Re: [PATCH v3 05/22] drm/tilcdc: Convert legacy panel binding via DT overlay at boot time

[Andrew Davis <afd@ti.com>]

On 1/6/26 11:42 AM, Kory Maincent (TI.com) wrote:
> To maintain backward compatibility while removing the deprecated
> tilcdc_panel driver, add a tilcdc_panel_legacy subdriver that converts
> the legacy "ti,tilcdc,panel" devicetree binding to the standard
> panel-dpi binding at early boot.
> 
> The conversion uses an embedded device tree overlay that is applied and
> modified during subsys_initcall. The process:
> 
> - Apply embedded overlay to create a tilcdc-panel-dpi node with
>    port/endpoint connections to the LCDC
> - Copy all properties from the legacy panel node to the new
>    tilcdc-panel-dpi node
> - Copy display-timings from the legacy panel
> - Convert legacy panel-info properties (invert-pxl-clk, sync-edge) to
>    standard display timing properties (pixelclk-active, syncclk-active)
> - Disable the legacy panel by removing its compatible property to
>    prevent the deprecated driver from binding
> 
> The result is a standard tilcdc-panel-dpi node with proper endpoints and
> timing properties, allowing the DRM panel infrastructure to work with
> legacy devicetrees without modification.
> 
> Other legacy panel-info properties are not migrated as they consistently
> use default values across all mainline devicetrees and can be hardcoded
> in the tilcdc driver.
> 
> This feature is optional via CONFIG_DRM_TILCDC_PANEL_LEGACY and should
> only be enabled for systems with legacy devicetrees containing
> "ti,tilcdc,panel" nodes.
> 
> Suggested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> Link: https://lore.kernel.org/all/1d9a9269-bfda-4d43-938b-2df6b82b9369@ideasonboard.com/
> Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com>
> ---
> 
> Using the approach of applying an overlay and then modifying the live
> device tree is the solution I found that requires no modification of the
> OF core. Dealing entirely with changesets would bring additional
> requirements such as phandle resolution management, which is internal to
> the OF framework. I intend to avoid OF core change to support this legacy
> binding.
> 
> Change in v3:
> - Use __free() macro instead of manual house cleaning.
> - Enable CONFIG_DRM_TILCDC_PANEL_LEGACY config by default.
> - Improve config description.
> - Rename "panel-dpi" to "tilcdc-panel-dpi" to avoid any future conflict.
> - Use OF changeset instead of modifying the live devicetree step by
>    step.
> - Add kfree to avoid memory leak.
> 
> Change in v2:
> - New patch.
> ---
>   drivers/gpu/drm/tilcdc/Kconfig                  |  16 ++
>   drivers/gpu/drm/tilcdc/Makefile                 |   2 +
>   drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c    | 185 ++++++++++++++++++++++++
>   drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso |  29 ++++
>   4 files changed, 232 insertions(+)
> 
> diff --git a/drivers/gpu/drm/tilcdc/Kconfig b/drivers/gpu/drm/tilcdc/Kconfig
> index 24f9a245ba593..4fca8058eb7db 100644
> --- a/drivers/gpu/drm/tilcdc/Kconfig
> +++ b/drivers/gpu/drm/tilcdc/Kconfig
> @@ -14,3 +14,19 @@ config DRM_TILCDC
>   	  controller, for example AM33xx in beagle-bone, DA8xx, or
>   	  OMAP-L1xx.  This driver replaces the FB_DA8XX fbdev driver.
>   
> +config DRM_TILCDC_PANEL_LEGACY
> +	bool "Support device tree blobs using TI LCDC Panel binding"
> +	default y
> +	depends on DRM_TILCDC
> +        depends on OF
> +        depends on BACKLIGHT_CLASS_DEVICE
> +        depends on PM

