Android Camera HAL3中预览preview模式下的数据流

本文均属自己阅读源码的点滴总结，转账请注明出处谢谢。欢迎和大家交流。qq:1037701636 email:gzzaigcn2009@163.comSoftware：系统源码Android5.1
前沿：    为了更好的梳理preview下buffer数据流的操作过程，前一文中对surface下的buffer相关的操作架构进行了描述。本文主要以此为基础，重点分析再Camera2Client和Camera3Device下是如何维护并读写这些视频帧缓存的。
1. Camera3Device::convertMetadataListToRequestListLocked函数结合上一博文中关于preview的控制流，定位到数据流主要的操作主要是对preview模式下将CameraMetadata mPreviewRequest转换为CaptureRequest的过程之中，回顾到mPreviewRequest是主要包含了当前preview下所需要Camera3Device来操作的OutputStream的index值。
2. Camera3Device::configureStreamsLocked函数    在configureStreamsLocked的函数中，主要关注的是Camera3Device对当前所具有的所有的mInputStreams和mOutputStreams进行Config的操作，分别包括startConfiguration/finishConfiguration两个状态。
2.1 mOutputStreams.editValueAt(i)->startConfiguration()这里的遍历所有输出stream即最终调用的函数入口为Camera3Stream::startConfiguration()，这里需要先看下Camera3OutputStream的整个结构，出现了Camera3Stream和Camera3IOStreamBase，两者是Input和Output stream所共有的，前者提供的更多的是对buffer的config、get/retrun buffer的操作，后者以维护当前的stream所拥有的buffer数目。另一个支路camera3_stream_t是一个和Camera HAL3底层进行stream信息交互的入口。


startConfiguration函数首先是判断当前stream的状态，对于已经config的不作处理，config的主要操作是getEndpointUsage：
status_t Camera3OutputStream::getEndpointUsage(uint32_t *usage) {

status_t res;
int32_t u = 0;
res = mConsumer->query(mConsumer.get(),
NATIVE_WINDOW_CONSUMER_USAGE_BITS, &u);
*usage = u;

return res;
}
这里的mConsumer其实就是之前创建的Surface的本体，每一个Stream在建立时createStream，都会传入一个ANativeWIndow类似的Consumer绑定到当前的stream中去。这里主要是完成当前window所管理的buffer的USAGE值，可参看grallo.h中的定义，由Gralloc模块负责指定当前buffer操作是由HW还是SW来完成以及不同的应用场合，在Gralloc模块中不同模块需求的buffer亦会有不同的分配、定义与处理方式： /* buffer will be used as an OpenGL ES texture */
GRALLOC_USAGE_HW_TEXTURE = 0x00000100,
/* buffer will be used as an OpenGL ES render target */
GRALLOC_USAGE_HW_RENDER = 0x00000200,
/* buffer will be used by the 2D hardware blitter */
GRALLOC_USAGE_HW_2D = 0x00000400,
/* buffer will be used by the HWComposer HAL module */
GRALLOC_USAGE_HW_COMPOSER = 0x00000800,
/* buffer will be used with the framebuffer device */
GRALLOC_USAGE_HW_FB = 0x00001000,
/* buffer will be used with the HW video encoder */
GRALLOC_USAGE_HW_VIDEO_ENCODER = 0x00010000,
/* buffer will be written by the HW camera pipeline */
GRALLOC_USAGE_HW_CAMERA_WRITE = 0x00020000,
/* buffer will be read by the HW camera pipeline */
GRALLOC_USAGE_HW_CAMERA_READ = 0x00040000,
/* buffer will be used as part of zero-shutter-lag queue */
GRALLOC_USAGE_HW_CAMERA_ZSL = 0x00060000,
/* mask for the camera access values */
GRALLOC_USAGE_HW_CAMERA_MASK = 0x00060000,
/* mask for the software usage bit-mask */
GRALLOC_USAGE_HW_MASK = 0x00071F00,

