Re: [PATCH v5 2/2] media: bcm2835-unicam: Fix RGB format / mbus code association

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Maxime,

Thank you for the patch.

On Mon, Feb 09, 2026 at 04:03:17PM +0100, Maxime Ripard wrote:
> From: Maxime Ripard <mripard@redhat.com>
> 
> The Unicam driver is a MIPI-CSI2 Receiver, that can capture RGB 4:4:4,
> YCbCr 4:2:2, and raw formats.
> 
> RGB 4:4:4 is converted to the MIPI-CSI2 RGB888 video format, and
> associated to the MEDIA_BUS_FMT_RGB888_1X24 media bus code.
> 
> However, V4L2_PIX_FMT_RGB24 is defined as having its color components in
> the R, G and B order, from left to right. MIPI-CSI2 however defines the
> RGB888 format with blue first, and that's what MEDIA_BUS_FMT_RGB888_1X24
> defines too.
> 
> This essentially means that the R and B will be swapped compared to what
> V4L2_PIX_FMT_RGB24 defines. The same situation occurs with
> V4L2_PIX_FMT_BGR24 being associated to MEDIA_BUS_FMT_BGR888_1X24.
> 
> In order to fix the swapped components, we need to change the
> association of V4L2_PIX_FMT_BGR24 to MEDIA_BUS_FMT_RGB888_1X24, and of
> V4L2_PIX_FMT_RGB24 to MEDIA_BUS_FMT_BGR888_1X24.
> 
> Since the media bus code is exposed to userspace, and validated by
> unicam's link_validate implementation, we need to explicitly accept (and
> warn) the old association still to preserve backward compatibility.
> 
> Signed-off-by: Maxime Ripard <mripard@redhat.com>
> ---
>  drivers/media/platform/broadcom/bcm2835-unicam.c | 36 +++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/platform/broadcom/bcm2835-unicam.c b/drivers/media/platform/broadcom/bcm2835-unicam.c
> index f10064107d543caf867249d0566a0f42d6d8c4c6..5e4850831c931d346146aa8e22c53f0655e462c9 100644
> --- a/drivers/media/platform/broadcom/bcm2835-unicam.c
> +++ b/drivers/media/platform/broadcom/bcm2835-unicam.c
> @@ -340,16 +340,16 @@ static const struct unicam_format_info unicam_image_formats[] = {
>  		.code		= MEDIA_BUS_FMT_RGB565_1X16,
>  		.depth		= 16,
>  		.csi_dt		= MIPI_CSI2_DT_RGB565,
>  	}, {
>  		.fourcc		= V4L2_PIX_FMT_RGB24, /* rgb */
> -		.code		= MEDIA_BUS_FMT_RGB888_1X24,
> +		.code		= MEDIA_BUS_FMT_BGR888_1X24,
>  		.depth		= 24,
>  		.csi_dt		= MIPI_CSI2_DT_RGB888,
>  	}, {
>  		.fourcc		= V4L2_PIX_FMT_BGR24, /* bgr */
> -		.code		= MEDIA_BUS_FMT_BGR888_1X24,
> +		.code		= MEDIA_BUS_FMT_RGB888_1X24,
>  		.depth		= 24,
>  		.csi_dt		= MIPI_CSI2_DT_RGB888,
>  	}, {
>  	/* Bayer Formats */
>  		.fourcc		= V4L2_PIX_FMT_SBGGR8,
> @@ -2153,12 +2153,40 @@ static int unicam_video_link_validate(struct media_link *link)
>  		if (WARN_ON(!fmtinfo)) {
>  			ret = -EPIPE;
>  			goto out;
>  		}
>  
> -		if (fmtinfo->code != format->code ||
> -		    fmt->height != format->height ||
> +		/*
> +		 * Unicam initially associated BGR24 to BGR888_1X24 and
> +		 * RGB24 to RGB888_1X24.
> +		 *
> +		 * In order to allow the applications using the old
> +		 * behaviour to run, let's accept the old combination,
> +		 * but warn about it.
> +		 */
> +		if (fmtinfo->code != format->code) {
> +			if (fmtinfo->fourcc == V4L2_PIX_FMT_BGR24 &&
> +			    format->code == MEDIA_BUS_FMT_BGR888_1X24) {
> +				dev_warn_once(node->dev->dev,
> +					      "MIPI-CSI media bus code for RGB88 is RGB888_1X24. The application must be fixed.");

s/RGB88/RGB888/

But I find this confusing, the message doesn't make it clear if RGB888 +
RGB888_1X24 is expected or is an error. I think the following would be
clearer.

