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lbwww/site/docs/uboot/index.md

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QEMU x86/ARM64 virtual machines are also supported, which should be easy targets to start tinkering on if you want to contribute.

Usage

When your board is powered on, U-Boot will ideally turn on the display and start printing console messages there. After a countdown of a few seconds it will proceed to automatically boot whatever it can find. U-Boot will fall back to an interactive prompt if its boot sequence fails or if you interrupt the countdown.

U-Boot supports UEFI to some extent, enough to run a GRUB package that would be installed by whatever OS you want to have on your device. The boot sequence checks for the standard UEFI removable media paths like /efi/boot/bootaa64.efi, so you should be able to use your desired OS' generic installer images. For details, see upstream documentation for UEFI on U-Boot.

Otherwise, the boot sequence also checks an extlinux.conf file that can configure which kernel, initramfs, device-tree file and kernel command line arguments should be used. See upstream documentation for Generic Distro Configuration Concept.

If you want to work inside the U-Boot shell, see an incomplete list of shell commands, or use the help command inside the prompt. Configuration is done via environment variables inside the shell, which can be saved to and automatically loaded from persistent storage configured at build-time.

WARNING: Environment variable storage has not been explicitly configured so far and is untested in the context of Libreboot. It may cause data loss or even brick your device by overwriting your disk's partition table, unexpected parts of the SPI ROM image, or do something else entirely.

Known issues

U-Boot integration in Libreboot is incomplete. Here is a list of known issues that affect all boards:

  • Branding is U-Boot with their logo (Libreboot only in version number)
  • Splash screen might be better instead of console messages
  • Cursor/drawing bugs with video improvements patches
  • Environment storage is likely dangerously broken
  • SMBIOS info is missing, Unknown, Unknown Product, etc.
  • Coreboot tables are ignored, unavailable to the OS
  • Re-inits display instead of using coreboot framebuffer
  • Chromebook FMAP layouts should match stock layout for GBB, VPD
  • Chromebook GBB, VPD are not handled, data may be unavailable to OS
  • U-Boot "coreboot*" defconfigs needs more work and an ARM64 version
  • UEFI support is incomplete

See also