Date:   Thu, 16 Mar 2023 12:07:55 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Ricardo Ribalda <ribalda@chromium.org>
Cc:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        linux-kernel@vger.kernel.org, "hn.chen" <hn.chen@sunplusit.com>,
        linux-media@vger.kernel.org
Subject: Re: [PATCH v9 6/6] media: uvcvideo: Fix hw timestamp handling for
 slow FPS
Message-ID: <20230316030755.GC1927922@google.com>
References: <20220920-resend-hwtimestamp-v9-0-55a89f46f6be@chromium.org>
 <20220920-resend-hwtimestamp-v9-6-55a89f46f6be@chromium.org>
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On (23/03/15 14:30), 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
> rollovers old, so the algorithm always provides good timestamps.
> 
> 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@sunplusit.com>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

[..]
> +	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;

A minor nit: there is a tab between `clock->count)` and `% clock->count` :)