}
这里都是在对查找到的BufferSlot进行初始化操作，可以看到只要buffer（sp
step6:图形缓存申请的实现mGraphicBufferAlloc->createGraphicBuffer()
那么这个函数是如何实现的呢？回来看看BufferQueue当初创建的时候把，在BufferQueue的构造函数里面，有一个图形缓存区分配的成员对象，他最终是由SurfaceFlinger来实现的。
sp composer(ComposerService::getComposerService());
mGraphicBufferAlloc = composer->createGraphicBufferAlloc();//创建GraphicBuffer virtual sp createGraphicBufferAlloc()
{
uint32_t n;
Parcel data, reply;
data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());
remote()->transact(BnSurfaceComposer::CREATE_GRAPHIC_BUFFER_ALLOC, data, &reply);//
return interface_cast(reply.readStrongBinder());//BpGraphicBufferAlloc
} status_t BnSurfaceComposer::onTransact(//内部函数由继承类SF来完成
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{ case CREATE_GRAPHIC_BUFFER_ALLOC: {
CHECK_INTERFACE(ISurfaceComposer, data, reply);
sp b = createGraphicBufferAlloc()->asBinder();//创建图像缓存
reply->writeStrongBinder(b);
} break; sp SurfaceFlinger::createGraphicBufferAlloc()
{
sp gba(new GraphicBufferAlloc());//图形缓存的申请
return gba;
}
在上述典型的Binder交互完成后，SF在服务端侧新建了一个图形缓存分配类对象后，将新建的gba写入Binder驱动，返回到客户端益BpBinder的形式存在
实际返回的是BpGraphicBufferAlloc的Binder代理，而
因此这里通过这个匿名的Binder代理，去请求BnGraphicBufferAlloc来完成
status_t BnGraphicBufferAlloc::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
case CREATE_GRAPHIC_BUFFER: {
CHECK_INTERFACE(IGraphicBufferAlloc, data, reply);
uint32_t w = data.readInt32();
uint32_t h = data.readInt32();
PixelFormat format = data.readInt32();
uint32_t usage = data.readInt32();
status_t error;
sp result =
createGraphicBuffer(w, h, format, usage, &error);
reply->writeInt32(error);
if (result != 0) {
reply->write(*result);
// We add a BufferReference to this parcel to make sure the
// buffer stays alive until the GraphicBuffer object on
// the other side has been created.
// This is needed so that the buffer handle can be
// registered before the buffer is destroyed on implementations
// that do not use file-descriptors to track their buffers.
reply->writeStrongBinder( new BufferReference(result) );
}
return NO_ERROR;
} break;
....
} 而之前存入的匿名binder对就是上述SF新建的GraphicBufferAlloc gba；而该类也正好继承了BpGraphicBufferAlloc这个对象，故有GraphicBufferAlloc::createGraphicBuffer来实现：
sp GraphicBufferAlloc::createGraphicBuffer(uint32_t w, uint32_t h,
PixelFormat format, uint32_t usage, status_t* error) {
sp graphicBuffer(new GraphicBuffer(w, h, format, usage));
status_t err = graphicBuffer->initCheck();
*error = err;
if (err != 0 || graphicBuffer->handle == 0) {
if (err == NO_MEMORY) {
GraphicBuffer::dumpAllocationsToSystemLog();
}
ALOGE("GraphicBufferAlloc::createGraphicBuffer(w=%d, h=%d) "
"failed (%s), handle=%p",
w, h, strerror(-err), graphicBuffer->handle);
return 0;
}
return graphicBuffer;
} 这里看到了一个图像缓存类GraphicBuffer，在这里所谓的图像缓存创建就是构造了这个对象
class GraphicBuffer
: public ANativeObjectBase<
ANativeWindowBuffer,
GraphicBuffer,
LightRefBase >, public Flattenable
{ 该类继承了本地窗口缓存ANativeWindowBuffer；
GraphicBuffer::GraphicBuffer(uint32_t w, uint32_t h,
