linux-kernel - Re: [PATCH RESEND v2 6/8] media: uvcvideo: Allow hw clock updates with buffers not full

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Date:   Fri, 30 Dec 2022 16:37:51 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        "hn.chen" <hn.chen@...plusit.com>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2 6/8] media: uvcvideo: Allow hw clock updates
 with buffers not full

Hi Ricardo,

Thank you for the patch.

On Fri, Dec 02, 2022 at 06:02:46PM +0100, Ricardo Ribalda wrote:
> With UVC 1.5 we get as little as one clock sample per frame. Which means
> that it takes 32 frames to move from the software timestamp to the
> hardware timestamp method.
> 
> This results in abrupt changes in the timestamping after 32 frames (~1
> second), resulting in noticeable artifacts when used for encoding.
> 
> With this patch we modify the update algorithm to work with whatever
> amount of values are available.

This too makes me thing that we should *really* move this to userspace.
It will but much easier to implement clock domain translation there,
with a better precision.

> Tested-by: HungNien Chen <hn.chen@...plusit.com>
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 75c32e232f5d..7c6448c6d706 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -742,10 +742,10 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
>  
>  	spin_lock_irqsave(&clock->lock, flags);
>  
> -	if (clock->count < clock->size)
> +	if (clock->count < 2)
>  		goto done;
>  
> -	first = &clock->samples[clock->head];
> +	first = &clock->samples[(clock->head - clock->count) % clock->size];
>  	last = &clock->samples[(clock->head - 1) % clock->size];
>  
>  	/* First step, PTS to SOF conversion. */
> @@ -760,6 +760,14 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
>  	if (y2 < y1)
>  		y2 += 2048 << 16;
>  
> +	/*
> +	 * Have at least 1/4 of a second of timestamps before we
> +	 * try to do any calculation. Otherwise we do not have enough
> +	 * precission.

s/precission/precision/

How did you determine 250ms was the right threshold ?

> +	 */
> +	if ((y2 - y1) < (256 << 16))
> +		goto done;
> +
>  	y = (u64)(y2 - y1) * (1ULL << 31) + (u64)y1 * (u64)x2
>  	  - (u64)y2 * (u64)x1;
>  	y = div_u64(y, x2 - x1);
> 

-- 
Regards,

Laurent Pinchart