Re: [PATCH v2 2/4] drm: mxsfb: Add optional DPI output bus-width configuration

Hi Francesco,

On 26-07-23, Francesco Dolcini wrote:
> From: Francesco Dolcini <[email protected]>
> 
> 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 optional property is absent, continue using the format reported
> by the downstream display device, preserving the existing behavior.
> 
> Signed-off-by: Francesco Dolcini <[email protected]>
> ---
> v2: use the common bus-width property instead of the legacy interface-pix-fmt
> ---
>  drivers/gpu/drm/mxsfb/mxsfb_drv.c | 24 ++++++++++++++++++++++++
>  drivers/gpu/drm/mxsfb/mxsfb_drv.h |  2 ++
>  drivers/gpu/drm/mxsfb/mxsfb_kms.c | 12 ++++++++++++
>  3 files changed, 38 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_drv.c b/drivers/gpu/drm/mxsfb/mxsfb_drv.c
> index 9b8fbda85d28..0545718a4b65 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>
> @@ -209,7 +212,10 @@ 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;
> +	struct device_node *ep;
> +	u32 bus_width = 0;

With this beeing set to '0' and ...

>  	int ret;
>  
>  	mxsfb = devm_kzalloc(&pdev->dev, sizeof(*mxsfb), GFP_KERNEL);
> @@ -236,6 +242,24 @@ 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) {
> +		of_property_read_u32(ep, "bus-width", &bus_width);

no bus-width property present, this ...

> +		of_node_put(ep);

Furthermore please see
20260723-v6-18-topic-imx93-parallel-display-v13-1-ccf3f9bbc0fc@nxp.com
for a proper error handling.

> +	}
> +
> +	switch (bus_width) {

switch should fail. Therefore I would recommend to set the default to
24 (bit) aka no limitation.

Regards,
  Marco

> +	case 16:
> +		mxsfb->bus_format = MEDIA_BUS_FMT_RGB565_1X16;
> +		break;
> +	case 18:
> +		mxsfb->bus_format = MEDIA_BUS_FMT_RGB666_1X18;
> +		break;
> +	case 24:
> +		mxsfb->bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +		break;
> +	}
> +
>  	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
> 
> 
> 

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