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Date: Wed, 29 May 2024 15:13:48 +0900
From: Tomasz Figa <tfiga@chromium.org>
To: Ricardo Ribalda <ribalda@chromium.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>, 
	Mauro Carvalho Chehab <mchehab@kernel.org>, linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, 
	"hn.chen" <hn.chen@sunplusit.com>, Hans Verkuil <hverkuil@xs4all.nl>, 
	Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Re: [PATCH v10 1/6] media: uvcvideo: Support timestamp lists of any
 size
Message-ID: <ga5jxpqrz33js4nzziuj234vxlkjhcanofqymjcy2vi2x7irjb@bkubcfb43ylm>
References: <20240323-resend-hwtimestamp-v10-0-b08e590d97c7@chromium.org>
 <20240323-resend-hwtimestamp-v10-1-b08e590d97c7@chromium.org>
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In-Reply-To: <20240323-resend-hwtimestamp-v10-1-b08e590d97c7@chromium.org>

On Sat, Mar 23, 2024 at 10:48:02AM +0000, Ricardo Ribalda wrote:
> The tail of the list lives at the position before the head. This is
> mathematically noted as:
> ```
> (head-1) mod size.
> ```
> Unfortunately C, does not have a modulus operator, but a remainder
> operator (%).
> The reminder operation has a different result than the modulus if
> (head -1) is a negative number and size is not a power of two.
> 
> Adding size to (head-1) allows the code to run with any value of size.
> 
> This does not change the current behaviour of the driver, as the size is
> always a power of two, but avoid tedious debugging if we ever change its
> size.
> 
> Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 7cbf4692bd875..659c9e9880a99 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -732,7 +732,7 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
>  		goto done;
>  
>  	first = &clock->samples[clock->head];
> -	last = &clock->samples[(clock->head - 1) % clock->size];
> +	last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
>  
>  	/* First step, PTS to SOF conversion. */
>  	delta_stc = buf->pts - (1UL << 31);
> 
> -- 
> 2.44.0.396.g6e790dbe36-goog
> 
>

It could be worth mentioning that basically head == 0 is the problematic
case here, but otherwise good catch.

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

Best regards,
Tomasz