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+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+Devicetree 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.rst[1]
+
+How overlays work
+-----------------
+
+A Devicetree'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 */
+	/dts-v1/;
+	/ {
+		compatible = "corp,foo";
+
+		/* shared resources */
+		res: res {
+		};
+
+		/* On chip peripherals */
+		ocp: ocp {
+			/* peripherals that are always instantiated */
+			peripheral1 { ... };
+		};
+	};
+    ---- foo.dts ---------------------------------------------------------------
+
+The overlay bar.dts,
+::
+
+    ---- bar.dts - overlay target location by label ----------------------------
+	/dts-v1/;
+	/plugin/;
+	&ocp {
+		/* bar peripheral */
+		bar {
+			compatible = "corp,bar";
+			... /* various properties and child nodes */
+		};
+	};
+    ---- bar.dts ---------------------------------------------------------------
+
+when loaded (and resolved as described in [1]) should 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.
+
+If the base DT was not compiled with the -@ option then the "&ocp" label
+will not be available to resolve the overlay node(s) to the proper location
+in the base DT. In this case, the target path can be provided. The target
+location by label syntax is preferred because the overlay can be applied to
+any base DT containing the label, no matter where the label occurs in the DT.
+
+The above bar.dts example modified to use target path syntax is::
+
+    ---- bar.dts - overlay target location by explicit path --------------------
+	/dts-v1/;
+	/plugin/;
+	&{/ocp} {
+		/* bar peripheral */
+		bar {
+			compatible = "corp,bar";
+			... /* various properties and child nodes */
+		}
+	};
+    ---- bar.dts ---------------------------------------------------------------
+
+
+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.
+
+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.
+
+A notifier callback for OF_OVERLAY_PRE_APPLY, OF_OVERLAY_POST_APPLY, or
+OF_OVERLAY_PRE_REMOVE may store pointers to a device tree node in the overlay
+or its content but these pointers must not persist past the notifier callback
+for OF_OVERLAY_POST_REMOVE.  The memory containing the overlay will be
+kfree()ed after OF_OVERLAY_POST_REMOVE notifiers are called.  Note that the
+memory will be kfree()ed even if the notifier for OF_OVERLAY_POST_REMOVE
+returns an error.
+
+The changeset notifiers in drivers/of/dynamic.c are a second type of notifier
+that could be triggered by applying or removing an overlay.  These notifiers
+are not allowed to store pointers to a device tree node in the overlay
+or its content.  The overlay code does not protect against such pointers
+remaining active when the memory containing the overlay is freed as a result
+of removing the overlay.
+
+Any other code that retains a pointer to the overlay nodes or data is
+considered to be a bug because after removing the overlay the pointer
+will refer to freed memory.
+
+Users of overlays must be especially aware of the overall operations that
+occur on the system to ensure that other kernel code does not retain any
+pointers to the overlay nodes or data.  Any example of an inadvertent use
+of such pointers is if a driver or subsystem module is loaded after an
+overlay has been applied, and the driver or subsystem scans the entire
+devicetree or a large portion of it, including the overlay nodes.