2.2 mHal3Device->ops->configure_streams(mHal3Device, &config);config是一个camera3_stream_configuration数据结构，他记录了一次和HAL3交互的stream的数量，已经当前每一个stream的属性配置相关的信息camer3_stream_t，包括stream中每一个buffer的属性值，stream的类型值等等，提交这些信息供hal3去分析处理。在高通平台中你可以看到，对于每一个stream在HAL3平台下均以一个Channel的形式存在。typedef struct camera3_stream_configuration {
uint32_t num_streams;
camera3_stream_t **streams;
} camera3_stream_configuration_t;




stream_type包括：CAMERA3_STREAM_OUTPUT、CAMERA3_STREAM_INPUT、CAMERA3_STREAM_BIDIRECTIONAL。format主要是指当前buffer支持的像素点存储格式，以HAL_PIXEL_FORMAT_IMPLEMENTATION_DEFINED居多，表明数据格式是由Gralloc模块来决定的。对于HAL3中对configureStreams接口的实现会放在后续介绍高通平台的实现机制时再做分析。
2.3 Camera3Stream::finishConfiguration该函数主要执行configureQueueLocked和registerBuffersLocked两个函数status_t Camera3OutputStream::configureQueueLocked() {
status_t res;

mTraceFirstBuffer = true;
if ((res = Camera3IOStreamBase::configureQueueLocked()) != OK) {
return res;
}

ALOG_ASSERT(mConsumer != 0, "mConsumer should never be NULL");

// Configure consumer-side ANativeWindow interface
res = native_window_api_connect(mConsumer.get(),
NATIVE_WINDOW_API_CAMERA);
if (res != OK) {
ALOGE("%s: Unable to connect to native window for stream %d",
__FUNCTION__, mId);
return res;
}

res = native_window_set_usage(mConsumer.get(), camera3_stream::usage);
if (res != OK) {
ALOGE("%s: Unable to configure usage %08x for stream %d",
__FUNCTION__, camera3_stream::usage, mId);
return res;
}

res = native_window_set_scaling_mode(mConsumer.get(),
NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
if (res != OK) {
ALOGE("%s: Unable to configure stream scaling: %s (%d)",
__FUNCTION__, strerror(-res), res);
return res;
}

if (mMaxSize == 0) {
// For buffers of known size
res = native_window_set_buffers_dimensions(mConsumer.get(),
camera3_stream::width, camera3_stream::height);
} else {
// For buffers with bounded size
res = native_window_set_buffers_dimensions(mConsumer.get(),
mMaxSize, 1);
}
if (res != OK) {
ALOGE("%s: Unable to configure stream buffer dimensions"
" %d x %d (maxSize %zu) for stream %d",
__FUNCTION__, camera3_stream::width, camera3_stream::height,
mMaxSize, mId);
return res;
}
res = native_window_set_buffers_format(mConsumer.get(),
camera3_stream::format);
if (res != OK) {
ALOGE("%s: Unable to configure stream buffer format %#x for stream %d",
__FUNCTION__, camera3_stream::format, mId);
return res;
}

int maxConsumerBuffers;
res = mConsumer->query(mConsumer.get(),
NATIVE_WINDOW_MIN_UNDEQUEUED_BUFFERS, &maxConsumerBuffers);//支持的最大buffer数量
if (res != OK) {
ALOGE("%s: Unable to query consumer undequeued"
" buffer count for stream %d", __FUNCTION__, mId);
return res;
}

ALOGV("%s: Consumer wants %d buffers, HAL wants %d", __FUNCTION__,
maxConsumerBuffers, camera3_stream::max_buffers);
if (camera3_stream::max_buffers == 0) {
ALOGE("%s: Camera HAL requested max_buffer count: %d, requires at least 1",
__FUNCTION__, camera3_stream::max_buffers);
return INVALID_OPERATION;
}

mTotalBufferCount = maxConsumerBuffers + camera3_stream::max_buffers;//至少2个buffer
mHandoutTotalBufferCount = 0;
mFrameCount = 0;
mLastTimestamp = 0;

