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SPDX-License-Identifier: GPL-2.02 3========================4Devicetree Overlay Notes5========================6 7This document describes the implementation of the in-kernel8device tree overlay functionality residing in drivers/of/overlay.c and is a9companion document to Documentation/devicetree/dynamic-resolution-notes.rst[1]10 11How overlays work12-----------------13 14A Devicetree's overlay purpose is to modify the kernel's live tree, and15have the modification affecting the state of the kernel in a way that16is reflecting the changes.17Since the kernel mainly deals with devices, any new device node that result18in an active device should have it created while if the device node is either19disabled 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 created82so a bar platform device will be registered and if a matching device driver83is loaded the device will be created as expected.84 85If the base DT was not compiled with the -@ option then the "&ocp" label86will not be available to resolve the overlay node(s) to the proper location87in the base DT. In this case, the target path can be provided. The target88location by label syntax is preferred because the overlay can be applied to89any 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 API107--------------------------------108 109The API is quite easy to use.110 1111) Call of_overlay_fdt_apply() to create and apply an overlay changeset. The112 return value is an error or a cookie identifying this overlay.113 1142) Call of_overlay_remove() to remove and cleanup the overlay changeset115 previously created via the call to of_overlay_fdt_apply(). Removal of an116 overlay changeset that is stacked by another will not be permitted.117 118Finally, if you need to remove all overlays in one-go, just call119of_overlay_remove_all() which will remove every single one in the correct120order.121 122There is the option to register notifiers that get called on123overlay operations. See of_overlay_notifier_register/unregister and124enum of_overlay_notify_action for details.125 126A notifier callback for OF_OVERLAY_PRE_APPLY, OF_OVERLAY_POST_APPLY, or127OF_OVERLAY_PRE_REMOVE may store pointers to a device tree node in the overlay128or its content but these pointers must not persist past the notifier callback129for OF_OVERLAY_POST_REMOVE. The memory containing the overlay will be130kfree()ed after OF_OVERLAY_POST_REMOVE notifiers are called. Note that the131memory will be kfree()ed even if the notifier for OF_OVERLAY_POST_REMOVE132returns an error.133 134The changeset notifiers in drivers/of/dynamic.c are a second type of notifier135that could be triggered by applying or removing an overlay. These notifiers136are not allowed to store pointers to a device tree node in the overlay137or its content. The overlay code does not protect against such pointers138remaining active when the memory containing the overlay is freed as a result139of removing the overlay.140 141Any other code that retains a pointer to the overlay nodes or data is142considered to be a bug because after removing the overlay the pointer143will refer to freed memory.144 145Users of overlays must be especially aware of the overall operations that146occur on the system to ensure that other kernel code does not retain any147pointers to the overlay nodes or data. Any example of an inadvertent use148of such pointers is if a driver or subsystem module is loaded after an149overlay has been applied, and the driver or subsystem scans the entire150devicetree or a large portion of it, including the overlay nodes.151