Device Tree Overlay Notes ------------------------- This document describes the implementation of the in-kernel device tree overlay functionality residing in drivers/of/overlay.c and is a companion document to Documentation/devicetree/dt-object-internal.txt[1] & Documentation/devicetree/dynamic-resolution-notes.txt[2] How overlays work ----------------- A Device Tree's overlay purpose is to modify the kernel's live tree, and have the modification affecting the state of the kernel in a way that is reflecting the changes. Since the kernel mainly deals with devices, any new device node that result in an active device should have it created while if the device node is either disabled or removed all together, the affected device should be deregistered. Lets take an example where we have a foo board with the following base tree which is taken from [1]. ---- foo.dts ----------------------------------------------------------------- /* FOO platform */ / { compatible = "corp,foo"; /* shared resources */ res: res { }; /* On chip peripherals */ ocp: ocp { /* peripherals that are always instantiated */ peripheral1 { ... }; } }; ---- foo.dts ----------------------------------------------------------------- The overlay bar.dts, when loaded (and resolved as described in [2]) should ---- bar.dts ----------------------------------------------------------------- /plugin/; /* allow undefined label references and record them */ / { .... /* various properties for loader use; i.e. part id etc. */ fragment@0 { target = <&ocp>; __overlay__ { /* bar peripheral */ bar { compatible = "corp,bar"; ... /* various properties and child nodes */ } }; }; }; ---- bar.dts ----------------------------------------------------------------- result in foo+bar.dts ---- foo+bar.dts ------------------------------------------------------------- /* FOO platform + bar peripheral */ / { compatible = "corp,foo"; /* shared resources */ res: res { }; /* On chip peripherals */ ocp: ocp { /* peripherals that are always instantiated */ peripheral1 { ... }; /* bar peripheral */ bar { compatible = "corp,bar"; ... /* various properties and child nodes */ } } }; ---- foo+bar.dts ------------------------------------------------------------- As a result of the overlay, a new device node (bar) has been created so a bar platform device will be registered and if a matching device driver is loaded the device will be created as expected. Overlay in-kernel API -------------------------------- The API is quite easy to use. 1. Call of_overlay_create() to create and apply an overlay. The return value is a cookie identifying this overlay. 2. Call of_overlay_destroy() to remove and cleanup the overlay previously created via the call to of_overlay_create(). Removal of an overlay that is stacked by another will not be permitted. Finally, if you need to remove all overlays in one-go, just call of_overlay_destroy_all() which will remove every single one in the correct order. Overlay DTS Format ------------------ The DTS of an overlay should have the following format: { /* ignored properties by the overlay */ fragment@0 { /* first child node */ target=; /* phandle target of the overlay */ or target-path="/path"; /* target path of the overlay */ __overlay__ { property-a; /* add property-a to the target */ node-a { /* add to an existing, or create a node-a */ ... }; }; } fragment@1 { /* second child node */ ... }; /* more fragments follow */ } Using the non-phandle based target method allows one to use a base DT which does not contain a __symbols__ node, i.e. it was not compiled with the -@ option. The __symbols__ node is only required for the target= method, since it contains the information required to map from a phandle to a tree location.