88 lines
4.0 KiB
Markdown
88 lines
4.0 KiB
Markdown
---
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title: U-Boot payload
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x-toc-enable: true
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...
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Libreboot has experimental support for using U-Boot as a coreboot
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payload since the [20221214](../../news/libreboot20221214.md) release.
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Refer to [docs/hardware/](../hardware/) for a complete list of U-Boot
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targets in Libreboot.
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U-Boot integration in Libreboot is currently at a proof-of-concept
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stage, with most boards completely untested and most likely not working.
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ROM images for them are mostly intended for further testing and
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development. If you have one of these machines and want to help fix
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things, you can ping `alpernebbi` on Libera IRC, who ported these boards
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to Libreboot.
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As of 14 December 2022, building of U-Boot images has been tested on
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Debian. Make sure you have the latest `lbmk` from the Git repository,
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and the build dependencies are installed like so, from `lbmk/` as root:
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./build dependencies debian
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This installs everything needed for `./build roms`, and part of the
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build process makes use of coreboot's own cross-compile toolchain.
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[QEMU x86/ARM64 virtual machines](../misc/emulation.md) are also
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supported, which should be easy targets to start tinkering on if you
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want to contribute.
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Usage
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=====
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When your board is powered on, U-Boot will ideally turn on the display
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and start printing console messages there. After a countdown of a few
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seconds it will proceed to automatically boot whatever it can find.
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U-Boot will fall back to an interactive prompt if its boot sequence
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fails or if you interrupt the countdown.
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U-Boot supports UEFI to some extent, enough to run a GRUB package that
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would be installed by whatever OS you want to have on your device. The
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boot sequence checks for the standard UEFI removable media paths like
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`/efi/boot/bootaa64.efi`, so you should be able to use your desired OS'
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generic installer images. For details, see upstream documentation for
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[UEFI on U-Boot](https://u-boot.readthedocs.io/en/latest/develop/uefi/uefi.html).
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Otherwise, the boot sequence also checks an `extlinux.conf` file that
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can configure which kernel, initramfs, device-tree file and kernel
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command line arguments should be used. See upstream documentation for
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[Generic Distro Configuration Concept](https://u-boot.readthedocs.io/en/latest/develop/distro.html).
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If you want to work inside the U-Boot shell, see an incomplete list of
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[shell commands](https://u-boot.readthedocs.io/en/latest/usage/index.html#shell-commands),
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or use the `help` command inside the prompt. Configuration is done via
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[environment variables](https://u-boot.readthedocs.io/en/latest/usage/environment.html)
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inside the shell, which can be saved to and automatically loaded from
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persistent storage configured at build-time.
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WARNING: Environment variable storage has not been explicitly configured
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so far and is untested in the context of Libreboot. It may cause data
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loss or even brick your device by overwriting your disk's partition
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table, unexpected parts of the SPI ROM image, or do something else
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entirely.
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Known issues
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============
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U-Boot integration in Libreboot is incomplete. Here is a list of known
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issues that affect all boards:
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- Branding is U-Boot with their logo (Libreboot only in version number)
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- Splash screen might be better instead of console messages
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- Cursor/drawing bugs with video improvements patches
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- Environment storage is likely dangerously broken
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- SMBIOS info is missing, `Unknown`, `Unknown Product`, etc.
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- Coreboot tables are ignored, unavailable to the OS
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- Re-inits display instead of using coreboot framebuffer
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- Chromebook FMAP layouts should match stock layout for GBB, VPD
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- Chromebook GBB, VPD are not handled, data may be unavailable to OS
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- U-Boot "coreboot\*" defconfigs needs more work and an ARM64 version
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- UEFI support is incomplete
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See also
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========
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- [U-Boot documentation](https://u-boot.readthedocs.io/en/latest/)
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- [U-Boot documentation (unmigrated files)](https://source.denx.de/u-boot/u-boot/-/tree/master/doc)
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- [U-Boot and generic distro boot](https://marcin.juszkiewicz.com.pl/2021/03/14/u-boot-and-generic-distro-boot/)
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