drm/tilcdc: Convert legacy panel binding via DT overlay at boot time

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/
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Kory Maincent (TI.com) <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20260123-feature_tilcdc-v5-5-5a44d2aa3f6f@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

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
+	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.

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

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;
+
+	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);

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>;
+		};
+	};
+};