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.. SPDX-License-Identifier: GPL-2.0+
.. Copyright (c) 2017, Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Device Tree Overlays
====================
Overlay Syntax
--------------
Device-tree overlays require a slightly different syntax compared to traditional
device-trees. Please refer to dt-object-internal.txt in the device-tree compiler
sources for information regarding the internal format of overlays:
https://git.kernel.org/pub/scm/utils/dtc/dtc.git/tree/Documentation/dt-object-internal.txt
Building Overlays
-----------------
In a nutshell overlays provides a means to manipulate a symbol a previous
device-tree or device-tree overlay has defined. It requires both the base
device-tree and all the overlays to be compiled with the *-@* command line
switch of the device-tree compiler so that symbol information is included.
Note
Support for *-@* option can only be found in dtc version 1.4.4 or newer.
Only version 4.14 or higher of the Linux kernel includes a built in version
of dtc that meets this requirement.
Building a binary device-tree overlay follows the same process as building a
traditional binary device-tree. For example:
**base.dts**
::
/dts-v1/;
/ {
foo: foonode {
foo-property;
};
};
.. code-block:: console
$ dtc -@ -I dts -O dtb -o base.dtb base.dts
**overlay.dtso**
::
/dts-v1/;
/plugin/;
/ {
fragment@1 {
target = <&foo>;
__overlay__ {
overlay-1-property;
bar: barnode {
bar-property;
};
};
};
};
.. code-block:: console
$ dtc -@ -I dts -O dtb -o overlay.dtbo overlay.dtso
Ways to Utilize Overlays in U-Boot
----------------------------------
There are two ways to apply overlays in U-Boot.
* Include and define overlays within a FIT image and have overlays
automatically applied.
* Manually load and apply overlays
The remainder of this document will discuss using overlays via the manual
approach. For information on using overlays as part of a FIT image please see:
doc/uImage.FIT/overlay-fdt-boot.txt
Manually Loading and Applying Overlays
--------------------------------------
1. Figure out where to place both the base device tree blob and the
overlay. Make sure you have enough space to grow the base tree without
overlapping anything.
::
=> setenv fdtaddr 0x87f00000
=> setenv fdtovaddr 0x87fc0000
2. Load the base binary device-tree and the binary device-tree overlay.
::
=> load ${devtype} ${bootpart} ${fdtaddr} ${bootdir}/base.dtb
=> load ${devtype} ${bootpart} ${fdtovaddr} ${bootdir}/overlay.dtbo
3. Set the base binary device-tree as the working fdt tree.
::
=> fdt addr $fdtaddr
4. Grow it enough so it can encompass all applied overlays
::
=> fdt resize 8192
5. You are now ready to apply the overlay.
::
=> fdt apply $fdtovaddr
6. Boot system like you would do with a traditional dtb.
For bootm:
::
=> bootm ${kerneladdr} - ${fdtaddr}
For bootz:
::
=> bootz ${kerneladdr} - ${fdtaddr}
Please note that in case of an error, both the base and overlays are going
to be invalidated, so keep copies to avoid reloading.
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