linux-kernel - Re: [PATCHv3 1/1 RESEND] media: imx: Round line size to 4 bytes

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Date:   Mon, 17 Oct 2022 11:28:57 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm, phone-devel@...r.kernel.org
Subject: Re: [PATCHv3 1/1 RESEND] media: imx: Round line size to 4 bytes

Hi Dorota,

Thank you for the patch.

On Sun, Oct 16, 2022 at 04:39:28PM +0200, Dorota Czaplejewicz wrote:
> Section 13.7.6.13 "CSI Image Parameter Register" of the
> i.MX 8M Quad Applications Processors Reference Manual
> states that the line size should be divisible by 8 bytes.
> However, the hardware also accepts sizes divisible by 4 bytes,
> which are needed to fully utilize the S5K3L6XX sensors.
> 
> This patch accepts line sizes divisible by 4 bytes.

The patch itself looks fine, but I always worry when we depart from the
documentation (sometimes that's a worry about documentation inaccuracies
:-)). In this particular case, we need to test the change on i.MX7 too,
as the alignment constraints could be platform-specific.

I would also very much appreciate feedback from NXP on this. There's a
risk of ill side-effects that I would prefer ruling out very clearly.

> Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
> ---
> Hello,
> 
> the Librem 5 is using an out-of-tree driver for s5k3l6xx, and rounding to 4 is optimal to operate it.
> 
> This revision improves the commit message.
> 
> Cheers,
> Dorota Czaplejewicz
> 
>  drivers/staging/media/imx/imx7-media-csi.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/media/imx/imx7-media-csi.c b/drivers/staging/media/imx/imx7-media-csi.c
> index a0553c24cce4..af821b410c3f 100644
> --- a/drivers/staging/media/imx/imx7-media-csi.c
> +++ b/drivers/staging/media/imx/imx7-media-csi.c
> @@ -999,10 +999,10 @@ static int imx7_csi_mbus_fmt_to_pix_fmt(struct v4l2_pix_format *pix,
>  	}
>  
>  	/* Round up width for minimum burst size */
> -	width = round_up(mbus->width, 8);
> +	width = round_up(mbus->width, 4);
>  
>  	/* Round up stride for IDMAC line start address alignment */
> -	stride = round_up((width * cc->bpp) >> 3, 8);
> +	stride = round_up((width * cc->bpp) >> 3, 4);
>  
>  	pix->width = width;
>  	pix->height = mbus->height;

-- 
Regards,

Laurent Pinchart