res = native_window_set_buffer_count(mConsumer.get(),
mTotalBufferCount);
if (res != OK) {
ALOGE("%s: Unable to set buffer count for stream %d",
__FUNCTION__, mId);
return res;
}

res = native_window_set_buffers_transform(mConsumer.get(),
mTransform);
if (res != OK) {
ALOGE("%s: Unable to configure stream transform to %x: %s (%d)",
__FUNCTION__, mTransform, strerror(-res), res);
}

return OK;
}如果你对SurfaceFlinger的架构熟悉的话，该代码会相对比较好理解。本质是根据当前stream设置的buffer属性，将这些属性值通过ANativeWindow这个接口传递给Consumer侧去维护：这里重点关注以下几个buffer的相关属性信息：比如native_window_set_buffer_count是设置当前Window所需要的buffer数目：    总的当前stream下的buffer个数总数为mTotalBufferCount = maxConsumerBuffers + camera3_stream::max_buffers。其中camera3_stream::max_buffer需要的buffer总数由configureStreams时HAL3底层的Device来决定的，高通平台下定义的camera3_stream::max_buffer数为7个，而maxConsumerBuffers指的是在所有buffer被dequeue时还应该保留的处于queue操作的buffer个数，即全dequeue时至少有maxConsumerBuffers个buffer是处于queue状态在被Consumer使用的。通过query NATIVE_WINDOW_MIN_UNDEQUEUED_BUFFERS来完成，一般默认是1个，即每个stream可以认为需要由8个buffer缓存块组成，实际可dequeue的为8个。比如native_window_set_buffers_transform一般是指定buffer的Consumer，即当前buffer显示的90/180/270°角度。

该过程本质是结合HAL3的底层buffer配置需求，反过来请求Buffer的Consumer端BufferQueueConsumer来设置相关的buffer属性。
registerBuffersLocked是一个比较重要的处理过程：
status_t Camera3Stream::registerBuffersLocked(camera3_device *hal3Device) {
ATRACE_CALL();

/**
* >= CAMERA_DEVICE_API_VERSION_3_2:
*
* camera3_device_t->ops->register_stream_buffers() is not called and must
* be NULL.
*/
if (hal3Device->common.version >= CAMERA_DEVICE_API_VERSION_3_2) {
ALOGV("%s: register_stream_buffers unused as of HAL3.2", __FUNCTION__);

if (hal3Device->ops->register_stream_buffers != NULL) {
ALOGE("%s: register_stream_buffers is deprecated in HAL3.2; "
"must be set to NULL in camera3_device::ops", __FUNCTION__);
return INVALID_OPERATION;
} else {
ALOGD("%s: Skipping NULL check for deprecated register_stream_buffers", __FUNCTION__);
}

return OK;
} else {
ALOGV("%s: register_stream_buffers using deprecated code path", __FUNCTION__);
}

status_t res;

size_t bufferCount = getBufferCountLocked();//获取buffer的数量,mTotalBufferCount，最少2个buffer

Vector<buffer_handle_t*> buffers;
buffers.insertAt(/*prototype_item*/NULL, /*index*/0, bufferCount);

camera3_stream_buffer_set bufferSet = camera3_stream_buffer_set();
bufferSet.stream = this;//新的bufferset指向camera3_stream_t
bufferSet.num_buffers = bufferCount;//当前stream下的buffer数
bufferSet.buffers = buffers.editArray();

Vector<camera3_stream_buffer_t> streamBuffers;
streamBuffers.insertAt(camera3_stream_buffer_t(), /*index*/0, bufferCount);

// Register all buffers with the HAL. This means getting all the buffers
// from the stream, providing them to the HAL with the
// register_stream_buffers() method, and then returning them back to the
// stream in the error state, since they won't have valid data.
//
// Only registered buffers can be sent to the HAL.

