linux-kernel - Re: [PATCH v9 6/6] media: uvcvideo: Fix hw timestamp handling for slow FPS

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Date: Fri, 22 Mar 2024 11:35:22 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org, "hn.chen" <hn.chen@...plusit.com>,
	linux-media@...r.kernel.org
Subject: Re: [PATCH v9 6/6] media: uvcvideo: Fix hw timestamp handling for
 slow FPS

Hi Ricardo,

Thank you for the patch.

On Wed, Mar 15, 2023 at 02:30:17PM +0100, Ricardo Ribalda wrote:
> In UVC 1.5 we get a single clock value per frame. With the current
> buffer size of 32, FPS slowers than 32 might roll-over twice.
> 
> The current code cannot handle two roll-over and provide invalid
> timestamps.
> 
> Revome all the samples from the circular buffer that are more than two

s/Revome/Remove/

> rollovers old, so the algorithm always provides good timestamps.

Wouldn't it be better to support multiple rollovers instead ?

> Note that we are removing values that are more than one second old,
> which means that there is enough distance between the two points that
> we use for the interpolation to provide good values.
> 
> Tested-by: HungNien Chen <hn.chen@...plusit.com>
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 24 ++++++++++++++++++++++++
>  drivers/media/usb/uvc/uvcvideo.h  |  1 +
>  2 files changed, 25 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 1db0d1bc80e6..c58b51207be6 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -471,8 +471,31 @@ static void uvc_video_clock_add_sample(struct uvc_clock *clock,
>  {
>  	unsigned long flags;
>  
> +	/*
> +	 * If we write new data on the position where we had the last
> +	 * overflow, remove the overflow pointer. There is no overflow
> +	 * on the whole circular buffer.
> +	 */
> +	if (clock->head == clock->last_sof_overflow)
> +		clock->last_sof_overflow = -1;
> +
>  	spin_lock_irqsave(&clock->lock, flags);
>  
> +	/* Handle overflows */
> +	if (clock->count > 0 && clock->last_sof > sample->dev_sof) {
> +		/*
> +		 * Remove data from the circular buffer that is older than the
> +		 * last overflow. We only support one overflow per circular
> +		 * buffer.
> +		 */
> +		if (clock->last_sof_overflow != -1) {
> +			clock->count = (clock->head - clock->last_sof_overflow
> +					+ clock->count)	% clock->count;
> +		}
> +		clock->last_sof_overflow = clock->head;
> +	}
> +
> +	/* Add sample */
>  	memcpy(&clock->samples[clock->head], sample, sizeof(*sample));
>  	clock->head = (clock->head + 1) % clock->size;
>  	clock->count = min(clock->count + 1, clock->size);
> @@ -605,6 +628,7 @@ 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 07b2fdb80adf..bf9f5162b833 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -499,6 +499,7 @@ 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