[PATCH v2 05/20] drm/tilcdc: Convert legacy panel binding via DT overlay at boot time

Kory Maincent (TI.com)
2025-12-11T16:39:57Z

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 panel-dpi node with placeholder
timing properties and port/endpoint connections to the LCDC
- Copy all properties from the legacy panel node to the new panel-dpi
- 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 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.

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 overlay 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 v2:
- New patch.
---
drivers/gpu/drm/tilcdc/Kconfig | 14 +++
drivers/gpu/drm/tilcdc/Makefile | 2 +
drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c | 159 ++++++++++++++++++++++++
drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso | 44 +++++++
4 files changed, 219 insertions(+)

diff --git a/drivers/gpu/drm/tilcdc/Kconfig b/drivers/gpu/drm/tilcdc/Kconfig
index 24f9a245ba593..4dcae0a06b819 100644
--- a/drivers/gpu/drm/tilcdc/Kconfig
+++ b/drivers/gpu/drm/tilcdc/Kconfig
@@ -14,3 +14,17 @@ 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 n
+ depends on DRM_TILCDC
+ depends on OF
+ depends on BACKLIGHT_CLASS_DEVICE
+ depends on PM
+ select OF_OVERLAY
+ select DRM_PANEL_SIMPLE
+ help
+ Choose this option if you need a kernel that is compatible
+ with device tree blobs using the obsolete "ti,tilcdc,panel"
+ binding. 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..a9651dd9f9935
--- /dev/null
+++ b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.c
@@ -0,0 +1,159 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2025 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 void __init
+tilcdc_panel_update_prop(struct device_node *node, char *name,
+ void *val, int length)
+{
+ struct property *prop;
+
+ prop = kzalloc(sizeof(*prop), GFP_KERNEL);
+ if (!prop)
+ return;
+
+ prop->name = kstrdup(name, GFP_KERNEL);
+ prop->length = length;
+ prop->value = kmemdup(val, length, GFP_KERNEL);
+ of_update_property(node, prop);
+}
+
+static int __init tilcdc_panel_copy_props(struct device_node *old_panel,
+ struct device_node *new_panel)
+{
+ struct device_node *child, *old_timing, *new_timing, *panel_info;
+ u32 invert_pxl_clk = 0, sync_edge = 0;
+ struct property *prop;
+
+ /* 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;
+
+ tilcdc_panel_update_prop(new_panel, prop->name,
+ prop->value, prop->length);
+ }
+
+ 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) {
+ of_node_put(child);
+ return -EINVAL;
+ }
+ }
+ of_node_put(child);
+
+ new_timing = of_get_child_by_name(new_panel, "panel-timing");
+ if (!new_timing)
+ return -EINVAL;
+
+ /* Copy all panel timing property to the new panel node */
+ for_each_property_of_node(old_timing, prop)
+ tilcdc_panel_update_prop(new_timing, prop->name,
+ prop->value, prop->length);
+
+ panel_info = of_get_child_by_name(old_panel, "panel-info");
+ if (!panel_info)
+ return -EINVAL;
+
+ /* 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)
+ tilcdc_panel_update_prop(new_timing, "pixelclk-active",
+ &(int){1}, sizeof(int));
+
+ if (!sync_edge)
+ tilcdc_panel_update_prop(new_timing, "syncclk-active",
+ &(int){1}, sizeof(int));
+
+ of_node_put(panel_info);
+ of_node_put(old_timing);
+ of_node_put(new_timing);
+ return 0;
+}
+
+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 *panel, *lcdc, *new_panel;
+ 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)) {
+ ret = -ENODEV;
+ goto out;
+ }
+
+ 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)
+ goto out;
+
+ new_panel = of_find_node_by_name(NULL, "panel-dpi");
+ if (!new_panel) {
+ ret = -ENODEV;
+ goto overlay_remove;
+ }
+
+ ret = tilcdc_panel_copy_props(panel, new_panel);
+ if (ret)
+ goto overlay_remove;
+
+ /* Remove compatible property to avoid any driver compatible match */
+ of_remove_property(panel, of_find_property(panel, "compatible",
+ NULL));
+overlay_remove:
+ of_overlay_remove(&ovcs_id);
+out:
+ of_node_put(new_panel);
+ of_node_put(panel);
+ of_node_put(lcdc);
+ 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..77f3ec9391d55
--- /dev/null
+++ b/drivers/gpu/drm/tilcdc/tilcdc_panel_legacy.dtso
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * DTS overlay for converting ti,tilcdc,panel binding to new binding.
+ *
+ * Copyright (C) Kory Maincent <kory.maincent@bootlin.com>
+ */
+
+/dts-v1/;
+/plugin/;
+
+&{/} {
+ panel-dpi {
+ compatible = "panel-dpi";
+ port {
+ panel_in: endpoint@0 {
+ remote-endpoint = <&lcd_0>;
+ };
+ };
+ panel-timing {
+ clock-frequency = <0>;
+ hactive = <0>;
+ vactive = <0>;
+ hfront-porch = <0>;
+ hback-porch = <0>;
+ hsync-len = <0>;
+ vfront-porch = <0>;
+ vback-porch = <0>;
+ vsync-len = <0>;
+ hsync-active = <0>;
+ vsync-active = <0>;
+ de-active = <0>;
+ pixelclk-active = <0>;
+ syncclk-active = <0>;
+ };
+ };
+};
+
+&lcdc {
+ port {
+ lcd_0: endpoint@0 {
+ remote-endpoint = <&panel_in>;
+ };
+ };
+};

--
2.43.0