uint32_t bufferIdx = 0;
for (; bufferIdx < bufferCount; bufferIdx++) {
res = getBufferLocked( &streamBuffers.editItemAt(bufferIdx) );//返回dequeue buffer出来的所有buffer
if (res != OK) {
ALOGE("%s: Unable to get buffer %d for registration with HAL",
__FUNCTION__, bufferIdx);
// Skip registering, go straight to cleanup
break;
}

sp<Fence> fence = new Fence(streamBuffers[bufferIdx].acquire_fence);
fence->waitForever("Camera3Stream::registerBuffers");//等待可写

buffers.editItemAt(bufferIdx) = streamBuffers[bufferIdx].buffer;//dequeue buffer出来的buffer handle
}
if (bufferIdx == bufferCount) {
// Got all buffers, register with HAL
ALOGV("%s: Registering %zu buffers with camera HAL",
__FUNCTION__, bufferCount);
ATRACE_BEGIN("camera3->register_stream_buffers");
res = hal3Device->ops->register_stream_buffers(hal3Device,
&bufferSet);//buffer绑定并register到hal层
ATRACE_END();
}

// Return all valid buffers to stream, in ERROR state to indicate
// they weren't filled.
for (size_t i = 0; i < bufferIdx; i++) {
streamBuffers.editItemAt(i).release_fence = -1;
streamBuffers.editItemAt(i).status = CAMERA3_BUFFER_STATUS_ERROR;
returnBufferLocked(streamBuffers[i], 0);//register后进行queue buffer
}

return res;
}
a 可以明确看到CAMERA_DEVICE_API_VERSION_3_2的版本才支持这个Device ops接口
b getBufferCountLocked获取当前stream下的允许的buffer总数c camera3_stream_buffer_t、camera3_stream_buffer_set和buffer_handle_t首先需要关注的结构是camera3_stream_buffer_t，用于描述每一个stream下的buffer自身的特性值，其中关键结构是buffer_handle_t值是每一个buffer在不同进程间共享的handle，此外acquire_fence和release_fence用来不同硬件模块对buffer读写时的同步。
camera3_stream_buffer_set是封装了当前stream下所有的buffer的信息：typedef struct camera3_stream_buffer_set {
/**
* The stream handle for the stream these buffers belong to
*/
camera3_stream_t *stream;
/**
* The number of buffers in this stream. It is guaranteed to be at least
* stream->max_buffers.
*/
uint32_t num_buffers;
/**
* The array of gralloc buffer handles for this stream. If the stream format
* is set to HAL_PIXEL_FORMAT_IMPLEMENTATION_DEFINED, the camera HAL device
* should inspect the passed-in buffers to determine any platform-private
* pixel format information.
*/
buffer_handle_t **buffers;

} camera3_stream_buffer_set_t;
三个变量分别保存stream的buffer个数，当前这个set集合所属的stream，以及他所包含的所有buffer的handle信息列表。
d getBufferLocked获取当前bufferstatus_t Camera3OutputStream::getBufferLocked(camera3_stream_buffer *buffer) {
ATRACE_CALL();
status_t res;

if ((res = getBufferPreconditionCheckLocked()) != OK) {
return res;
}

ANativeWindowBuffer* anb;
int fenceFd;

/**
* Release the lock briefly to avoid deadlock for below scenario:
* Thread 1: StreamingProcessor::startStream -> Camera3Stream::isConfiguring().
* This thread acquired StreamingProcessor lock and try to lock Camera3Stream lock.
* Thread 2: Camera3Stream::returnBuffer->StreamingProcessor::onFrameAvailable().
* This thread acquired Camera3Stream lock and bufferQueue lock, and try to lock
* StreamingProcessor lock.
* Thread 3: Camera3Stream::getBuffer(). This thread acquired Camera3Stream lock
* and try to lock bufferQueue lock.
* Then there is circular locking dependency.
*/
sp<ANativeWindow> currentConsumer = mConsumer;
mLock.unlock();

res = currentConsumer->dequeueBuffer(currentConsumer.get(), &anb, &fenceFd);
mLock.lock();
if (res != OK) {
ALOGE("%s: Stream %d: Can't dequeue next output buffer: %s (%d)",
__FUNCTION__, mId, strerror(-res), res);
return res;
}