PixelFormat reqFormat, uint32_t reqUsage)
: BASE(), mOwner(ownData), mBufferMapper(GraphicBufferMapper::get()),
mInitCheck(NO_ERROR), mIndex(-1)
{
width =
height =
stride =
format =
usage = 0;
handle = NULL;
mInitCheck = initSize(w, h, reqFormat, reqUsage);//内部实现缓存的申请
}
GraphicBuffer有一个mBufferMapper对象，缓存的映射，看看他的初始化：
class GraphicBufferMapper : public Singleton
{
public:
static inline GraphicBufferMapper& get() { return getInstance(); }
status_t registerBuffer(buffer_handle_t handle);
status_t unregisterBuffer(buffer_handle_t handle);
status_t lock(buffer_handle_t handle,
int usage, const Rect& bounds, void** vaddr);
status_t unlock(buffer_handle_t handle);
// dumps information about the mapping of this handle
void dump(buffer_handle_t handle);
status_t get_phy_addess(buffer_handle_t handle, void** vaddr);
private:
friend class Singleton;
GraphicBufferMapper();
gralloc_module_t const *mAllocMod;
}; 这个的get()返回的是一个GraphicBufferMapper对象，且为单列模式。
step7: 该对象将来完成图形缓存的映射，也就是图形缓存区内存映射到应用程序。看她的构造函数：
GraphicBufferMapper::GraphicBufferMapper()
: mAllocMod(0)
{
hw_module_t const* module;
int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module);
ALOGE_IF(err, "FATAL: can't find the %s module", GRALLOC_HARDWARE_MODULE_ID);
if (err == 0) {
mAllocMod = (gralloc_module_t const *)module;
}
}
很清楚这个是获取FrameBuffer的HAL模块gralloc的handle到module中，重点来看构造函数里的initSize函数：
status_t GraphicBuffer::initSize(uint32_t w, uint32_t h, PixelFormat format,
uint32_t reqUsage)
{
GraphicBufferAllocator& allocator = GraphicBufferAllocator::get();
status_t err = allocator.alloc(w, h, format, reqUsage, &handle, &stride);//图像缓冲区的分配
if (err == NO_ERROR) {
this->width = w;
this->height = h;
this->format = format;
this->usage = reqUsage;
}
return err;
}
这里又出现了一个图形缓存分配器的类，类似于GraphicBufferMapper函数，来看他的构造过程：
GraphicBufferAllocator::GraphicBufferAllocator()
: mAllocDev(0)
{
hw_module_t const* module;
int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module);//调用HAL层
ALOGE_IF(err, "FATAL: can't find the %s module", GRALLOC_HARDWARE_MODULE_ID);
if (err == 0) {
gralloc_open(module, &mAllocDev);//获得buffer分配模块mAllocDev，使用的是GPU这个模块ID
}
}这里是打开了HAL层的gralloc模块到mAllocDev中，调用alloc函数，看看他完成了什么？
status_t GraphicBufferAllocator::alloc(uint32_t w, uint32_t h, PixelFormat format,
int usage, buffer_handle_t* handle, int32_t* stride)
{
......
BufferLiberatorThread::maybeWaitForLiberation();
err = mAllocDev->alloc(mAllocDev, w, h, format, usage, handle, stride);
if (err != NO_ERROR) {
ALOGW("WOW! gralloc alloc failed, waiting for pending frees!");
BufferLiberatorThread::waitForLiberation();
err = mAllocDev->alloc(mAllocDev, w, h, format, usage, handle, stride);
}
ALOGW_IF(err, "alloc(%u, %u, %d, %08x, ...) failed %d (%s)",
w, h, format, usage, err, strerror(-err));
if (err == NO_ERROR) {
Mutex::Autolock _l(sLock);
KeyedVector& list(sAllocList);
int bpp = bytesPerPixel(format);
if (bpp < 0) {
// probably a HAL custom format. in any case, we don't know
// what its pixel size is.
