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

Ricardo Ribalda
2026-03-24T07:24:05Z

Hi Laurent

On Tue, 24 Mar 2026 at 01:20, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> 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.

SGTM. I will change this locally and upload it after someone reviews
2/2 to avoid bloating the list

Thanks!
>
> 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

--
Ricardo Ribalda