/**
* FenceFD now owned by HAL except in case of error,
* in which case we reassign it to acquire_fence
*/
handoutBufferLocked(*buffer, &(anb->handle), /*acquireFence*/fenceFd,
/*releaseFence*/-1, CAMERA3_BUFFER_STATUS_OK, /*output*/true);

return OK;
}
该函数主要是从由ANativeWindow从Consumer端dequeue获取一个buffer，本质上这个过程中首次执行是会有Consumer端去分配一个由实际物理空间的给当前的一个buffer的。

接着执行handoutBufferLocked，填充camera3_stream_buffer这个结构体，其中设置的acquireFence为-1值表明hal3的这个buffer可被Framewrok直接使用，而acquireFence表示HAL3如何想使用这个buffer时需要等待其变为1，因为buffer分配和handler返回不一定是一致同步的。还会切换当前buffer的状态CAMERA3_BUFFER_STATUS_OK。void Camera3IOStreamBase::handoutBufferLocked(camera3_stream_buffer &buffer,
buffer_handle_t *handle,
int acquireFence,
int releaseFence,
camera3_buffer_status_t status,
bool output) {
/**
* Note that all fences are now owned by HAL.
*/

// Handing out a raw pointer to this object. Increment internal refcount.
incStrong(this);
buffer.stream = this;
buffer.buffer = handle;
buffer.acquire_fence = acquireFence;
buffer.release_fence = releaseFence;
buffer.status = status;

// Inform tracker about becoming busy
if (mHandoutTotalBufferCount == 0 && mState != STATE_IN_CONFIG &&
mState != STATE_IN_RECONFIG) {
/**
* Avoid a spurious IDLE->ACTIVE->IDLE transition when using buffers
* before/after register_stream_buffers during initial configuration
* or re-configuration.
*
* TODO: IN_CONFIG and IN_RECONFIG checks only make sense for <HAL3.2
*/
sp<StatusTracker> statusTracker = mStatusTracker.promote();
if (statusTracker != 0) {
statusTracker->markComponentActive(mStatusId);
}
}
mHandoutTotalBufferCount++;//统计dequeuebuffer的数量

if (output) {
mHandoutOutputBufferCount++;
}
}
e hal3Device->ops->register_stream_buffers(hal3Device,&bufferSet);//buffer绑定并register到hal层将所属的stream下的所有buffer有关的信息，主要是每个buffer的buffer_handle_t值，交给HAL3层去实现。比如高通HAL3平台每一个Channel对应于Camera3Device端的stream，而每一个stream的buffer在不同的Channel下面却是一个个的stream，这是高通的实现方式。
f 在完成register所有buffer后，设置每一个buffer状态为从CAMERA3_BUFFER_STATUS_OK切换到CAMERA3_BUFFER_STATUS_ERROR表明这个buffer都是可用的，目的在于执行returnBufferLocked是为了将这些因为register而出列的所有buffer再次cancelbuffer操作。Camera3OutputStream::returnBufferLocked->Camera3IOStreamBase::returnAnyBufferLocked->Camera3OutputStream::returnBufferCheckedLocked
status_t Camera3OutputStream::returnBufferCheckedLocked(//result返回时调用
const camera3_stream_buffer &buffer,
nsecs_t timestamp,
bool output,
/*out*/
sp<Fence> *releaseFenceOut) {

(void)output;
ALOG_ASSERT(output, "Expected output to be true");

status_t res;
sp<Fence> releaseFence;

/**
* Fence management - calculate Release Fence
*/
if (buffer.status == CAMERA3_BUFFER_STATUS_ERROR) {
if (buffer.release_fence != -1) {
ALOGE("%s: Stream %d: HAL should not set release_fence(%d) when "
"there is an error", __FUNCTION__, mId, buffer.release_fence);
close(buffer.release_fence);
}

