[libcamera-devel] [PATCH] libcamera: v4l2_videodevice: Explain multiplanar bytesused reasoning

[Laurent Pinchart]

Hi Kieran,

Thank you for the patch.

On Thu, Oct 14, 2021 at 09:54:05AM +0100, Kieran Bingham wrote:
> The ternary operation used to get the total bytesused of a V4L2 single
> planar format which is stored in a multiplanar buffer can easily be
> mis-read to think it's a bug, and appears to be reading the value of the
> first of N planes as the total.
> 
> Directly explain the reasoning for why it looks like the condition is
> inverted, as it is correct that the total bytes used is stored in only
> the first plane of the multiplanar buffer.
> 
> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> ---
>  src/libcamera/v4l2_videodevice.cpp | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index ba5f88cd41ed..f4752f8bb23f 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -1711,6 +1711,7 @@ FrameBuffer *V4L2VideoDevice::dequeueBuffer()
>  			return buffer;
>  		}
>  
> +		/* Single planar format stored in a multiplanar buffer */

Now I find this a bit confusing :-) How about the following ?

		/*
		 * With a V4L2 single-planar format, all the data is stored in a
		 * single memory plane. The number of bytes used is conveyed
		 * through that plane when using the V4L2 multi-planar API, or
		 * set directly in the buffer when using the V4L2 single-planar
		 * API.
		 */

It's a bit verbose perhaps ?

>  		unsigned int bytesused = multiPlanar ? planes[0].bytesused
>  				       : buf.bytesused;
>  

-- 
Regards,

Laurent Pinchart