1 .. SPDX-License-Identifier: GPL-2.0
2
3 ========================
4 Devicetree Overlay Notes
5 ========================
6
7 This document describes the implementation of the in-kernel
8 device tree overlay functionality residing in drivers/of/overlay.c and is a
9 companion document to Documentation/devicetree/dynamic-resolution-notes.rst[1]
10
11 How overlays work
12 -----------------
13
14 A Devicetree's overlay purpose is to modify the kernel's live tree, and
15 have the modification affecting the state of the kernel in a way that
16 is reflecting the changes.
17 Since the kernel mainly deals with devices, any new device node that result
18 in an active device should have it created while if the device node is either
19 disabled or removed all together, the affected device should be deregistered.
20
21 Lets 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
41 The 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
56 when 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
81 As a result of the overlay, a new device node (bar) has been created
82 so a bar platform device will be registered and if a matching device driver
83 is loaded the device will be created as expected.
84
85 If the base DT was not compiled with the -@ option then the "&ocp" label
86 will not be available to resolve the overlay node(s) to the proper location
87 in the base DT. In this case, the target path can be provided. The target
88 location by label syntax is preferred because the overlay can be applied to
89 any base DT containing the label, no matter where the label occurs in the DT.
90
91 The 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
106 Overlay in-kernel API
107 --------------------------------
108
109 The API is quite easy to use.
110
111 1) 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
114 2) 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
118 Finally, if you need to remove all overlays in one-go, just call
119 of_overlay_remove_all() which will remove every single one in the correct
120 order.
121
122 There is the option to register notifiers that get called on
123 overlay operations. See of_overlay_notifier_register/unregister and
124 enum of_overlay_notify_action for details.
125
126 A notifier callback for OF_OVERLAY_PRE_APPLY, OF_OVERLAY_POST_APPLY, or
127 OF_OVERLAY_PRE_REMOVE may store pointers to a device tree node in the overlay
128 or its content but these pointers must not persist past the notifier callback
129 for OF_OVERLAY_POST_REMOVE. The memory containing the overlay will be
130 kfree()ed after OF_OVERLAY_POST_REMOVE notifiers are called. Note that the
131 memory will be kfree()ed even if the notifier for OF_OVERLAY_POST_REMOVE
132 returns an error.
133
134 The changeset notifiers in drivers/of/dynamic.c are a second type of notifier
135 that could be triggered by applying or removing an overlay. These notifiers
136 are not allowed to store pointers to a device tree node in the overlay
137 or its content. The overlay code does not protect against such pointers
138 remaining active when the memory containing the overlay is freed as a result
139 of removing the overlay.
140
141 Any other code that retains a pointer to the overlay nodes or data is
142 considered to be a bug because after removing the overlay the pointer
143 will refer to freed memory.
144
145 Users of overlays must be especially aware of the overall operations that
146 occur on the system to ensure that other kernel code does not retain any
147 pointers to the overlay nodes or data. Any example of an inadvertent use
148 of such pointers is if a driver or subsystem module is loaded after an
149 overlay has been applied, and the driver or subsystem scans the entire
150 devicetree or a large portion of it, including the overlay nodes.
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