src/kernel/linux/v4.19/Documentation/devicetree/overlay-notes.txt

Device Tree Overlay Notes
-------------------------

This document describes the implementation of the in-kernel
device tree overlay functionality residing in drivers/of/overlay.c and is a
companion document to Documentation/devicetree/dynamic-resolution-notes.txt[1]

How overlays work
-----------------

A Device Tree's overlay purpose is to modify the kernel's live tree, and
have the modification affecting the state of the kernel in a way that
is reflecting the changes.
Since the kernel mainly deals with devices, any new device node that result
in an active device should have it created while if the device node is either
disabled or removed all together, the affected device should be deregistered.

Lets take an example where we have a foo board with the following base tree:

---- foo.dts -----------------------------------------------------------------
	/* FOO platform */
	/ {
		compatible = "corp,foo";

		/* shared resources */
		res: res {
		};

		/* On chip peripherals */
		ocp: ocp {
			/* peripherals that are always instantiated */
			peripheral1 { ... };
		}
	};
---- foo.dts -----------------------------------------------------------------

The overlay bar.dts, when loaded (and resolved as described in [1]) should

---- bar.dts -----------------------------------------------------------------
/plugin/;	/* allow undefined label references and record them */
/ {
	....	/* various properties for loader use; i.e. part id etc. */
	fragment@0 {
		target = <&ocp>;
		__overlay__ {
			/* bar peripheral */
			bar {
				compatible = "corp,bar";
				... /* various properties and child nodes */
			}
		};
	};
};
---- bar.dts -----------------------------------------------------------------

result in foo+bar.dts

---- foo+bar.dts -------------------------------------------------------------
	/* FOO platform + bar peripheral */
	/ {
		compatible = "corp,foo";

		/* shared resources */
		res: res {
		};

		/* On chip peripherals */
		ocp: ocp {
			/* peripherals that are always instantiated */
			peripheral1 { ... };

			/* bar peripheral */
			bar {
				compatible = "corp,bar";
				... /* various properties and child nodes */
			}
		}
	};
---- foo+bar.dts -------------------------------------------------------------

As a result of the overlay, a new device node (bar) has been created
so a bar platform device will be registered and if a matching device driver
is loaded the device will be created as expected.

Overlay in-kernel API
--------------------------------

The API is quite easy to use.

1. Call of_overlay_fdt_apply() to create and apply an overlay changeset. The
return value is an error or a cookie identifying this overlay.

2. Call of_overlay_remove() to remove and cleanup the overlay changeset
previously created via the call to of_overlay_fdt_apply(). Removal of an
overlay changeset that is stacked by another will not be permitted.

Finally, if you need to remove all overlays in one-go, just call
of_overlay_remove_all() which will remove every single one in the correct
order.

In addition, there is the option to register notifiers that get called on
overlay operations. See of_overlay_notifier_register/unregister and
enum of_overlay_notify_action for details.

Note that a notifier callback is not supposed to store pointers to a device
tree node or its content beyond OF_OVERLAY_POST_REMOVE corresponding to the
respective node it received.

Overlay DTS Format
------------------

The DTS of an overlay should have the following format:

{
	/* ignored properties by the overlay */

	fragment@0 {	/* first child node */

		target=<phandle>;	/* phandle target of the overlay */
	or
		target-path="/path";	/* target path of the overlay */

		__overlay__ {
			property-a;	/* add property-a to the target */
			node-a {	/* add to an existing, or create a node-a */
				...
			};
		};
	}
	fragment@1 {	/* second child node */
		...
	};
	/* more fragments follow */
}

Using the non-phandle based target method allows one to use a base DT which does
not contain a __symbols__ node, i.e. it was not compiled with the -@ option.
The __symbols__ node is only required for the target=<phandle> method, since it
contains the information required to map from a phandle to a tree location.