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x86: Add documentation for the chain-load feature

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Add a few notes about this feature, which is aimed for development.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
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Simon Glass 2020-04-26 09:13:01 -06:00 committed by Bin Meng
parent 86ee14f58b
commit 249154672d
1 changed files with 28 additions and 0 deletions
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doc/arch/x86.rst
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@ -712,6 +712,34 @@ to load a 'u-boot-payload.efi', see below test logs on QEMU.
See :doc:`../uefi/u-boot_on_efi` and :doc:`../uefi/uefi` for details of
EFI support in U-Boot.
Chain-loading
-------------
U-Boot can be chain-loaded from another bootloader, such as coreboot or
Slim Bootloader. Typically this is done by building for targets 'coreboot' or
'slimbootloader'.
For example, at present we have a 'coreboot' target but this runs very
different code from the bare-metal targets, such as coral. There is very little
in common between them.
It is useful to be able to boot the same U-Boot on a device, with or without a
first-stage bootloader. For example, with chromebook_coral, it is helpful for
testing to be able to boot the same U-Boot (complete with FSP) on bare metal
and from coreboot. It allows checking of things like CPU speed, comparing
registers, ACPI tables and the like.
To do this you can use ll_boot_init() in appropriate places to skip init that
has already been done by the previous stage. This works by setting a
GD_FLG_NO_LL_INIT flag when U-Boot detects that it is running from another
bootloader.
With this feature, you can build a bare-metal target and boot it from
coreboot, for example.
Note that this is a development feature only. It is not intended for use in
production environments. Also it is not currently part of the automated tests
so may break in the future.
TODO List
---------
- Audio