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0290c4ca25
This patch is aimed primarily at drivers/of/overlay.c, but those
changes also have a small impact in a few other files.
overlay.c is difficult to read and maintain. Improve readability:
- Rename functions, types and variables to better reflect what
they do and to be consistent with names in other places,
such as the device tree overlay FDT (flattened device tree),
and make the algorithms more clear
- Use the same names consistently throughout the file
- Update comments for name changes
- Fix incorrect comments
This patch is intended to not introduce any functional change.
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
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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