android: Document the structures and functions for post-processing

Specifically document:
 - CameraDevice::sendCaptureResults()
 - CameraDevice::completeDescriptor()
 - CameraDevice::streamProcessingComplete()

 - CameraStream::PostProcessorWorker class
 - Camera3RequestDescriptor::StreamBuffer structure

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/android/camera_device.cpp

@@ -1164,6 +1164,17 @@ void CameraDevice::requestComplete(Request *request)
 	}
 }
 
+/**
+ * \brief Complete the Camera3RequestDescriptor
+ * \param[in] descriptor The Camera3RequestDescriptor that has completed
+ *
+ * The function marks the Camera3RequestDescriptor as 'complete'. It shall be
+ * called when all the streams in the Camera3RequestDescriptor have completed
+ * capture (or have been generated via post-processing) and the request is ready
+ * to be sent back to the framework.
+ *
+ * \context This function is \threadsafe.
+ */
 void CameraDevice::completeDescriptor(Camera3RequestDescriptor *descriptor)
 {
 	MutexLocker lock(descriptorsMutex_);
@@ -1172,6 +1183,19 @@ void CameraDevice::completeDescriptor(Camera3RequestDescriptor *descriptor)
 	sendCaptureResults();
 }
 
+/**
+ * \brief Sequentially send capture results to the framework
+ *
+ * Iterate over the descriptors queue to send completed descriptors back to the
+ * framework, in the same order as they have been queued. For each complete
+ * descriptor, populate a locally-scoped camera3_capture_result_t from the
+ * descriptor, send the capture result back by calling the
+ * process_capture_result() callback, and remove the descriptor from the queue.
+ * Stop iterating if the descriptor at the front of the queue is not complete.
+ *
+ * This function should never be called directly in the codebase. Use
+ * completeDescriptor() instead.
+ */
 void CameraDevice::sendCaptureResults()
 {
 	while (!descriptors_.empty() && !descriptors_.front()->isPending()) {
@@ -1231,6 +1255,19 @@ void CameraDevice::setBufferStatus(Camera3RequestDescriptor::StreamBuffer &strea
 	}
 }
 
+/**
+ * \brief Handle post-processing completion of a stream in a capture request
+ * \param[in] streamBuffer The StreamBuffer for which processing is complete
+ * \param[in] status Stream post-processing status
+ *
+ * This function is called from the post-processor's thread whenever a camera
+ * stream has finished post processing. The corresponding entry is dropped from
+ * the descriptor's pendingStreamsToProcess_ map.
+ *
+ * If the pendingStreamsToProcess_ map is then empty, all streams requiring to
+ * be generated from post-processing have been completed. Mark the descriptor as
+ * complete using completeDescriptor() in that case.
+ */
 void CameraDevice::streamProcessingComplete(Camera3RequestDescriptor::StreamBuffer *streamBuffer,
 					    Camera3RequestDescriptor::Status status)
 {

src/android/camera_request.cpp

@@ -52,6 +52,44 @@ Camera3RequestDescriptor::Camera3RequestDescriptor(
 
 Camera3RequestDescriptor::~Camera3RequestDescriptor() = default;
 
+/**
+ * \struct Camera3RequestDescriptor::StreamBuffer
+ * \brief Group information for per-stream buffer of Camera3RequestDescriptor
+ *
+ * A capture request placed to the libcamera HAL can contain multiple streams.
+ * Each stream will have an associated buffer to be filled. StreamBuffer
+ * tracks this buffer with contextual information which aids in the stream's
+ * generation. The generation of the stream will depend on its type (refer to
+ * the CameraStream::Type documentation).
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::stream
+ * \brief Pointer to the corresponding CameraStream
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::camera3Buffer
+ * \brief Native handle to the buffer
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::frameBuffer
+ * \brief Encapsulate the dmabuf handle inside a libcamera::FrameBuffer for
+ * direct streams
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::fence
+ * \brief Acquire fence of the buffer
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::status
+ * \brief Track the status of the buffer
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::internalBuffer
+ * \brief Pointer to a buffer internally handled by CameraStream (if any)
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::srcBuffer
+ * \brief Pointer to the source frame buffer used for post-processing
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::dstBuffer
+ * \brief Pointer to the destination frame buffer used for post-processing
+ *
+ * \var Camera3RequestDescriptor::StreamBuffer::request
+ * \brief Back pointer to the Camera3RequestDescriptor to which the StreamBuffer belongs
+ */
 Camera3RequestDescriptor::StreamBuffer::StreamBuffer(
 	CameraStream *cameraStream, const camera3_stream_buffer_t &buffer,
 	Camera3RequestDescriptor *requestDescriptor)

src/android/camera_stream.cpp

@@ -244,6 +244,18 @@ void CameraStream::putBuffer(FrameBuffer *buffer)
 	buffers_.push_back(buffer);
 }
 
+/**
+ * \class CameraStream::PostProcessorWorker
+ * \brief Post-process a CameraStream in an internal thread
+ *
+ * If the association between CameraStream and camera3_stream_t dictated by
+ * CameraStream::Type is internal or mapped, the stream is generated by post
+ * processing of a libcamera stream. Such a request is queued to a
+ * PostProcessorWorker in CameraStream::process(). A queue of post-processing
+ * requests is maintained by the PostProcessorWorker and it will run the
+ * post-processing on an internal thread as soon as any request is available on
+ * its queue.
+ */
 CameraStream::PostProcessorWorker::PostProcessorWorker(PostProcessor *postProcessor)
 	: postProcessor_(postProcessor)
 {