linux/Documentation/devicetree/overlay-notes.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3========================
   4Devicetree Overlay Notes
   5========================
   6
   7This document describes the implementation of the in-kernel
   8device tree overlay functionality residing in drivers/of/overlay.c and is a
   9companion document to Documentation/devicetree/dynamic-resolution-notes.rst[1]
  10
  11How overlays work
  12-----------------
  13
  14A Devicetree's overlay purpose is to modify the kernel's live tree, and
  15have the modification affecting the state of the kernel in a way that
  16is reflecting the changes.
  17Since the kernel mainly deals with devices, any new device node that result
  18in an active device should have it created while if the device node is either
  19disabled or removed all together, the affected device should be deregistered.
  20
  21Lets take an example where we have a foo board with the following base tree::
  22
  23    ---- foo.dts ---------------------------------------------------------------
  24        /* FOO platform */
  25        /dts-v1/;
  26        / {
  27                compatible = "corp,foo";
  28
  29                /* shared resources */
  30                res: res {
  31                };
  32
  33                /* On chip peripherals */
  34                ocp: ocp {
  35                        /* peripherals that are always instantiated */
  36                        peripheral1 { ... };
  37                };
  38        };
  39    ---- foo.dts ---------------------------------------------------------------
  40
  41The overlay bar.dts,
  42::
  43
  44    ---- bar.dts - overlay target location by label ----------------------------
  45        /dts-v1/;
  46        /plugin/;
  47        &ocp {
  48                /* bar peripheral */
  49                bar {
  50                        compatible = "corp,bar";
  51                        ... /* various properties and child nodes */
  52                };
  53        };
  54    ---- bar.dts ---------------------------------------------------------------
  55
  56when loaded (and resolved as described in [1]) should result in foo+bar.dts::
  57
  58    ---- foo+bar.dts -----------------------------------------------------------
  59        /* FOO platform + bar peripheral */
  60        / {
  61                compatible = "corp,foo";
  62
  63                /* shared resources */
  64                res: res {
  65                };
  66
  67                /* On chip peripherals */
  68                ocp: ocp {
  69                        /* peripherals that are always instantiated */
  70                        peripheral1 { ... };
  71
  72                        /* bar peripheral */
  73                        bar {
  74                                compatible = "corp,bar";
  75                                ... /* various properties and child nodes */
  76                        };
  77                };
  78        };
  79    ---- foo+bar.dts -----------------------------------------------------------
  80
  81As a result of the overlay, a new device node (bar) has been created
  82so a bar platform device will be registered and if a matching device driver
  83is loaded the device will be created as expected.
  84
  85If the base DT was not compiled with the -@ option then the "&ocp" label
  86will not be available to resolve the overlay node(s) to the proper location
  87in the base DT. In this case, the target path can be provided. The target
  88location by label syntax is preferred because the overlay can be applied to
  89any base DT containing the label, no matter where the label occurs in the DT.
  90
  91The above bar.dts example modified to use target path syntax is::
  92
  93    ---- bar.dts - overlay target location by explicit path --------------------
  94        /dts-v1/;
  95        /plugin/;
  96        &{/ocp} {
  97                /* bar peripheral */
  98                bar {
  99                        compatible = "corp,bar";
 100                        ... /* various properties and child nodes */
 101                }
 102        };
 103    ---- bar.dts ---------------------------------------------------------------
 104
 105
 106Overlay in-kernel API
 107--------------------------------
 108
 109The API is quite easy to use.
 110
 1111) Call of_overlay_fdt_apply() to create and apply an overlay changeset. The
 112   return value is an error or a cookie identifying this overlay.
 113
 1142) Call of_overlay_remove() to remove and cleanup the overlay changeset
 115   previously created via the call to of_overlay_fdt_apply(). Removal of an
 116   overlay changeset that is stacked by another will not be permitted.
 117
 118Finally, if you need to remove all overlays in one-go, just call
 119of_overlay_remove_all() which will remove every single one in the correct
 120order.
 121
 122In addition, there is the option to register notifiers that get called on
 123overlay operations. See of_overlay_notifier_register/unregister and
 124enum of_overlay_notify_action for details.
 125
 126Note that a notifier callback is not supposed to store pointers to a device
 127tree node or its content beyond OF_OVERLAY_POST_REMOVE corresponding to the
 128respective node it received.
 129