linux-kernel - Re: [PATCH v2] media: uvcvideo: Implement V4L2_EVENT_FRAME

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Message-ID: <ZUnpE1Wfe2dImqU0@valkosipuli.retiisi.eu>
Date:   Tue, 7 Nov 2023 07:36:51 +0000
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Esker Wong <esker@...omium.org>
Cc:     Ricardo Ribalda <ribalda@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media: uvcvideo: Implement V4L2_EVENT_FRAME_SYNC

Hi Esker,

On Tue, Nov 07, 2023 at 01:06:20PM +0800, Esker Wong wrote:
> [send again in text mode]
> Hi Sakari,
> 
> Sequence number is important to us. We need it to measure the latency
> from this event to the time we display the frame.

I think we could also add the sequence number to V4L2_EVENT_VSYNC.

Cc Hans.

> On Mon, Nov 6, 2023 at 7:06 PM Sakari Ailus <sakari.ailus@....fi> 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@...omium.org>
> > > Tested-by: Esker Wong <esker@...omium.org>
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > > 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@chromium.org
> > > ---
> > >  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.
> >
> > 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.
> >
> > 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

-- 
Regards,

Sakari Ailus