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_fdt_apply() to create and apply an overlay changeset. The
91return value is an error or a cookie identifying this overlay.
92
932. Call of_overlay_remove() to remove and cleanup the overlay changeset
94previously created via the call to of_overlay_fdt_apply(). Removal of an
95overlay changeset that is stacked by another will not be permitted.
96
97Finally, if you need to remove all overlays in one-go, just call
98of_overlay_remove_all() which will remove every single one in the correct
99order.
100
101In addition, there is the option to register notifiers that get called on
102overlay operations. See of_overlay_notifier_register/unregister and
103enum of_overlay_notify_action for details.
104
105Note that a notifier callback is not supposed to store pointers to a device
106tree node or its content beyond OF_OVERLAY_POST_REMOVE corresponding to the
107respective node it received.
108
109Overlay DTS Format
110------------------
111
112The DTS of an overlay should have the following format:
113
114{
115	/* ignored properties by the overlay */
116
117	fragment@0 {	/* first child node */
118
119		target=<phandle>;	/* phandle target of the overlay */
120	or
121		target-path="/path";	/* target path of the overlay */
122
123		__overlay__ {
124			property-a;	/* add property-a to the target */
125			node-a {	/* add to an existing, or create a node-a */
126				...
127			};
128		};
129	}
130	fragment@1 {	/* second child node */
131		...
132	};
133	/* more fragments follow */
134}
135
136Using the non-phandle based target method allows one to use a base DT which does
137not contain a __symbols__ node, i.e. it was not compiled with the -@ option.
138The __symbols__ node is only required for the target=<phandle> method, since it
139contains the information required to map from a phandle to a tree location.
140