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doc: Document how to apply fdt overlays

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We have the capability to apply overlays on the command line but
we didn't have a document explaining how.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Reviewed-by: Łukasz Majewski
Acked-by: Simon Glass <sjg@chromium.org>
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Pantelis Antoniou 2017-09-04 23:12:18 +03:00 committed by Simon Glass
parent 6b54e50b5a
commit d80162cfc5
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U-Boot FDT Overlay usage (without FIT images)
=============================================
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.
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 blob and overlay blobs
=> load ${devtype} ${bootpart} ${fdtaddr} ${bootdir}/base.dtb
=> load ${devtype} ${bootpart} ${fdtovaddr} ${bootdir}/overlay.dtb
3. Set it as the working fdt tree.
=> fdtaddr $fdtaddr
4. Grow it enough so it can 'fit' all the applied overlays
=> fdt resize 8192
5. You are now ready to apply the overlay.
=> fdt apply $fdtovaddr
Please note that in case of an error, both the base and overlays are going
to be invalidated, so keep copies to avoid reloading.
Pantelis Antoniou
pantelis.antoniou@konsulko.com
11/7/2017