Re: [PATCH v2] media: uvcvideo: Implement V4L2_EVENT_FRAME_SYNC

[Ricardo Ribalda]

Hi Sakari

On Mon, 6 Nov 2023 at 12:06, Sakari Ailus <sakari.ailus@xxxxxx> wrote:
>
> Hi Ricardo,
>
> On Mon, Nov 06, 2023 at 10:52:27AM +0000, Ricardo Ribalda wrote:
> > Add support for the frame_sync event, so user-space can become aware
> > earlier of new frames.
> >
> > Suggested-by: Esker Wong <esker@xxxxxxxxxxxx>
> > Tested-by: Esker Wong <esker@xxxxxxxxxxxx>
> > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > ---
> > We have measured a latency of around 30msecs between frame sync
> > and dqbuf.
> > ---
> > Changes in v2:
> > - Suggested by Laurent. Split sequence++ and event init.
> > - Link to v1: https://lore.kernel.org/r/20231020-uvc-event-v1-1-3baa0e9f6952@xxxxxxxxxxxx
> > ---
> >  drivers/media/usb/uvc/uvc_v4l2.c  | 2 ++
> >  drivers/media/usb/uvc/uvc_video.c | 7 +++++++
> >  2 files changed, 9 insertions(+)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> > index f4988f03640a..9f3fb5fd2375 100644
> > --- a/drivers/media/usb/uvc/uvc_v4l2.c
> > +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> > @@ -1352,6 +1352,8 @@ static int uvc_ioctl_subscribe_event(struct v4l2_fh *fh,
> >       switch (sub->type) {
> >       case V4L2_EVENT_CTRL:
> >               return v4l2_event_subscribe(fh, sub, 0, &uvc_ctrl_sub_ev_ops);
> > +     case V4L2_EVENT_FRAME_SYNC:
> > +             return v4l2_event_subscribe(fh, sub, 0, NULL);
> >       default:
> >               return -EINVAL;
> >       }
> > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > index 28dde08ec6c5..4f3a510ca4fe 100644
> > --- a/drivers/media/usb/uvc/uvc_video.c
> > +++ b/drivers/media/usb/uvc/uvc_video.c
> > @@ -1073,9 +1073,16 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
> >        * that discontinuous sequence numbers always indicate lost frames.
> >        */
> >       if (stream->last_fid != fid) {
> > +             struct v4l2_event event = {
> > +                     .type = V4L2_EVENT_FRAME_SYNC,
> > +             };
> > +
> >               stream->sequence++;
> >               if (stream->sequence)
> >                       uvc_video_stats_update(stream);
> > +
> > +             event.u.frame_sync.frame_sequence = stream->sequence,
> > +             v4l2_event_queue(&stream->vdev, &event);
>
> uvc_video_decode_start() is called when the reception of the entire frame
> has been completed. However, the documentation for V4L2_EVENT_FRAME_SYNC
> says that the event is "Triggered immediately when the reception of a frame
> has begun.". The functionality here doesn't seem to fit to this patch.

If I understand correctly, there are two types of transfers, BULK and ISOC.

For bulk frames are typically divided in multiple packages and the
event is generated in the first package. So I guess we fit the
description

For isoc we receive the whole frame at once and we trigger the event
before we process it... It is maybe a bit more borderline
implementation of the event description, but not very far away.


>
> Wouldn't V4L2_EVENT_VSYNC be a better fit, even if we don't really have a
> concept of vertical sync in the case of USB? That event doesn't have the
> sequence though but I guess it's not an issue at least if your case.

All that said, If Esker does not need the sequence number I have no
issue converting it. @Esker Wong WDYT?

>
> Another technically correct option could be to create a new event for this
> but I'm not sure it's worth it.
>
> >       }
> >
> >       uvc_video_clock_decode(stream, buf, data, len);
> >
>
> --
> Regards,
>
> Sakari Ailus



-- 
Ricardo Ribalda