bpp = 0;
}
alloc_rec_t rec;
rec.w = w;
rec.h = h;
rec.s = *stride;
rec.format = format;
rec.usage = usage;
rec.size = h * stride[0] * bpp;
list.add(*handle, rec);
}
return err;
} 这里是调用了Gralloc模块的alloc回调函数来完成对内存图形缓存区的申请。
step8:回到HAL层看看gralloc模块做了什么？
static int gralloc_alloc(alloc_device_t* dev,
int w, int h, int format, int usage,
buffer_handle_t* pHandle, int* pStride)
{
if (!pHandle || !pStride)
return -EINVAL;
size_t size, stride;
int align = 4;
int bpp = 0;
switch (format) {
case HAL_PIXEL_FORMAT_RGBA_8888:
case HAL_PIXEL_FORMAT_RGBX_8888:
case HAL_PIXEL_FORMAT_BGRA_8888:
bpp = 4;
break;
case HAL_PIXEL_FORMAT_RGB_888:
bpp = 3;
break;
case HAL_PIXEL_FORMAT_RGB_565:
case HAL_PIXEL_FORMAT_RGBA_5551:
case HAL_PIXEL_FORMAT_RGBA_4444:
case HAL_PIXEL_FORMAT_RAW_SENSOR:
bpp = 2;
break;
default:
return -EINVAL;
}
size_t bpr = (w*bpp + (align-1)) & ~(align-1);
size = bpr * h;
stride = bpr / bpp;
int err;
if (usage & GRALLOC_USAGE_HW_FB) {
err = gralloc_alloc_framebuffer(dev, size, usage, pHandle);//分配系统缓存帧
} else {
err = gralloc_alloc_buffer(dev, size, usage, pHandle);//分配的是内存缓存
}
if (err < 0) {
return err;
}
*pStride = stride;
return 0;
}这里看到usage有一种是硬件帧缓存，另一个是开辟单独的匿名内存块。当然数据直接写入FramerBuffer是最快的，但往往一个帧缓存是远远不够的，故而这里将还会创建匿名的pmem来作为图像缓冲区，但帧缓存只有一个。这里的buffer_handle_t *pHandle最终可以理解为是图形缓存在当前应用程序mmap后的用户空间地址。使得后续的图像渲染等直接对用户空间操作即可。
到这里我们返回到了SurfaceTextureClient::dequeueBuffer函数中去，完成mSurfaceTexture->dequeueBuffer函数返回后，可以看到如果当前的在服务端从新分配了图像缓存后ISurfaceTexture::BUFFER_NEEDS_REALLOCATION，将调用requestBuffer函数，来看看为何还要这么处理？
依旧是BpSurfaceTexture来处理
virtual status_t requestBuffer(int bufferIdx, sp* buf) {
Parcel data, reply;
data.writeInterfaceToken(ISurfaceTexture::getInterfaceDescriptor());
data.writeInt32(bufferIdx);
status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
if (result != NO_ERROR) {
return result;
}
bool nonNull = reply.readInt32();
if (nonNull) {
*buf = new GraphicBuffer();//应用程序侧也新建一个图形缓存
reply.read(**buf);//buf生成
}
result = reply.readInt32();
return result;
}
对应在BnSurfaceTexture侧的响应如下：
status_t BnSurfaceTexture::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case REQUEST_BUFFER: {
CHECK_INTERFACE(ISurfaceTexture, data, reply);
int bufferIdx = data.readInt32();
sp buffer;
int result = requestBuffer(bufferIdx, &buffer);
reply->writeInt32(buffer != 0);
if (buffer != 0) {
reply->write(*buffer);
}
reply->writeInt32(result);
return NO_ERROR;
} break; 通过客户端传递过来的数据，即一个buffer的索引id，在BufferQueue里调用requestBuffer函数
status_t BufferQueue::requestBuffer(int slot, sp* buf) {
int maxBufferCount = getMaxBufferCountLocked();
if (slot < 0 || maxBufferCount <= slot) {
ST_LOGE("requestBuffer: slot index out of range [0, %d]: %d",
maxBufferCount, slot);
return BAD_VALUE;
} else if (mSlots[slot].mBufferState != BufferSlot::DEQUEUED) {
// XXX: I vaguely recall there was some reason this can be valid, but
// for the life of me I can't recall under what circumstances that's
// the case.
ST_LOGE("requestBuffer: slot %d is not owned by the client (state=%d)",
slot, mSlots[slot].mBufferState);
return BAD_VALUE;
}
mSlots[slot].mRequestBufferCalled = true;
*buf = mSlots[slot].mGraphicBuffer;
return NO_ERROR;
} 上述函数通过这个buffer的索引值，找到对应的BufferSlot后，返回的是这个他维护着的服务端的sp
status_t Parcel::write(const Flattenable& val)
{ err = val.flatten(buf, len, fds, fd_count);//对buffer进行写入调用，GraphicBuffer::flatten
.............
