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

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Date:   Mon, 6 Nov 2023 12:39:25 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Esker Wong <esker@...omium.org>
Subject: Re: [PATCH] media: uvcvideo: Implement V4L2_EVENT_FRAME_SYNC

Hi Ricardo,

Thank you for the patch.

On Fri, Oct 20, 2023 at 06:41:45AM +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.

Not surprising, especially for large resolutions. I'm curious though,
what do you use this event for ?

> ---
>  drivers/media/usb/uvc/uvc_v4l2.c  | 2 ++
>  drivers/media/usb/uvc/uvc_video.c | 8 +++++++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> 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..1d4b4807b005 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1073,9 +1073,15 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
>  	 * that discontinuous sequence numbers always indicate lost frames.
>  	 */
>  	if (stream->last_fid != fid) {
> -		stream->sequence++;
> +		struct v4l2_event event = {
> +			.type = V4L2_EVENT_FRAME_SYNC,
> +			.u.frame_sync.frame_sequence =  ++stream->sequence,

Extra space before ++.

It's easy to miss the ++ there when reading the code, would the
following be more readable ?

		struct v4l2_event event = {
			.type = V4L2_EVENT_FRAME_SYNC,
		};

		stream->sequence++;
		if (stream->sequence)
			uvc_video_stats_update(stream);

		.u.frame_sync.frame_sequence = stream->sequence;
		v4l2_event_queue(&stream->vdev, &event);

> +		};
> +
>  		if (stream->sequence)
>  			uvc_video_stats_update(stream);
> +
> +		v4l2_event_queue(&stream->vdev, &event);
>  	}
>  
>  	uvc_video_clock_decode(stream, buf, data, len);
> 
> ---
> base-commit: ce55c22ec8b223a90ff3e084d842f73cfba35588
> change-id: 20231020-uvc-event-d3d1bbbdcb2f

-- 
Regards,

Laurent Pinchart