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

On Mon, 5 Jan 2026 17:22:20 +0100
Herve Codina [off-list ref] wrote:

Hi Luca, Kory,

On Wed, 17 Dec 2025 15:23:26 +0100
"Luca Ceresoli" [off-list ref] (by way of Kory Maincent
[off-list ref]) wrote:

quoted

Hi,

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

Yes sure.

Here is my global review.

Depending on the discussion on things I have spotted, I will go deeper in
patch details.

...

quoted

quoted

+
+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);  

I would use OF changesets to perform the modification.

OF changesets are kind of atomic. You first prepare all modifications in a
changeset and then you apply the changeset.
If something goes wrong, the changeset is removed.

Also, if something goes wrong during the changeset preparation, you can abort
without any modification on the live device-tree.

Ok, I will take a look at changeset to use it to copy all the properties to new
the panel-dpi node.

...

quoted

quoted

+
+	ret = of_overlay_fdt_apply(dtbo_start, dtbo_size, &ovcs_id,
NULL);
+	if (ret)
+		goto out;  

As soon as the overlay is applied, the driver handling the panel-dti node
can be probed.

Modifying some properties after applying the overlay could be not seen by the
driver.

I disagree here. This driver is loaded at subsys_initcall (initcall 4) while
the panel is loaded at device_initcall (initcall 6) so the panel driver won't
probe in-between.

...

quoted

quoted

+subsys_initcall(tilcdc_panel_legacy_init);  

IMHO, the call to tilcdc_panel_legacy_init() will be too late.

subsys initcalls are called after arch initcalls.

During arch initcalls, of_platform_populate_init() is called
https://elixir.bootlin.com/linux/v6.19-rc3/source/drivers/of/platform.c#L599

The root node is populated and handled by the platform bus.

Later at subsys initcall, the tilcdc_panel_legacy_init() function is called.
This function starts by applying the overlay and so a new node (panel-dpi)
is added at the root node.

This trigger an OF_RECONFIG_CHANGE_ADD event handled by the platform bus.
https://elixir.bootlin.com/linux/v6.19-rc3/source/drivers/of/platform.c#L731

If the "panel-dpi" compatible driver is available, its probe() is called but
the panel-dpi DT node is not fully correct. Indeed, tilcdc_panel_copy_props()
has not be called yet.

As said before, I don't think this will happen due to the initcall difference
value.

Also, the legacy compatible string is removed after the
of_platform_populate_init() call. The legacy driver could have been already
probed.

Same here.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com