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

Luca Ceresoli
2025-12-17T14:23:38Z

Hi,

Cc: Hervé, can you review the DT overlay aspects?

On Thu Dec 11, 2025 at 5:38 PM CET, 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:

I was a bit skeptical about the overlay idea initially. But after having
seen the code and thought about any alternative solution, I think you
probably took the simplest approach.

> - 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>

It's fair to add before your SoB line:
Suggested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://lore.kernel.org/all/1d9a9269-bfda-4d43-938b-2df6b82b9369@ideasonboard.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

'default' defaults to 'n', you can drop this line.

However I think it should instead be enabled by default. You propose to
entirely remove the tilcdc panel driver in the next patch, so any users
without DRM_TILCDC_PANEL_LEGACY in their defconfig would be broken. For
this reason, I propose to enable DRM_TILCDC_PANEL_LEGACY in all cases where
the tilcdc_panel was compiled in, which I guess means:

default DRM_TILCDC

Except I think if DRM_TILCDC=m, DRM_TILCDC_PANEL_LEGACY should be =y. I
don't know how to do that in Kconfig. But I'm not really sure about this
last topic.

> + 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.

Maybe mention here what it does?

For example, rewording your commit message:

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.

> 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");

There's some housekeeping code in this function to ensure you put all the
device_node refs. It would be simpler and less error prone to use a cleanup
action. E.g.:

- struct device_node *child, *old_timing, *new_timing, *panel_info;

- child = of_get_child_by_name(old_panel, "display-timings");
+ struct device_node *child __free(device_node) = 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;

tilcdc_panel_update_prop() has previously done various allocations which
will not be freed if you return here. You shoudl probably do all the
of_get_*() at the top, and if they all succeed start copying data along
with with the needed allocations.

> + /* 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);

Is it correct to remove the overlay here? Won't it remove what you have
just added?

> +out:
> + of_node_put(new_panel);
> + of_node_put(panel);
> + of_node_put(lcdc);

Here too you can use cleanup actions, even though the current code is
slightly simpler than tilcdc_panel_copy_props as far as of_node_put() is
concerned.

> + 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>;
> + };
> + };
> +};

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering