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

[Sakari Ailus <sakari.ailus@iki.fi>]

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@chromium.org>
> Tested-by: Esker Wong <esker@chromium.org>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.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