/**
* Reassign release fence as the acquire fence in case of error
*/
releaseFence = new Fence(buffer.acquire_fence);
} else {
res = native_window_set_buffers_timestamp(mConsumer.get(), timestamp);
if (res != OK) {
ALOGE("%s: Stream %d: Error setting timestamp: %s (%d)",
__FUNCTION__, mId, strerror(-res), res);
return res;
}

releaseFence = new Fence(buffer.release_fence);
}

int anwReleaseFence = releaseFence->dup();

/**
* Release the lock briefly to avoid deadlock with
* StreamingProcessor::startStream -> Camera3Stream::isConfiguring (this
* thread will go into StreamingProcessor::onFrameAvailable) during
* queueBuffer
*/
sp<ANativeWindow> currentConsumer = mConsumer;
mLock.unlock();

/**
* Return buffer back to ANativeWindow
*/
if (buffer.status == CAMERA3_BUFFER_STATUS_ERROR) {
// Cancel buffer
res = currentConsumer->cancelBuffer(currentConsumer.get(),
container_of(buffer.buffer, ANativeWindowBuffer, handle),
anwReleaseFence);//Register buffer locked所在的事情，cancelbuffer dequeue的buffer
if (res != OK) {
ALOGE("%s: Stream %d: Error cancelling buffer to native window:"
" %s (%d)", __FUNCTION__, mId, strerror(-res), res);
}
} else {
if (mTraceFirstBuffer && (stream_type == CAMERA3_STREAM_OUTPUT)) {
{
char traceLog[48];
snprintf(traceLog, sizeof(traceLog), "Stream %d: first full buffer\n", mId);
ATRACE_NAME(traceLog);
}
mTraceFirstBuffer = false;
}

res = currentConsumer->queueBuffer(currentConsumer.get(),
container_of(buffer.buffer, ANativeWindowBuffer, handle),
anwReleaseFence);//queuebuffer，送显ANativeWindowBuffer
if (res != OK) {
ALOGE("%s: Stream %d: Error queueing buffer to native window: "
"%s (%d)", __FUNCTION__, mId, strerror(-res), res);
}
}
mLock.lock();
if (res != OK) {
close(anwReleaseFence);
}

*releaseFenceOut = releaseFence;

return res;
}
该函数对于首次register的处理来说，他处理的buffer均是CAMERA3_BUFFER_STATUS_ERROR，调用了cancelBuffer将所有buffer的状态都还原为free的状态，依次说明目前的buffer均是可用的，之前均不涉及到对buffer的数据流的操作。

3 buffer数据流的dequeue操作上述步骤2主要是将每一个Stream下全部的buffer信息全部register到下层的HAL3中，为后续对buffer的数据流读写操作奠定基础。那么preview模式下我们又是如何去获得一帧完成的视频流的呢？触发点就是preview模式下的Request，前面提到过一个mPreviewRequest至少包含一个StreamProcessor和一个CallbackProcessor的两路stream，每路stream拥有不同的buffer数量。比如要从HAL3获取一帧图像数据，最简单的思路就是从StreamProcessor下的Outputstream流中下发一个可用的buffer地址，然后HAL3填充下数据，Framework就可以拥有一帧数据了。根据这个思路，回顾到前一博文中每次会不断的下发一个Request命令包到HAL3中，在这里我们就可以看到这个buffer地址身影。Camera3Device::RequestThread::threadLoop() 下的部分代码：
outputBuffers.insertAt(camera3_stream_buffer_t(), 0,
nextRequest->mOutputStreams.size());//Streamprocess,Callbackprocessor
request.output_buffers = outputBuffers.array();//camera3_stream_buffer_t
for (size_t i = 0; i < nextRequest->mOutputStreams.size(); i++) {
res = nextRequest->mOutputStreams.editItemAt(i)->
getBuffer(&outputBuffers.editItemAt(i));//等待获取buffer,内部是dequeue一根buffer填充到camera3_stream_buffer_t
if (res != OK) {
// Can't get output buffer from gralloc queue - this could be due to
// abandoned queue or other consumer misbehavior, so not a fatal
// error
ALOGE("RequestThread: Can't get output buffer, skipping request:"
" %s (%d)", strerror(-res), res);
Mutex::Autolock l(mRequestLock);
if (mListener != NULL) {
mListener->notifyError(
ICameraDeviceCallbacks::ERROR_CAMERA_REQUEST,
nextRequest->mResultExtras);
}
cleanUpFailedRequest(request, nextRequest, outputBuffers);
return true;
}
request.num_output_buffers++;//一般一根OutStream对应一个buffer,故总的out_buffer数目
}在这个下发到HAL3的camera3_capture_request中，可以看到 const camera3_stream_buffer_t *output_buffers，下面的代码可以说明这一次的Request的output_buffers是打包了当前Camera3Device所拥有的mOutputStreams。 outputBuffers.insertAt(camera3_stream_buffer_t(), 0,
nextRequest->mOutputStreams.size());//Streamprocess,Callbackprocessor对于每一个OutputStream他会给她分配一个buffer handle。关注下面的处理代码：nextRequest->mOutputStreams.editItemAt(i)->
getBuffer(&outputBuffers.editItemAt(i))nextRequest->mOutputStreams.editItemAt(i)是获取一个Camera3OutputStream对象，然后对getBuffer而言传入的是这个Camera3OutputStream所对应的这次buffer的输入位置，这个camera3_stream_buffer是需要从Camera3OutputStream对象中去获取的。status_t Camera3Stream::getBuffer(camera3_stream_buffer *buffer) {
ATRACE_CALL();
Mutex::Autolock l(mLock);
status_t res = OK;

