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