Spaces -> tabs

> +	select OF_OVERLAY
> +	select DRM_PANEL_SIMPLE
> +	help
> +	  Modifies the live device tree at early boot to convert the legacy
> +	  "ti,tilcdc,panel" devicetree node to the standard panel-dpi node.
> +	  This allows to maintain backward compatibility for boards which
> +	  were using the deprecated tilcdc_panel driver.
> +	  If you find "ti,tilcdc,panel"-string from your DTB, you probably
> +	  need this. Otherwise you do not.
> diff --git a/drivers/gpu/drm/tilcdc/Makefile b/drivers/gpu/drm/tilcdc/Makefile
> index f5190477de721..6d6a08b5adf40 100644
> --- a/drivers/gpu/drm/tilcdc/Makefile
> +++ b/drivers/gpu/drm/tilcdc/Makefile
> @@ -11,3 +11,5 @@ tilcdc-y := \
>   	tilcdc_drv.o
>   
>   obj-$(CONFIG_DRM_TILCDC)	+= tilcdc.o
> +obj-$(CONFIG_DRM_TILCDC_PANEL_LEGACY)	+= tilcdc_panel_legacy.o \
> +					   tilcdc_panel_legacy.dtbo.o
> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c
> new file mode 100644
> index 0000000000000..37a69b3cf04b2
> --- /dev/null
> +++ b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c
> @@ -0,0 +1,185 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2025 Bootlin
> + * Author: Kory Maincent <kory.maincent@bootlin.com>
> + *
> + * To support the legacy "ti,tilcdc,panel" binding, the devicetree has to
> + * be transformed to the new panel-dpi binding with the endpoint associated.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/of.h>
> +#include <linux/of_fdt.h>
> +#include <linux/slab.h>
> +
> +/* Embedded dtbo symbols created by cmd_wrap_S_dtb in scripts/Makefile.lib */
> +extern char __dtbo_tilcdc_panel_legacy_begin[];
> +extern char __dtbo_tilcdc_panel_legacy_end[];
> +
> +static int __init
> +tilcdc_panel_update_prop(struct of_changeset *ocs, struct device_node *node,
> +			 char *name, void *val, int length)
> +{
> +	struct property *prop;
> +
> +	prop = kzalloc(sizeof(*prop), GFP_KERNEL);
> +	if (!prop)
> +		return -ENOMEM;
> +
> +	prop->name = kstrdup(name, GFP_KERNEL);
> +	prop->length = length;
> +	prop->value = kmemdup(val, length, GFP_KERNEL);
> +	if (!prop->name || !prop->value) {
> +		kfree(prop->name);
> +		kfree(prop->value);
> +		kfree(prop);
> +		return -ENOMEM;
> +	}
> +
> +	return of_changeset_update_property(ocs, node, prop);
> +}
> +
> +static int __init tilcdc_panel_copy_props(struct device_node *old_panel,
> +					  struct device_node *new_panel)
> +{
> +	struct device_node *old_timing __free(device_node) = NULL;
> +	struct device_node *new_timing __free(device_node) = NULL;
> +	struct device_node *panel_info __free(device_node) = NULL;
> +	struct device_node *child __free(device_node) = NULL;
> +	u32 invert_pxl_clk = 0, sync_edge = 0;
> +	struct of_changeset ocs;
> +	struct property *prop;
> +	int ret;
> +
> +	child = of_get_child_by_name(old_panel, "display-timings");
> +	if (!child)
> +		return -EINVAL;
> +
> +	/* The default display timing is the one specified as native-mode.
> +	 * If no native-mode is specified then the first node is assumed
> +	 * to be the native mode.
> +	 */
> +	old_timing = of_parse_phandle(child, "native-mode", 0);
> +	if (!old_timing) {
> +		old_timing = of_get_next_child(child, NULL);
> +		if (!old_timing)
> +			return -EINVAL;
> +	}
> +
> +	panel_info = of_get_child_by_name(old_panel, "panel-info");
> +	if (!panel_info)
> +		return -EINVAL;
> +
> +	of_changeset_init(&ocs);
> +
> +	/* Copy all panel properties to the new panel node */
> +	for_each_property_of_node(old_panel, prop) {
> +		if (!strncmp(prop->name, "compatible", sizeof("compatible")))
> +			continue;
> +
> +		ret = tilcdc_panel_update_prop(&ocs, new_panel, prop->name,
> +					       prop->value, prop->length);
> +		if (ret)
> +			goto destroy_ocs;
> +	}
> +
> +	new_timing = of_changeset_create_node(&ocs, new_panel, "panel-timing");
> +	if (!new_timing) {
> +		ret = -ENODEV;
> +		goto destroy_ocs;
> +	}
> +
> +	/* Copy all panel timing properties to the new panel node */
> +	for_each_property_of_node(old_timing, prop) {
> +		ret = tilcdc_panel_update_prop(&ocs, new_timing, prop->name,
> +					       prop->value, prop->length);
> +		if (ret)
> +			goto destroy_ocs;
> +	}
> +
> +	/* Looked only for these two parameter as all the other are always
> +	 * set to default and not related to common DRM properties.
> +	 */
> +	of_property_read_u32(panel_info, "invert-pxl-clk", &invert_pxl_clk);
> +	of_property_read_u32(panel_info, "sync-edge", &sync_edge);
> +
> +	if (!invert_pxl_clk) {
> +		ret = tilcdc_panel_update_prop(&ocs, new_timing, "pixelclk-active",
> +					       &(u32){cpu_to_be32(1)}, sizeof(u32));
> +		if (ret)
> +			goto destroy_ocs;
> +	}
> +
> +	if (!sync_edge) {
> +		ret = tilcdc_panel_update_prop(&ocs, new_timing, "syncclk-active",
> +					       &(u32){cpu_to_be32(1)}, sizeof(u32));
> +		if (ret)
> +			goto destroy_ocs;
> +	}
> +
> +	/* Remove compatible property to avoid any driver compatible match */
> +	of_changeset_remove_property(&ocs, old_panel,
> +				     of_find_property(old_panel, "compatible", NULL));
> +
> +	of_changeset_apply(&ocs);
> +	return 0;
> +
> +destroy_ocs:
> +	of_changeset_destroy(&ocs);
> +	return ret;
> +}
> +
> +static const struct of_device_id tilcdc_panel_of_match[] __initconst = {
> +	{ .compatible = "ti,tilcdc,panel", },
> +	{},
> +};
> +
> +static const struct of_device_id tilcdc_of_match[] __initconst = {
> +	{ .compatible = "ti,am33xx-tilcdc", },
> +	{ .compatible = "ti,da850-tilcdc", },
> +	{},
> +};
> +
> +static int __init tilcdc_panel_legacy_init(void)
> +{
> +	struct device_node *new_panel __free(device_node) = NULL;
> +	struct device_node *panel __free(device_node) = NULL;
> +	struct device_node *lcdc __free(device_node) = NULL;
> +	void *dtbo_start;
> +	u32 dtbo_size;
> +	int ovcs_id;
> +	int ret;
> +
> +	lcdc = of_find_matching_node(NULL, tilcdc_of_match);
> +	panel = of_find_matching_node(NULL, tilcdc_panel_of_match);
> +
> +	if (!of_device_is_available(panel) ||
> +	    !of_device_is_available(lcdc))
> +		return 0;
> +
> +	dtbo_start = __dtbo_tilcdc_panel_legacy_begin;
> +	dtbo_size = __dtbo_tilcdc_panel_legacy_end -
> +		    __dtbo_tilcdc_panel_legacy_begin;
> +
> +	ret = of_overlay_fdt_apply(dtbo_start, dtbo_size, &ovcs_id, NULL);
> +	if (ret)
> +		return ret;

