media: nxp: imx8-isi: Fix scale factor calculation for hardware rounding

commit 5eb54da upstream.

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: cf21f32 ("media: nxp: Add i.MX8 ISI driver")
Cc: stable@vger.kernel.org
Signed-off-by: Guoniu Zhou <guoniu.zhou@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patch.msgid.link/20260323-isi-v3-2-8df53b24e622@oss.nxp.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Guoniu Zhou

drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c

@@ -112,7 +112,14 @@ 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.
+	 */
+	return min_t(u32, DIV_ROUND_UP(from * 0x1000, to * *dec),
+		     ISI_DOWNSCALE_THRESHOLD);
 }
 
 static void mxc_isi_channel_set_scaling(struct mxc_isi_pipe *pipe,