[PATCH v3 2/2] media: nxp: imx8-isi: Fix scale factor calculation for hardware rounding

[Laurent Pinchart]

Hi Guoniu,

Thank you for the patch.

On Mon, Mar 23, 2026 at 04:33:31PM +0800, Guoniu Zhou wrote:
> From: Guoniu Zhou <guoniu.zhou at nxp.com>
> 
> The ISI hardware rounds the actual output size up to an integer, as
> described in i.MX93 Reference Manual section 57.7.8 (Channel 0 Scale
> Factor). The scale factor must be calculated to ensure the theoretical
> output value rounds up to exactly the desired size.
> 
> Fixes: cf21f328fcaf ("media: nxp: Add i.MX8 ISI driver")
> Cc: stable at vger.kernel.org
> Signed-off-by: Guoniu Zhou <guoniu.zhou at nxp.com>
> ---
>  drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c b/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c
> index 37e59d687ed7..a2edac8292a7 100644
> --- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c
> +++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c
> @@ -112,7 +112,16 @@ static u32 mxc_isi_channel_scaling_ratio(unsigned int from, unsigned int to,
>  	else
>  		*dec = 8;
>  
> -	return min_t(u32, from * 0x1000 / (to * *dec), ISI_DOWNSCALE_THRESHOLD);
> +	/*
> +	 * The ISI rounds output dimensions up to the next integer (i.MX93 RM
> +	 * section 57.7.8). Calculate the scale factor such that the theoretical
> +	 * output (input / scale_factor) rounds up to exactly the desired output.

I assume you have verified this applies to the ISI in the i.MX8 family.

> +	 *
> +	 * Example from the reference manual: Scaling 800 to 720 lines
> +	 *   - scale = 0x11C8: 800/0x1C8 = 719.859375 -> 720 (correct)
> +	 *   - scale = 0x11C7: 800/0x1C7 = 720.017578 -> 721 (one extra line)

I think the first paragraph is clear enough, the example is not
required.

> +	 */
> +	return min_t(u32, DIV_ROUND_UP(from * 0x1000, to * *dec), ISI_DOWNSCALE_THRESHOLD);

Line wrap please.

	return min_t(u32, DIV_ROUND_UP(from * 0x1000, to * *dec),
		     ISI_DOWNSCALE_THRESHOLD);

Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

No need to send a new version, I'll handle those changes.

>  }
>  
>  static void mxc_isi_channel_set_scaling(struct mxc_isi_pipe *pipe,

-- 
Regards,

Laurent Pinchart