Documentation: Add documentation for device tree overlays

Document the recently gained device tree overlay features.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.barebox.org/20210624085223.14616-18-s.hauer@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/user/devicetree.rst b/Documentation/user/devicetree.rst
index 4be6a1e703..91afffdcda 100644
--- a/Documentation/user/devicetree.rst
+++ b/Documentation/user/devicetree.rst
@@ -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
+overlays to.
+
+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
+from it.
+
+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:
+
+``global.of.overlay.dir``
+  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.
+``global.of.overlay.compatible``
+  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.
+``global.of.overlay.filepattern``
+  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
+  applied.
+``global.of.overlay.filter``
+  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.