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x86: Mention running U-Boot in 64-bit mode in the README 

This feature is not supported. Document this, and add some details on how it
might be implemented.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
This commit is contained in:
Simon Glass 2016-07-17 01:23:46 -06:00 committed by Bin Meng
parent 007adbc2f9
commit 37b4a9098c
1 changed files with 18 additions and 0 deletions
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doc/README.x86
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@ -1049,11 +1049,29 @@ provides the same EFI run-time services) is not currently supported on x86.
See README.efi for details of EFI support in U-Boot.
64-bit Support
--------------
U-Boot supports booting a 64-bit kernel directly and is able to change to
64-bit mode to do so. It also supports (with CONFIG_EFI_STUB) booting from
both 32-bit and 64-bit UEFI. However, U-Boot itself is currently always built
in 32-bit mode. Some access to the full memory range is provided with
arch_phys_memset().
The development work to make U-Boot itself run in 64-bit mode has not yet
been attempted. The best approach would likely be to build a 32-bit SPL
image for U-Boot, with CONFIG_SPL_BUILD. This could then handle the early CPU
init in 16-bit and 32-bit mode, running the FSP and any other binaries that
are needed. Then it could change to 64-bit model and jump to U-Boot proper.
Given U-Boot's extensive 64-bit support this has not been a high priority,
but it would be a nice addition.
TODO List
---------
- Audio
- Chrome OS verified boot
- Support for CONFIG_EFI_LOADER
- Building U-Boot to run in 64-bit mode
References
----------