Re: [PATCH v5 1/2] media: uvcvideo: Fix sequence number when no EOF

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Mon, Mar 23, 2026 at 09:53:52AM +0000, Ricardo Ribalda wrote:
> If the driver could not detect the EOF, the sequence number is increased
> twice:
>  1) When we enter uvc_video_decode_start() with the old buffer and FID has
>    flipped => We return -EAGAIN and last_fid is not flipped
>  2) When we enter uvc_video_decode_start() with the new buffer.
> 
> Fix this issue by moving the new frame detection logic earlier in
> uvc_video_decode_start().
> 
> This also has some nice side affects:
> 
> - The error status from the new packet will no longer get propagated
>   to the previous frame-buffer.
> - uvc_video_clock_decode() will no longer update the previous frame
>   buf->stf with info from the new packet.
> - uvc_video_clock_decode() and uvc_video_stats_decode() will no longer
>   get called twice for the same packet.
> 
> Cc: stable@kernel.org
> Fixes: 650b95feee35 ("[media] uvcvideo: Generate discontinuous sequence numbers when frames are lost")
> Reported-by: Hans de Goede <hansg@kernel.org>
> Closes: https://lore.kernel.org/linux-media/CANiDSCuj4cPuB5_v2xyvAagA5FjoN8V5scXiFFOeD3aKDMqkCg@mail.gmail.com/T/#me39fb134e8c2c085567a31548c3403eb639625e4
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 92 ++++++++++++++++++++-------------------
>  1 file changed, 47 insertions(+), 45 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 40c76c051da2..eddb4821b205 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1168,6 +1168,53 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
>  	header_len = data[0];
>  	fid = data[1] & UVC_STREAM_FID;
>  
> +	/*
> +	 * Mark the buffer as done if we're at the beginning of a new frame.
> +	 * End of frame detection is better implemented by checking the EOF
> +	 * bit (FID bit toggling is delayed by one frame compared to the EOF
> +	 * bit), but some devices don't set the bit at end of frame (and the
> +	 * last payload can be lost anyway). We thus must check if the FID has
> +	 * been toggled.
> +	 *
> +	 * stream->last_fid is initialized to -1, and buf->bytesused to 0,
> +	 * so the first isochronous frame will never trigger an end of frame
> +	 * detection.
> +	 *
> +	 * Empty buffers (bytesused == 0) don't trigger end of frame detection
> +	 * as it doesn't make sense to return an empty buffer. This also
> +	 * avoids detecting end of frame conditions at FID toggling if the
> +	 * previous payload had the EOF bit set.
> +	 */
> +	if (fid != stream->last_fid && buf && buf->bytesused != 0) {
> +		uvc_dbg(stream->dev, FRAME,
> +			"Frame complete (FID bit toggled)\n");
> +		buf->state = UVC_BUF_STATE_READY;
> +
> +		return -EAGAIN;
> +	}
> +
> +	/*
> +	 * Some cameras, when running two parallel streams (one MJPEG alongside
> +	 * another non-MJPEG stream), are known to lose the EOF packet for a frame.
> +	 * We can detect the end of a frame by checking for a new SOI marker, as
> +	 * the SOI always lies on the packet boundary between two frames for
> +	 * these devices.
> +	 */
> +	if (stream->dev->quirks & UVC_QUIRK_MJPEG_NO_EOF &&
> +	    (stream->cur_format->fcc == V4L2_PIX_FMT_MJPEG ||
> +	    stream->cur_format->fcc == V4L2_PIX_FMT_JPEG)) {
> +		const u8 *packet = data + header_len;
> +
> +		if (len >= header_len + 2 &&
> +		    packet[0] == 0xff && packet[1] == JPEG_MARKER_SOI &&
> +		    buf && buf->bytesused != 0) {

How about moving the buf && buf->bytesused != 0 to the outer condition,
so that we don't inspect the packet when we don't need to ? It will also
make the two end of frame detection blocks more similar.

The patch otherwise looks fine, I don't see anything below this code
that would need to be performed before.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> +			buf->state = UVC_BUF_STATE_READY;
> +			buf->error = 1;
> +			stream->last_fid ^= UVC_STREAM_FID;
> +			return -EAGAIN;
> +		}
> +	}
> +
>  	/*
>  	 * Increase the sequence number regardless of any buffer states, so
>  	 * that discontinuous sequence numbers always indicate lost frames.
> @@ -1224,51 +1271,6 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
>  		buf->state = UVC_BUF_STATE_ACTIVE;
>  	}
>  
> -	/*
> -	 * Mark the buffer as done if we're at the beginning of a new frame.
> -	 * End of frame detection is better implemented by checking the EOF
> -	 * bit (FID bit toggling is delayed by one frame compared to the EOF
> -	 * bit), but some devices don't set the bit at end of frame (and the
> -	 * last payload can be lost anyway). We thus must check if the FID has
> -	 * been toggled.
> -	 *
> -	 * stream->last_fid is initialized to -1, so the first isochronous
> -	 * frame will never trigger an end of frame detection.
> -	 *
> -	 * Empty buffers (bytesused == 0) don't trigger end of frame detection
> -	 * as it doesn't make sense to return an empty buffer. This also
> -	 * avoids detecting end of frame conditions at FID toggling if the
> -	 * previous payload had the EOF bit set.
> -	 */
> -	if (fid != stream->last_fid && buf->bytesused != 0) {
> -		uvc_dbg(stream->dev, FRAME,
> -			"Frame complete (FID bit toggled)\n");
> -		buf->state = UVC_BUF_STATE_READY;
> -		return -EAGAIN;
> -	}
> -
> -	/*
> -	 * Some cameras, when running two parallel streams (one MJPEG alongside
> -	 * another non-MJPEG stream), are known to lose the EOF packet for a frame.
> -	 * We can detect the end of a frame by checking for a new SOI marker, as
> -	 * the SOI always lies on the packet boundary between two frames for
> -	 * these devices.
> -	 */
> -	if (stream->dev->quirks & UVC_QUIRK_MJPEG_NO_EOF &&
> -	    (stream->cur_format->fcc == V4L2_PIX_FMT_MJPEG ||
> -	    stream->cur_format->fcc == V4L2_PIX_FMT_JPEG)) {
> -		const u8 *packet = data + header_len;
> -
> -		if (len >= header_len + 2 &&
> -		    packet[0] == 0xff && packet[1] == JPEG_MARKER_SOI &&
> -		    buf->bytesused != 0) {
> -			buf->state = UVC_BUF_STATE_READY;
> -			buf->error = 1;
> -			stream->last_fid ^= UVC_STREAM_FID;
> -			return -EAGAIN;
> -		}
> -	}
> -
>  	stream->last_fid = fid;
>  
>  	return header_len;
> 

-- 
Regards,

Laurent Pinchart