[PATCH v1 2/4] drm: mxsfb: Allow optional LCDIF interface format override

[Alexander Stein]

Hi Francesco,

thanks for the patch.

Am Freitag, 17. Juli 2026, 14:18:44 CEST schrieb Francesco Dolcini:
> From: Francesco Dolcini <francesco.dolcini at toradex.com>
> 
> LCDIF programs LCD_DATABUS_WIDTH from the selected media bus format. The
> format reported by the downstream panel or bridge describes the display
> input, but it does not describe how the LCDIF data pins are physically
> wired on the board.
> 
> These can differ. For example, a 16-bit LCDIF bus can be connected to a
> 24-bit display by wiring the available color bits to the corresponding
> display inputs. In that case, using the display's 24-bit format to
> configure LCDIF selects the wrong data-bus mode and changes the assignment
> of color bits on the LCD_DATA pins.
> 
> Read the optional interface-pix-fmt property from the LCDIF node and use
> it to select the media bus format used to configure LCDIF. This allows
> the LCDIF bus mode to describe the physical interface independently of
> the downstream display format.
> 
> When the optional property is absent, continue using the format reported
> by the downstream display device, preserving the existing behavior.
> 
> This follows the same approach already implemented in
> drivers/gpu/drm/imx/ipuv3/parallel-display.c.
> 
> Signed-off-by: Francesco Dolcini <francesco.dolcini at toradex.com>

Nice, this work on my imx6ul based hardware where a 18-bit display is
attached as a 24-bit one.
Using interface-pix-fmt = "rgb24" this works as expected.

Tested-by: Alexander Stein <alexander.stein at ew.tq-group.com>

> ---
>  drivers/gpu/drm/mxsfb/mxsfb_drv.c | 16 ++++++++++++++++
>  drivers/gpu/drm/mxsfb/mxsfb_drv.h |  2 ++
>  drivers/gpu/drm/mxsfb/mxsfb_kms.c |  8 ++++++++
>  3 files changed, 26 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_drv.c b/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> index 9b8fbda85d28..edf347968fd2 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> @@ -12,7 +12,9 @@
>  #include <linux/clk.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/io.h>
> +#include <linux/media-bus-format.h>
>  #include <linux/module.h>
> +#include <linux/of.h>
>  #include <linux/platform_device.h>
>  #include <linux/property.h>
>  #include <linux/pm_runtime.h>
> @@ -209,7 +211,10 @@ static int mxsfb_load(struct drm_device *drm,
>  		      const struct mxsfb_devdata *devdata)
>  {
>  	struct platform_device *pdev = to_platform_device(drm->dev);
> +	struct device_node *np = pdev->dev.of_node;
>  	struct mxsfb_drm_private *mxsfb;
> +	u32 bus_format = 0;
> +	const char *fmt;
>  	int ret;
>  
>  	mxsfb = devm_kzalloc(&pdev->dev, sizeof(*mxsfb), GFP_KERNEL);
> @@ -236,6 +241,17 @@ static int mxsfb_load(struct drm_device *drm,
>  	if (IS_ERR(mxsfb->clk_disp_axi))
>  		mxsfb->clk_disp_axi = NULL;
>  
> +	ret = of_property_read_string(np, "interface-pix-fmt", &fmt);
> +	if (!ret) {
> +		if (!strcmp(fmt, "rgb24"))
> +			bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +		else if (!strcmp(fmt, "rgb565"))
> +			bus_format = MEDIA_BUS_FMT_RGB565_1X16;
> +		else if (!strcmp(fmt, "rgb666"))
> +			bus_format = MEDIA_BUS_FMT_RGB666_1X18;
> +	}
> +	mxsfb->bus_format = bus_format;
> +
>  	ret = dma_set_mask_and_coherent(drm->dev, DMA_BIT_MASK(32));
>  	if (ret)
>  		return ret;
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_drv.h b/drivers/gpu/drm/mxsfb/mxsfb_drv.h
> index d160d921b25f..f64800f06a69 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_drv.h
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_drv.h
> @@ -46,6 +46,8 @@ struct mxsfb_drm_private {
>  	struct drm_connector		*connector;
>  	struct drm_bridge		*bridge;
>  
> +	u32 bus_format;

Indent broken.

> +
>  	bool				crc_active;
>  };
>  
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> index d8ebebc5314b..8b482727aed5 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> @@ -386,6 +386,14 @@ static void mxsfb_crtc_atomic_enable(struct drm_crtc *crtc,
>  	if (!bus_format)
>  		bus_format = MEDIA_BUS_FMT_RGB888_1X24;
>  
> +	/*
> +	 * Prefer bus format set via optional "interface-pix-fmt" DT property,
> +	 * over panel or next bridge bus format. This is necessary to support
> +	 * 24bit DPI panels connected to a 18bit interface, for example.

Maybe rephrase and state "mismatching interface" as it goes both ways.

Best regards,
Alexander

> +	 */
> +	if (mxsfb->bus_format)
> +		bus_format = mxsfb->bus_format;
> +
>  	mxsfb_crtc_mode_set_nofb(mxsfb, bridge_state, bus_format);
>  
>  	/* Write cur_buf as well to avoid an initial corrupt frame */
> 


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