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authorMauro Carvalho Chehab <mchehab+huawei@kernel.org>2020-04-15 16:45:22 +0200
committerRob Herring <robh@kernel.org>2020-05-04 17:09:52 -0500
commit642e6e5ce03e5b02d658e87d6c21d881b1ce8f62 (patch)
tree0367f0da7d89483797d3af6522ad5acbf278be23 /Documentation/devicetree/overlay-notes.rst
parent218e1b3d10f1face9f1684f713346072fea3d3ec (diff)
docs: dt: convert overlay-notes.txt to ReST format
- Add a SPDX header; - Adjust document title; - Some whitespace fixes and new line breaks; - Mark literal blocks as such; - Add it to devicetree/index.rst. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Signed-off-by: Rob Herring <robh@kernel.org>
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+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+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.rst[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 */
+ /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.
+
+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.