linux-kernel - Re: [PATCH RESEND v2 8/8] media: uvcvideo: Fix hw timestampt handling for slow FPS

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Date:   Fri, 30 Dec 2022 16:51:01 +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 8/8] media: uvcvideo: Fix hw timestampt
 handling for slow FPS

Hi Ricardo,

Thank you for the patch.

On Fri, Dec 02, 2022 at 06:02:48PM +0100, Ricardo Ribalda wrote:
> In UVC 1.5, when working with FPS under 32, there is a chance that the
> circular buffer contains two dev_sof overflows,

An explanation of why this is the case would be good in the commit
message. Also, by overflow, are you talking about the SOF counter
rolling over ?

> but the clock interpolator
> is only capable of handle a single overflow.
> 
> Remove all the samples from the circular buffer that are two overflows
> old.

I would rather support multiple roll-over in the clock interpolator.
Dropping older sampls will lead to less predictable behaviour and harder
to debug issues.

> Tested-by: HungNien Chen <hn.chen@...plusit.com>
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 15 +++++++++++++++
>  drivers/media/usb/uvc/uvcvideo.h  |  1 +
>  2 files changed, 16 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index c81a8362d582..6b6bd521d6c2 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -471,6 +471,20 @@ static void uvc_video_clock_add_sample(struct uvc_clock *clock,
>  
>  	spin_lock_irqsave(&clock->lock, flags);
>  
> +	/* Delete last overflows */
> +	if (clock->head == clock->last_sof_overflow)
> +		clock->last_sof_overflow = -1;
> +
> +	/* Handle overflows */
> +	if (clock->count > 0 && clock->last_sof > sample->dev_sof) {
> +		/* Remove data from the last^2 overflows */
> +		if (clock->last_sof_overflow != -1)
> +			clock->count = (clock->head - clock->last_sof_overflow)
> +								% clock->count;
> +		clock->last_sof_overflow = clock->head;
> +	}
> +
> +	/* Add sample */
>  	memcpy(&clock->samples[clock->head], sample, sizeof(*sample));
>  	clock->last_sof = sample->dev_sof;
>  	clock->head = (clock->head + 1) % clock->size;
> @@ -594,6 +608,7 @@ static void uvc_video_clock_reset(struct uvc_clock *clock)
>  	clock->head = 0;
>  	clock->count = 0;
>  	clock->last_sof = -1;
> +	clock->last_sof_overflow = -1;
>  	clock->sof_offset = -1;
>  }
>  
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index 14daa7111953..d8c520ce5a86 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -647,6 +647,7 @@ struct uvc_streaming {
>  		unsigned int head;
>  		unsigned int count;
>  		unsigned int size;
> +		unsigned int last_sof_overflow;
>  
>  		u16 last_sof;
>  		u16 sof_offset;

-- 
Regards,

Laurent Pinchart