linux-kernel - Re: [PATCH v9 4/6] media: uvcvideo: Allow hw clock updates with buffers not full

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Message-ID: <20230316030851.GD1927922@google.com>
Date:   Thu, 16 Mar 2023 12:08:51 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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 4/6] media: uvcvideo: Allow hw clock updates with
 buffers not full

On (23/03/15 14:30), 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>
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>

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

> +	 * 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))

A minor nit: extra space `( (` :)