Re: [PATCH v5 2/4] drm: mxsfb: Add optional DPI output bus-width configuration

[Frank Li <Frank.li@oss.nxp.com>]

On Thu, Sep 17, 2026 at 04:00:37PM +0200, Francesco Dolcini wrote:
> From: Francesco Dolcini <francesco.dolcini@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 bus-width endpoint property from the LCDIF output port
> 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 property is absent, continue using the format reported by the
> downstream display device, preserving the previous behavior. The
> devicetree binding restricts bus-width to 16, 18 or 24; any other value
> means the devicetree is broken, so fail the probe instead of silently
> falling back to the previous behavior.
>
> Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>

Reviewed-by: Frank Li <Frank.Li@nxp.com>

> ---
> v5: fail probe on invalid bus-width instead of falling back to the
>     previous behavior; factor the property parsing into its own
>     mxsfb_of_bus_fmt() helper. Drop the r-b/t-b tags below since they
>     were given for the previous silent-fallback behavior and for a
>     different code shape; needs re-review.
> v4: add r-b and t-b tags
> v3: improve commit message and make it clear that the code assumes that
>     the DT is correct, when the DT contains an invalid property value,
>     the code safely fall-back to the previous behavior
> v2: use the common bus-width property instead of the legacy interface-pix-fmt
>
> Dropped for v5:
> Reviewed-by: Frank Li <Frank.Li@nxp.com>
> Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
> Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> # TQMa6UL[L]x
> ---
>  drivers/gpu/drm/mxsfb/mxsfb_drv.c | 43 +++++++++++++++++++++++++++++++
>  drivers/gpu/drm/mxsfb/mxsfb_drv.h |  2 ++
>  drivers/gpu/drm/mxsfb/mxsfb_kms.c | 12 +++++++++
>  3 files changed, 57 insertions(+)
>
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_drv.c b/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> index 8b4a500347fb..641691fc92c8 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> @@ -12,7 +12,10 @@
>  #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/of_graph.h>
>  #include <linux/platform_device.h>
>  #include <linux/property.h>
>  #include <linux/pm_runtime.h>
> @@ -207,11 +210,41 @@ static void mxsfb_irq_uninstall(struct drm_device *dev)
>  	free_irq(mxsfb->irq, dev);
>  }
>
> +static int mxsfb_of_bus_fmt(const struct device_node *ep, u32 *bus_format)
> +{
> +	u32 bus_width;
> +	int ret;
> +
> +	ret = of_property_read_u32(ep, "bus-width", &bus_width);
> +	if (ret == -EINVAL)
> +		return 0;
> +	if (ret)
> +		return ret;
> +
> +	switch (bus_width) {
> +	case 16:
> +		*bus_format = MEDIA_BUS_FMT_RGB565_1X16;
> +		break;
> +	case 18:
> +		*bus_format = MEDIA_BUS_FMT_RGB666_1X18;
> +		break;
> +	case 24:
> +		*bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>  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;
> +	struct device_node *ep;
>  	int ret;
>
>  	mxsfb = devm_kzalloc(&pdev->dev, sizeof(*mxsfb), GFP_KERNEL);
> @@ -238,6 +271,16 @@ static int mxsfb_load(struct drm_device *drm,
>  	if (IS_ERR(mxsfb->clk_disp_axi))
>  		mxsfb->clk_disp_axi = NULL;
>
> +	ep = of_graph_get_next_endpoint(np, NULL);
> +	if (ep) {
> +		ret = mxsfb_of_bus_fmt(ep, &mxsfb->bus_format);
> +		of_node_put(ep);
> +		if (ret) {
> +			dev_err(drm->dev, "Invalid bus-width endpoint property\n");
> +			return ret;
> +		}
> +	}
> +
>  	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..bdc47ce5be79 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;
> +
>  	bool				crc_active;
>  };
>
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> index d8ebebc5314b..1d78c486d147 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> @@ -386,6 +386,18 @@ static void mxsfb_crtc_atomic_enable(struct drm_crtc *crtc,
>  	if (!bus_format)
>  		bus_format = MEDIA_BUS_FMT_RGB888_1X24;
>
> +	/*
> +	 * Prefer the bus format derived from the OF graph endpoint "bus-width"
> +	 * property when available. Otherwise, use the bus format reported by
> +	 * the downstream bridge or panel.
> +	 *
> +	 * This supports mismatched display and interface bus widths, such as
> +	 * a 24-bit panel connected through an 18-bit interface or an 18-bit
> +	 * panel connected through a 24-bit interface.
> +	 */
> +	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 */
> --
> 2.47.3
>
>