				dev_warn_once(node->dev->dev,
					      "Incorrect pixel format BGR24 for BGR888_1X24. Fix your application to use RGB24.");

> +			} else if (fmtinfo->fourcc == V4L2_PIX_FMT_RGB24 &&
> +				   format->code == MEDIA_BUS_FMT_RGB888_1X24) {
> +				dev_warn_once(node->dev->dev,
> +					      "MIPI-CSI media bus code for BGR888 is BGR888_1X24. The application must be fixed.");

				dev_warn_once(node->dev->dev,
					      "Incorrect pixel format RGB24 for RGB888_1X24. Fix your application to use BGR24.");

Or you could combine those two (untested):

diff --git a/drivers/media/platform/broadcom/bcm2835-unicam.c b/drivers/media/platform/broadcom/bcm2835-unicam.c
index f10064107d54..e9f191536765 100644
--- a/drivers/media/platform/broadcom/bcm2835-unicam.c
+++ b/drivers/media/platform/broadcom/bcm2835-unicam.c
@@ -2148,22 +2148,38 @@ static int unicam_video_link_validate(struct media_link *link)
 		const struct v4l2_pix_format *fmt = &node->fmt.fmt.pix;
 		const struct unicam_format_info *fmtinfo;

-		fmtinfo = unicam_find_format_by_fourcc(fmt->pixelformat,
-						       UNICAM_SD_PAD_SOURCE_IMAGE);
+		fmtinfo = unicam_find_format_by_code(format->code,
+						     UNICAM_SD_PAD_SOURCE_IMAGE);
 		if (WARN_ON(!fmtinfo)) {
 			ret = -EPIPE;
 			goto out;
 		}

-		if (fmtinfo->code != format->code ||
-		    fmt->height != format->height ||
+		if (fmtinfo->fourcc != fmt->pixelformat) {
+			if ((fmtinfo->fourcc == V4L2_PIX_FMT_BGR24 &&
+			     format->code == MEDIA_BUS_FMT_BGR888_1X24) ||
+			    (fmtinfo->fourcc == V4L2_PIX_FMT_RGB24 &&
+			     format->code == MEDIA_BUS_FMT_RGB888_1X24)) {
+                                dev_warn_once(node->dev->dev,
+                                              "Incorrect pixel format %p4CC for 0x%04x. Fix your application to use %p4CC.\n",
+                                              &fmt->pixelformat, format->code, &fmtinfo->fourcc);
+			} else {
+				dev_dbg(node->dev->dev,
+					"image: format mismatch: 0x%04x <=> %p4CCx\n",
+					format->code, &fmt->pixelformat);
+				ret = -EPIPE
+				goto out;
+			}
+		}
+
+		if (fmt->height != format->height ||
 		    fmt->width != format->width ||
 		    fmt->field != format->field) {
 			dev_dbg(node->dev->dev,
-				"image: (%u x %u) 0x%08x %s != (%u x %u) 0x%08x %s\n",
-				fmt->width, fmt->height, fmtinfo->code,
+				"image: (%u x %u) %s != (%u x %u) %s\n",
+				fmt->width, fmt->height,
 				v4l2_field_names[fmt->field],
-				format->width, format->height, format->code,
+				format->width, format->height,
 				v4l2_field_names[format->field]);
 			ret = -EPIPE;
 		}

> +			} else {
> +				dev_dbg(node->dev->dev,
> +					"image: (%u x %u) 0x%08x %s != (%u x %u) 0x%08x %s\n",
> +					fmt->width, fmt->height, fmtinfo->code,
> +					v4l2_field_names[fmt->field],
> +					format->width, format->height, format->code,
> +					v4l2_field_names[format->field]);

As this message is printed specifically due to a format mismatch, I
would drop the size and field information:

				dev_dbg(node->dev->dev,
					"image: format mismatch: 0x%04x <=> %p4CC\n",
					format->code, &fmt->pixelformat);

> +				ret = -EPIPE;
> +				goto out;
> +			}
> +		}
> +
> +		if (fmt->height != format->height ||
>  		    fmt->width != format->width ||
>  		    fmt->field != format->field) {
>  			dev_dbg(node->dev->dev,
>  				"image: (%u x %u) 0x%08x %s != (%u x %u) 0x%08x %s\n",
>  				fmt->width, fmt->height, fmtinfo->code,

And here you can drop the format.

I like the approach in this v5. As far as I can see, it won't break
userspace, will warn of incorrect format usage, and implements the
backward compatibility in the driver that initially got it wrong,
unicam. I'm happy with it.

-- 
Regards,

Laurent Pinchart