Looking over the users of `of_overlay_fdt_apply()` they all seem
to start with a begin and end address, then have to convert to start
and size. I wonder if this function should be converted to just take
begin and end addresses instead.. (not a problem for this series, just
thinking out loud).

Andrew

> +
> +	new_panel = of_find_node_by_name(NULL, "tilcdc-panel-dpi");
> +	if (!new_panel) {
> +		ret = -ENODEV;
> +		goto overlay_remove;
> +	}
> +
> +	ret = tilcdc_panel_copy_props(panel, new_panel);
> +	if (ret)
> +		goto overlay_remove;
> +
> +	return 0;
> +
> +overlay_remove:
> +	of_overlay_remove(&ovcs_id);
> +	return ret;
> +}
> +
> +subsys_initcall(tilcdc_panel_legacy_init);
> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso
> new file mode 100644
> index 0000000000000..ae71d10f5ec13
> --- /dev/null
> +++ b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso
> @@ -0,0 +1,29 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * DTS overlay for converting ti,tilcdc,panel binding to new binding.
> + *
> + * Copyright (C) 2025 Bootlin
> + * Author: Kory Maincent <kory.maincent@bootlin.com>
> + */
> +
> +/dts-v1/;
> +/plugin/;
> +
> +&{/} {
> +	tilcdc-panel-dpi {
> +		compatible = "panel-dpi";
> +		port {
> +			panel_in: endpoint@0 {
> +				remote-endpoint = <&lcd_0>;
> +			};
> +		};
> +	};
> +};
> +
> +&lcdc {
> +	port {
> +		lcd_0: endpoint@0 {
> +			remote-endpoint = <&panel_in>;
> +		};
> +	};
> +};
>