}
之所以哪呢过将这个buffer写入在于GraphicBuffer的特殊性，该类继承了一个Flattenable类，最终调用下面的函数
status_t GraphicBuffer::flatten(void* buffer, size_t size,//发送端打包
int fds[], size_t count) const
{
size_t sizeNeeded = GraphicBuffer::getFlattenedSize();
if (size < sizeNeeded) return NO_MEMORY;
size_t fdCountNeeded = GraphicBuffer::getFdCount();
if (count < fdCountNeeded) return NO_MEMORY;
int* buf = static_cast(buffer);
buf[0] = 'GBFR';
buf[1] = width;
buf[2] = height;
buf[3] = stride;
buf[4] = format;
buf[5] = usage;
buf[6] = 0;
buf[7] = 0;
if (handle) {
buf[6] = handle->numFds;
buf[7] = handle->numInts;
native_handle_t const* const h = handle;
memcpy(fds, h->data, h->numFds*sizeof(int));
memcpy(&buf[8], h->data + h->numFds, h->numInts*sizeof(int));
}
return NO_ERROR;
}
上述函数将GraphicBuffer的信息初始化到buf数组里面，最终数据都写入到reply中返回。
而同样在requestbuffer的客户端处，是对接收到的数据的解析，解析使用下面的过程实现：
BpSurfaceTexture(const sp& impl)
: BpInterface(impl)
{
}
virtual status_t requestBuffer(int bufferIdx, sp* buf) {
Parcel data, reply;
data.writeInterfaceToken(ISurfaceTexture::getInterfaceDescriptor());
data.writeInt32(bufferIdx);
status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
if (result != NO_ERROR) {
return result;
}
bool nonNull = reply.readInt32();
if (nonNull) {
*buf = new GraphicBuffer();//应用程序侧也新建一个图形缓存
reply.read(**buf);//buf生成
}
result = reply.readInt32();
return result;
}
首先是新建一个应用程序客户端侧的一个GraphicBuffer对象，然后利用服务端返回的reply信息填充并初始化GraphicBuffer
status_t Parcel::read(Flattenable& val) const
{
// size
if (err == NO_ERROR) {
err = val.unflatten(buf, len, fds, fd_count);//解析出信息到buffer
}
...
}status_t GraphicBuffer::unflatten(void const* buffer, size_t size,
int fds[], size_t count)//接收端解析
{
if (size < 8*sizeof(int)) return NO_MEMORY;
int const* buf = static_cast(buffer);
if (buf[0] != 'GBFR') return BAD_TYPE;
const size_t numFds = buf[6];
const size_t numInts = buf[7];
const size_t sizeNeeded = (8 + numInts) * sizeof(int);
if (size < sizeNeeded) return NO_MEMORY;
size_t fdCountNeeded = 0;
if (count < fdCountNeeded) return NO_MEMORY;
if (handle) {
// free previous handle if any
free_handle();
}
if (numFds || numInts) {
width = buf[1];
height = buf[2];
stride = buf[3];
format = buf[4];
usage = buf[5];
native_handle* h = native_handle_create(numFds, numInts);//创建本地的图像缓存buffer
memcpy(h->data, fds, numFds*sizeof(int));//文件描述符
memcpy(h->data + numFds, &buf[8], numInts*sizeof(int));//数据
handle = h;
} else {
width = height = stride = format = usage = 0;
handle = NULL;
}
mOwner = ownHandle;
if (handle != 0) {
status_t err = mBufferMapper.registerBuffer(handle);//buffer注册，即将这个图像缓存mmap映射到当前的用户进程
if (err != NO_ERROR) {
ALOGE("unflatten: registerBuffer failed: %s (%d)",
strerror(-err), err);
return err;
}
}
return NO_ERROR;
} 在flatten和unflatten函数其实就是为了满足Binder数据的通信协议Parcel而设计的(用于传输对象变量)，在unflatten函数中完成了对数据的解析后获得了一个handle，利用这个handle进行了registerBuffer的操作。
status_t GraphicBufferMapper::registerBuffer(buffer_handle_t handle)
{
ATRACE_CALL();
status_t err;
err = mAllocMod->registerBuffer(mAllocMod, handle);//Client端可以将指定的内存区域映射到自己的进程空间
ALOGW_IF(err, "registerBuffer(%p) failed %d (%s)",
handle, err, strerror(-err));
return err;
}这里又回到了客户端侧的，的确这里很好奇的是在服务端侧也有过GraphicBuffer的存在，而客户端侧却还要创建一个GraphicBuffer，这个原因是什么呢？
来看看Gralloc模块对registerBuffer的实现吧：
int gralloc_register_buffer(gralloc_module_t const* module,
buffer_handle_t handle)
{
if (private_handle_t::validate(handle) < 0)
return -EINVAL;
// if this handle was created in this process, then we keep it as is.
