[PATCH 16/17] Documentation: Add documentation for device tree overlays

Sascha Hauer s.hauer at pengutronix.de
Tue Jun 22 22:16:31 PDT 2021

Document the recently gained device tree overlay features.

Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
 Documentation/user/devicetree.rst | 37 +++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Documentation/user/devicetree.rst b/Documentation/user/devicetree.rst
index 4be6a1e703..d78394b7aa 100644
--- a/Documentation/user/devicetree.rst
+++ b/Documentation/user/devicetree.rst
@@ -75,3 +75,40 @@ 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:
+  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 using
+  bootloader spec this variable will be temporarily overwritten with
+  ``$mountpoint/overlays`` where ``$mountpoint`` is the path where the filesystem
+  containing the bootloader spec entries are located.
+  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. When empty, all files are applied.

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