linux-kernel - Re: [PATCH v10 4/6] media: uvcvideo: Allow hw clock updates with buffers not full

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Date: Wed, 29 May 2024 17:03:08 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org, linux-kernel@...r.kernel.org, 
	"hn.chen" <hn.chen@...plusit.com>, Hans Verkuil <hverkuil@...all.nl>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v10 4/6] media: uvcvideo: Allow hw clock updates with
 buffers not full

On Sat, Mar 23, 2024 at 10:48:05AM +0000, 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.
> 
> Tested-by: HungNien Chen <hn.chen@...plusit.com>
> Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index d6ca383f643e3..af25b9f1b53fe 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -768,10 +768,10 @@ 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) % clock->size];
>  	last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
>  
>  	/* First step, PTS to SOF conversion. */
> @@ -786,6 +786,18 @@ 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
> +	 * precision. This value was determined by running Android CTS
> +	 * on different devices.
> +	 *
> +	 * dev_sof runs at 1KHz, and we have a fixed point precision of
> +	 * 16 bits.
> +	 */
> +	if ((y2 - y1) < ((1000 / 4) << 16))
> +		goto done;

Not a comment for this patch directly, but...

This kind of makes me wonder if we don't want to have some documentation
that specifies what the userspace can expect from the timestamps, so
that this isn't changed randomly in the future breaking what was fixed
by this patch.

Anyway:

Reviewed-by: Tomasz Figa <tfiga@...omium.org>

Best regards,
Tomasz