int err = 0;
private_handle_t* hnd = (private_handle_t*)handle;
if (hnd->pid != getpid()) {
void *vaddr;
err = gralloc_map(module, handle, &vaddr);
}
return err;
}调用gralloc模块中的gralloc_map完成mmap的相关操作：
static int gralloc_map(gralloc_module_t const* module,
buffer_handle_t handle,
void** vaddr)
{
private_handle_t* hnd = (private_handle_t*)handle;
if (!(hnd->flags & private_handle_t::PRIV_FLAGS_FRAMEBUFFER)) {
size_t size = hnd->size;
void* mappedAddress = mmap(0, size,
PROT_READ|PROT_WRITE, MAP_SHARED, hnd->fd, 0);//将匿名共享内存mmap到用户空间
if (mappedAddress == MAP_FAILED) {
ALOGE("Could not mmap %s", strerror(errno));
return -errno;
}
hnd->base = intptr_t(mappedAddress) + hnd->offset;
//ALOGD("gralloc_map() succeeded fd=%d, off=%d, size=%d, vaddr=%p",
// hnd->fd, hnd->offset, hnd->size, mappedAddress);
}
*vaddr = (void*)hnd->base;
return 0;
}这里的逻辑应该是handle维护着映射到用户空间的虚拟地址，hnd->base就包含了这个信息，而hnd->fd应该是一个内存设备的描述符。最终这里就通过这个handle将服务端申请并分配的图形内存缓冲区（无论是帧缓存还是匿名的ashmem）共享到客户端，两者都以GrallocBuffer对象的形式存在。
到此为止分析基于OpenGL ES的ANativeWindow和ANativeWindowBuffer的dequeueBuffer的分析基本完成了，下面以2个简易的流程图来进行总结，方便自己理解，图1是OpenGL ES所需要的在应用层的Surface创建的一个过程，如果图片看不清，可以下载地址SurfaceFlinger应用端创建surface的过程图：
图2： 图形缓存GraphicBuffer的申请与分配图
这里继续补充在应用程序侧和服务侧的GraphicBuffer，这两个图形缓存区的区别与联系：
1.在BufferQueue的构造函数中有mGraphicBufferAlloc = composer->createGraphicBufferAlloc()，显然是由SF进程来完成这个对象的创建的，故SF维护着mGraphicBufferAlloc这个匿名的本地Binder服务，并传递给BufferQueue维护。
2.BufferQueue中请求createGraphicBuffer时，还是有SF来完成，故缓存的实际创建还是有SF进程来完成的，由createGraphicBuffer里的new GraphicBuffer()；在SF侧创建实际的图形缓存。
3.交给GraphicBufferAllocator来完成gralloc的图形缓存的申请并完成映射。分为gralloc_alloc_framebuffer和gralloc_alloc_buffer两种，前者是直接在帧缓存中分配出图像缓存，后者是直接申请一个匿名共享内存来做为图形缓存区。最终申请并完成映射到SF处的图像缓存以buffer_handle_t的类型维护着。
4.在服务端SF那里有了图像缓存后，应用端势必也需要有服务端图像缓冲区的信息，将实际的图像内存维护到应用侧的用户空间。这个过程是在requestBuffer来完成的：
实际是有GraphicBuffer来完成，其中GraphicBufferMapper来维护Gralloc模块的打开，以及gralloc_module_t回调registerBuffer。gralloc的registerBuffer实际就是用来用户空间注册缓存，而注册实际只是一个mmap的过程，映射到用户空间。
这样在内核中开辟的一个内存空间，在服务端被映射，在应用端也完成映射，故使得最终的操作都是同一块内存图形区域，建立了紧密的联系。