+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
Device Tree Overlay Notes
--------------------------
+=========================
This document describes the implementation of the in-kernel
device tree overlay functionality residing in drivers/of/overlay.c and is a
in an active device should have it created while if the device node is either
disabled or removed all together, the affected device should be deregistered.
-Lets take an example where we have a foo board with the following base tree:
+Lets take an example where we have a foo board with the following base tree::
----- foo.dts -----------------------------------------------------------------
+ ---- foo.dts ---------------------------------------------------------------
/* FOO platform */
/dts-v1/;
/ {
peripheral1 { ... };
};
};
----- foo.dts -----------------------------------------------------------------
+ ---- foo.dts ---------------------------------------------------------------
The overlay bar.dts,
+::
----- bar.dts - overlay target location by label ------------------------------
+ ---- bar.dts - overlay target location by label ----------------------------
/dts-v1/;
/plugin/;
&ocp {
... /* various properties and child nodes */
};
};
----- bar.dts -----------------------------------------------------------------
+ ---- bar.dts ---------------------------------------------------------------
-when loaded (and resolved as described in [1]) should result in foo+bar.dts
+when loaded (and resolved as described in [1]) should result in foo+bar.dts::
----- foo+bar.dts -------------------------------------------------------------
+ ---- foo+bar.dts -----------------------------------------------------------
/* FOO platform + bar peripheral */
/ {
compatible = "corp,foo";
};
};
};
----- foo+bar.dts -------------------------------------------------------------
+ ---- foo+bar.dts -----------------------------------------------------------
As a result of the overlay, a new device node (bar) has been created
so a bar platform device will be registered and if a matching device driver
location by label syntax is preferred because the overlay can be applied to
any base DT containing the label, no matter where the label occurs in the DT.
-The above bar.dts example modified to use target path syntax is:
+The above bar.dts example modified to use target path syntax is::
----- bar.dts - overlay target location by explicit path ----------------------
+ ---- bar.dts - overlay target location by explicit path --------------------
/dts-v1/;
/plugin/;
&{/ocp} {
... /* various properties and child nodes */
}
};
----- bar.dts -----------------------------------------------------------------
+ ---- bar.dts ---------------------------------------------------------------
Overlay in-kernel API
The API is quite easy to use.
-1. Call of_overlay_fdt_apply() to create and apply an overlay changeset. The
-return value is an error or a cookie identifying this overlay.
+1) Call of_overlay_fdt_apply() to create and apply an overlay changeset. The
+ return value is an error or a cookie identifying this overlay.
-2. Call of_overlay_remove() to remove and cleanup the overlay changeset
-previously created via the call to of_overlay_fdt_apply(). Removal of an
-overlay changeset that is stacked by another will not be permitted.
+2) Call of_overlay_remove() to remove and cleanup the overlay changeset
+ previously created via the call to of_overlay_fdt_apply(). Removal of an
+ overlay changeset that is stacked by another will not be permitted.
Finally, if you need to remove all overlays in one-go, just call
of_overlay_remove_all() which will remove every single one in the correct