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

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@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>

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 0187d4ab97e8e28fca9013f6864a094e08f2c570..16b20ea2d1db7221e3028bd36e9796177fbf3394 100644 (file)
--- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c
+++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-hw.c
@@ -112,7 +112,14 @@ 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,