[PATCH 17/18] Documentation: Add documentation for device tree overlays
s.hauer at pengutronix.de
Thu Jun 24 01:52:22 PDT 2021
Document the recently gained device tree overlay features.
Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
Documentation/user/devicetree.rst | 43 +++++++++++++++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/Documentation/user/devicetree.rst b/Documentation/user/devicetree.rst
index 4be6a1e703..91afffdcda 100644
@@ -75,3 +75,46 @@ It is important to know that these commands normally work on the internal
devicetree. If you want to modify the devicetree the kernel is started with
see the -f options to of_property and of_node. This option will register the
operation for later execution on the Kernel devicetree.
+Device tree overlays
+barebox has support for device tree overlays. barebox knows two different trees,
+the live tree and the device tree the kernel is started with. Both can be applied
+Device tree overlays on the live tree
+While the live tree can be patched by board code, barebox does not
+detect any changes to the live tree. To let the overlays have any effect, board
+code must make sure the live tree is patched before the devices are instanciated
+Device tree overlays on the kernel device tree
+Overlays can be applied to the kernel device tree before it is handed over to
+the kernel. The behaviour is controlled by different variables:
+ Overlays are read from this directory. barebox will try to apply all overlays
+ found here if not limited by one of the other variables below. When the path
+ given here is an absolute path it is used as is. A relative path is relative
+ to ``/`` or relative to the rootfs when using bootloader spec.
+ This is a space separated list of compatibles. Only overlays matching one of
+ these compatibles will be applied. When this list is empty then all overlays
+ will be applied. Overlays that don't have a compatible are considered being
+ always compatible.
+ This is a space separated list of file patterns. An overlay is only applied
+ when its filename matches one of the patterns. The patterns can contain
+ ``*`` and ``?`` as wildcards. The default is ``*`` which means all files are
+ This is a space separated list of filters to apply. There are two generic filters:
+ ``filepattern`` matches ``global.of.overlay.filepattern`` above, ``compatible`` matches
+ ``global.of.overlay.compatible`` above. The default is ``filepattern compatible``
+ which means the two generic filters are active. This list may be replaced or
+ supplemented by board specific filters.
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