Re: [PATCH] media: uvcvideo: Fix buffer sequence in frame gaps

[Laurent Pinchart]

On Tue, Mar 10, 2026 at 11:54:24AM +0000, Ricardo Ribalda wrote:
> In UVC, the FID flips with every frame. For every FID flip, we increase
> the sequence number. Userpace use that information to figure out if
> there has been a gap between frames.
> 
> Now, we only update the sequence number on the first FID flip, which
> results in frames being incorrectly numbered just after a frame gap.

The sequence number is increased on every FID flip. I assume you mean
that we don't set the buffer sequence number in some cases. "on the
first FID flip" doesn't seem right. Could you please improve the commit
message to describe more clearly what the problem is ?

> This patch rewrites the sequence number of the buffer in those
> situations.
> 
> Cc: stable@xxxxxxxxxx
> Fixes: 650b95feee35 ("[media] uvcvideo: Generate discontinuous sequence numbers when frames are lost")
> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 40c76c051da2..ec769a504569 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1176,6 +1176,14 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
>  		stream->sequence++;
>  		if (stream->sequence)
>  			uvc_video_stats_update(stream);
> +
> +		/*
> +		 * If we have not received any data and FID flips, update the
> +		 * sequence number of the buffer to tell userspace exactly
> +		 * where the frame gap is.
> +		 */
> +		if (buf && !buf->bytesused)
> +			buf->buf.sequence = stream->sequence;

Do we still need to set buf->buf.sequence below ? And how about the
buffer timestamp, isn't it also not set in the case that this patch
addresses ?

>  	}
>  
>  	uvc_video_clock_decode(stream, buf, data, len);
> 
> ---
> base-commit: a7da7fb57f2a787412da1a62292a17fa00fbfbdf
> change-id: 20260310-uvc-fid-e1e55447b6f1

-- 
Regards,

Laurent Pinchart