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

Laurent Pinchart
2026-03-24T00:20:25Z

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