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1Device Tree Overlay Notes
2-------------------------
3
4This document describes the implementation of the in-kernel
5device tree overlay functionality residing in drivers/of/overlay.c and is a
6companion document to Documentation/devicetree/dynamic-resolution-notes.txt[1]
7
8How overlays work
9-----------------
10
11A Device Tree's overlay purpose is to modify the kernel's live tree, and
12have the modification affecting the state of the kernel in a way that
13is reflecting the changes.
14Since the kernel mainly deals with devices, any new device node that result
15in an active device should have it created while if the device node is either
16disabled or removed all together, the affected device should be deregistered.
17
18Lets take an example where we have a foo board with the following base tree:
19
20---- foo.dts -----------------------------------------------------------------
21 /* FOO platform */
22 / {
23 compatible = "corp,foo";
24
25 /* shared resources */
26 res: res {
27 };
28
29 /* On chip peripherals */
30 ocp: ocp {
31 /* peripherals that are always instantiated */
32 peripheral1 { ... };
33 }
34 };
35---- foo.dts -----------------------------------------------------------------
36
37The overlay bar.dts, when loaded (and resolved as described in [1]) should
38
39---- bar.dts -----------------------------------------------------------------
40/plugin/; /* allow undefined label references and record them */
41/ {
42 .... /* various properties for loader use; i.e. part id etc. */
43 fragment@0 {
44 target = <&ocp>;
45 __overlay__ {
46 /* bar peripheral */
47 bar {
48 compatible = "corp,bar";
49 ... /* various properties and child nodes */
50 }
51 };
52 };
53};
54---- bar.dts -----------------------------------------------------------------
55
56result 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
85Overlay in-kernel API
86--------------------------------
87
88The API is quite easy to use.
89
901. Call of_overlay_create() to create and apply an overlay. The return value
91is a cookie identifying this overlay.
92
932. Call of_overlay_destroy() to remove and cleanup the overlay previously
94created via the call to of_overlay_create(). Removal of an overlay that
95is stacked by another will not be permitted.
96
97Finally, if you need to remove all overlays in one-go, just call
98of_overlay_destroy_all() which will remove every single one in the correct
99order.
100
101Overlay DTS Format
102------------------
103
104The DTS of an overlay should have the following format:
105
106{
107 /* ignored properties by the overlay */
108
109 fragment@0 { /* first child node */
110
111 target=<phandle>; /* phandle target of the overlay */
112 or
113 target-path="/path"; /* target path of the overlay */
114
115 __overlay__ {
116 property-a; /* add property-a to the target */
117 node-a { /* add to an existing, or create a node-a */
118 ...
119 };
120 };
121 }
122 fragment@1 { /* second child node */
123 ...
124 };
125 /* more fragments follow */
126}
127
128Using the non-phandle based target method allows one to use a base DT which does
129not contain a __symbols__ node, i.e. it was not compiled with the -@ option.
130The __symbols__ node is only required for the target=<phandle> method, since it
131contains the information required to map from a phandle to a tree location.