// This function should be only called when the stream is configured already.
if (mState != STATE_CONFIGURED) {
ALOGE("%s: Stream %d: Can't get buffers if stream is not in CONFIGURED state %d",
__FUNCTION__, mId, mState);
return INVALID_OPERATION;
}

// Wait for new buffer returned back if we are running into the limit.
if (getHandoutOutputBufferCountLocked() == camera3_stream::max_buffers) {//dequeue过多时等待queue的释放
ALOGV("%s: Already dequeued max output buffers (%d), wait for next returned one.",
__FUNCTION__, camera3_stream::max_buffers);
res = mOutputBufferReturnedSignal.waitRelative(mLock, kWaitForBufferDuration);
if (res != OK) {
if (res == TIMED_OUT) {
ALOGE("%s: wait for output buffer return timed out after %lldms", __FUNCTION__,
kWaitForBufferDuration / 1000000LL);
}
return res;
}
}

res = getBufferLocked(buffer);
if (res == OK) {
fireBufferListenersLocked(*buffer, /*acquired*/true, /*output*/true);
}

return res;
}
上述的代码先是检查dequeue了的buffer是否已经达到本stream申请的buffer数目的最大值，如果已经全部dequeue的话就得wait到当前已经有buffer return并且queue操作后，在处理完成后才允许将从buffer队列中再次执行dequeue操作。    随后调用getBufferLocked通过2.2(d）小节可以知道是从buffer队列中获取一个可用的buffer，并填充这个camera3_stream_buffer值。这样处理完的结果是，下发的Request包含所有模块下的outputstream，同时每个stream都配备了一个camera3_stream_buffer供底层HAL3.0去处理，而这个buffer在Camera3Device模式下，可以是交互的是帧图像数据，可以是参数控制命令，也可以是其他的3A信息，这些不同的信息一般归不同的模块管理，也就是不同的stream来处理。
4 buffer数据流的queue操作    dequeue出来的buffer信息已经随着Request下发到了HAL3层，在Camera3Device架构下，可以使用一个Callback接口将数据从HAL3回传到Camera所在的Framework层。Camera3Device私有继承了一个Callback接口camera3_callback_ops数据结构，分别预留了notify和process_capture_result。前者是用于回调一些shutter已经error等信息，后者以Callback数据流为主，这个回调接口通过device->initialize(camera3_device, this)来完成注册。void Camera3Device::sProcessCaptureResult(const camera3_callback_ops *cb,
const camera3_capture_result *result) {
Camera3Device *d =
const_cast<Camera3Device*>(static_cast<const Camera3Device*>(cb));
d->processCaptureResult(result);
}
返回的buffer所有信息均包含在camera3_capture_result中，该函数的处理过程相对比较复杂，如果只定位queue buffer的入口可直接到returnOutputBuffers中去：void Camera3Device::returnOutputBuffers(
const camera3_stream_buffer_t *outputBuffers, size_t numBuffers,
nsecs_t timestamp) {
for (size_t i = 0; i < numBuffers; i++)//对每一个buffer所属的stream进行分析
{
Camera3Stream *stream = Camera3Stream::cast(outputBuffers[i].stream);//该buffer对应的camera3_stream
status_t res = stream->returnBuffer(outputBuffers[i], timestamp);//Camera3OutPutStream，每一各stream对应的return
// Note: stream may be deallocated at this point, if this buffer was
// the last reference to it.
if (res != OK) {
ALOGE("Can't return buffer to its stream: %s (%d)",
strerror(-res), res);
}
}
}因为在下发Request时，每一个buffer均包含所述的stream信息，当buffer数据返回到Framework层时，我们又可以转到Camera3OutPutStream来处理这个return的buffer。status_t Camera3Stream::returnBuffer(const camera3_stream_buffer &buffer,
nsecs_t timestamp) {
ATRACE_CALL();
Mutex::Autolock l(mLock);

