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