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fdt: Add a Kconfig for boards with a prior stage

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When U-Boot is started from another firmware program, not just a prior
phase of U-Boot, special behaviour is typically used. In particular, the
device tree may come from that prior stage.

At present this is sort-of indicated by OF_BOARD, although the
correlation is not 1:1, since that option simply means that the board has
a custom mechanism for obtaining the device tree. For example, sandbox
defines OF_BOARD. Also the board_fdt_blob_setup() function can in fact
make use of the devicetree in U-Boot if it wishes, as used by
dragonboard410c until very recently.

Add an explicit Kconfig for this situation. Update the OF_BOARD option to
more-accurately reflect what it is doing, e.g. for sandbox.

Drop the docs in the README as it is out of date.

Signed-off-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Simon Glass
2021-12-16 20:59:35 -07:00
committed by Tom Rini
parent ff66e7bb73
commit 275b4832f6
3 changed files with 29 additions and 34 deletions
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33
dts/Kconfig
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@@ -98,13 +98,36 @@ endchoice
config OF_BOARD
bool "Provided by the board (e.g a previous loader) at runtime"
default y if SANDBOX
default y if SANDBOX || OF_HAS_PRIOR_STAGE
help
If this option is enabled, the device tree will be provided by
the board at runtime if the board supports it. The device tree bundled
with the image (if any) will be overridden / ignored.
If this option is enabled, the device tree is provided at runtime by
a custom function called board_fdt_blob_setup(). The board must
implement this function if it wishes to provide special behaviour.
A device tree file must be provided in the tree.
With this option, the device tree build by U-Boot may be overridden or
ignored. See OF_HAS_PRIOR_STAGE.
Note: Boards which use this to handle a device tree passed from an
earlier stage should enable OF_HAS_PRIOR_STAGE.
config OF_HAS_PRIOR_STAGE
bool
help
Indicates that a prior stage of the firmware (before U-Boot proper)
makes use of device tree and this board normally boots with that prior
stage, that provides a devicetree to U-Boot.
This means that the device tree built in U-Boot should not be packaged
in the firmware image. Instead, the prior stage's device tree should
be so packaged. At runtime, the prior stage reads this, does any
necessary fix-ups, then passes it to U-Boot. See OF_BOARD.
This option does not preclude using the U-Boot device tree, e.g. for
development purposes, but it is not recommended, and likely will not
even work, for production systems.
Note: This option must be set in Kconfig and cannot be enabled or
disabled in the board's defconfig file.
config DEFAULT_DEVICE_TREE
string "Default Device Tree for DT control"