/**
* TODO: Check that the state is valid first.
*
* <HAL3.2 IN_CONFIG and IN_RECONFIG in addition to CONFIGURED.
* >= HAL3.2 CONFIGURED only
*
* Do this for getBuffer as well.
*/
status_t res = returnBufferLocked(buffer, timestamp);//以queue buffer为主
if (res == OK) {
fireBufferListenersLocked(buffer, /*acquired*/false, /*output*/true);
mOutputBufferReturnedSignal.signal();
}

return res;
}在这里看看registerBuffersLocked，参考前面对这个函数他是register完所有的buffer时被调用，在这里其本质处理的buffer状态不在是CAMERA3_BUFFER_STATUS_ERROR，而是CAMERA3_BUFFER_STATUS_OK故执行的是将会queuebuffer的操作。
5 buffer数据真正的被Consumer处理    在queuebuffer的操作时，参考前一博文Android5.1中surface和CpuConsumer下生产者和消费者间的处理框架简述很容易知道真正的Consumer需要开始工作了，对于preview模式下的当然是由SurfaceFlinger的那套机制去处理。而在Camera2Client和Camera3Device下你还可以看到CPUConsumer的存在，比如：void CallbackProcessor::onFrameAvailable(const BufferItem& /*item*/) {
Mutex::Autolock l(mInputMutex);
if (!mCallbackAvailable) {
mCallbackAvailable = true;
mCallbackAvailableSignal.signal();//数据callback线程处理
}
}在这里，你就可以去处理那些处于queue状态的buffer数据，比如这里的Callback将这帧数据上传会APP。bool CallbackProcessor::threadLoop() {
status_t res;

{
Mutex::Autolock l(mInputMutex);
while (!mCallbackAvailable) {
res = mCallbackAvailableSignal.waitRelative(mInputMutex,
kWaitDuration);
if (res == TIMED_OUT) return true;
}
mCallbackAvailable = false;
}

do {
sp<Camera2Client> client = mClient.promote();
if (client == 0) {
res = discardNewCallback();
} else {
res = processNewCallback(client);//callback 处理新的一帧
}
} while (res == OK);

return true;
}

l.mRemoteCallback->dataCallback(CAMERA_MSG_PREVIEW_FRAME,
callbackHeap->mBuffers[heapIdx], NULL);//处理成API的需求后，回调Preview frame

6 总结到这里，整个preview预览的视频流基本介绍完毕了，主要框架虽然负责，但仔细看看也就是buffer的queue与dequeue操作，真正的HAL3的实现才是最为复杂的。后续还会简单介绍下整个take picture的过程，数据的回调处理在后续中还会继续分析。

下面贴一图是整个Camera3架构下基于Request和result的处理流序图：