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doc: overlays: Tweak documentation regarding overlays

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Pull some information regarding overlays from commit messages and
put them directly within the documentation. Also add some information
regarding required dtc version to properly use overlays.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Franklin S Cooper Jr 2017-09-04 23:12:19 +03:00 committed by Simon Glass
parent d80162cfc5
commit 56fc7032e1
2 changed files with 87 additions and 6 deletions
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85
doc/README.fdt-overlays
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@ -1,9 +1,76 @@
U-Boot FDT Overlay usage (without FIT images)
U-Boot FDT Overlay usage
=============================================
FDT overlays are now available for use even without FIT images.
It is not as automagic as with FIT images though and require a few
manual steps.
Overlays Syntax
---------------
Overlays require slightly different syntax compared to traditional overlays.
Please refer to dt-object-internal.txt in the dtc 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 dtb
or overlay has defined. It requires both the base and all the overlays
to be compiled with the -@ command line switch 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 an overlay follows the same process as building a traditional dtb.
For example:
base.dts
--------
/dts-v1/;
/ {
foo: foonode {
foo-property;
};
};
$ dtc -@ -I dts -O dtb -o base.dtb base.dts
bar.dts
-------
/dts-v1/;
/plugin/;
/ {
fragment@1 {
target = <&foo>;
__overlay__ {
overlay-1-property;
bar: barnode {
bar-property;
};
};
};
};
$ dtc -@ -I dts -O dtb -o bar.dtb bar.dts
Ways to Utilize Overlays in U-boot
----------------------------------
There are two ways to apply overlays in U-boot.
1. Include and define overlays within a FIT image and have overlays
automatically applied.
2. 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
@ -29,6 +96,16 @@ overlapping anything.
=> 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.

8
doc/uImage.FIT/overlay-fdt-boot.txt
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@ -1,5 +1,5 @@
U-Boot FDT Overlay usage
========================
U-Boot FDT Overlay FIT usage
============================
Introduction
------------
@ -8,6 +8,10 @@ of similar boards and their expansion options. The same kernel on DT enabled
platforms can support this easily enough by providing a DT blob upon boot
that matches the desired configuration.
This document focuses on specifically using overlays as part of a FIT image.
General information regarding overlays including its syntax and building it
can be found in doc/README.fdt-overlays
Configuration without overlays
------------------------------