[libcamera-devel] [RFC PATCH v1 11/12] libcamera: v4l2_videodevice: Split planes when dequeuing buffer

[Kieran Bingham]

On 02/09/2021 05:23, Laurent Pinchart wrote:
> When dequeueing a buffer from a V4L2VideoDevice, the number of planes in
> the FrameBuffer may not match the number of V4L2 buffer planes if the
> PixelFormat is multi-planar (has multiple colour planes) and the V4L2
> format is single-planar (has a single buffer plane). In this case, we
> need to split the single V4L2 buffer plane into FrameBuffer planes. Do
> so, and add checks to reject invalid V4L2 buffers in case of a driver
> issue.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/libcamera/v4l2_videodevice.cpp | 54 ++++++++++++++++++++++++------
>  1 file changed, 43 insertions(+), 11 deletions(-)
> 
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index 79cb792117d5..b80b9038a914 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -7,6 +7,7 @@
>  
>  #include "libcamera/internal/v4l2_videodevice.h"
>  
> +#include <algorithm>
>  #include <array>
>  #include <fcntl.h>
>  #include <iomanip>
> @@ -1637,18 +1638,49 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
>  	buffer->metadata_.timestamp = buf.timestamp.tv_sec * 1000000000ULL
>  				    + buf.timestamp.tv_usec * 1000ULL;
>  
> -	if (multiPlanar) {
> -		if (buf.length > buffer->metadata_.planes().size()) {
> -			LOG(V4L2, Error)
> -				<< "Invalid number of planes (" << buf.length
> -				<< " != " << buffer->metadata_.planes().size() << ")";
> -			return nullptr;
> +	if (V4L2_TYPE_IS_CAPTURE(buf.type)) {

Should we swap this around and return the buffer early to lower the
indents below?

 if (!V4L2_TYPE_IS_CAPTURE(buf.type)) {
	/* Output buffer is completed */
	return buffer;
 }


> +		unsigned int numV4l2Planes = multiPlanar ? buf.length : 1;
> +		FrameMetadata &metadata = buffer->metadata_;
> +
> +		if (numV4l2Planes != metadata.planes().size()) {
> +			/*
> +			 * If we have a multi-planar buffer with a V4L2
> +			 * single-planar format, split the V4L2 buffer across
> +			 * the buffer planes. Only the last plane may have less
> +			 * bytes used than its length.
> +			 */
> +			if (numV4l2Planes != 1) {
> +				LOG(V4L2, Error)
> +					<< "Invalid number of planes (" << numV4l2Planes
> +					<< " != " << metadata.planes().size() << ")";
> +				return nullptr;
> +			}
> +
> +			unsigned int bytesused = multiPlanar ? planes[0].bytesused
> +					       : buf.bytesused;
> +
> +			for (auto [i, plane] : utils::enumerate(buffer->planes())) {
> +				if (!bytesused) {
> +					LOG(V4L2, Error)
> +						<< "Dequeued buffer is too small";

Presumably if we got here - it's likely something (hardware) already
overwrote memory it likely didn't own?

> +					return nullptr;

I'm still very weary of returning nullptrs on dequeue when a buffer is
removed from queuedBuffers_


Wouldn't it be better to return the buffer, but mark it as corrupted or
invalid or such ?


> +				}
> +
> +				metadata.planes()[i].bytesused =
> +					std::min(plane.length, bytesused);
> +				bytesused -= metadata.planes()[i].bytesused;
> +			}
> +		} else if (multiPlanar) {
> +			/*
> +			 * If we use the multi-planar API, fill in the planes.
> +			 * The number of planes in the frame buffer and in the
> +			 * V4L2 buffer is guaranteed to be equal at this point.
> +			 */
> +			for (unsigned int i = 0; i < numV4l2Planes; ++i)
> +				metadata.planes()[i].bytesused = planes[i].bytesused;
> +		} else {
> +			metadata.planes()[0].bytesused = buf.bytesused;
>  		}
> -
> -		for (unsigned int nplane = 0; nplane < buf.length; nplane++)
> -			buffer->metadata_.planes()[nplane].bytesused = planes[nplane].bytesused;
> -	} else {
> -		buffer->metadata_.planes()[0].bytesused = buf.bytesused;
>  	}
>  
>  	return buffer;
>