View source for U-Boot

'''U-Boot''' is a common '''''platform firmware''''' implementation for embedded and embedded-like devices.

It may also be used as a bootloader, but with modern U-Boot, [[UEFI]] boot is generally possible too.

== U-Boot in Nixpkgs ==
Mainline U-Boot is packaged in Nixpkgs.

* {{Nixpkgs Link|pkgs/misc/uboot/default.nix}} 

Vendor forks of U-Boot are generally not packaged in Nixpkgs, and generally unwelcome.

U-Boot may be patched for compatibility reasons mainly, and work should be taken to send them to mainline U-Boot.

=== Building a packaged U-Boot ===
You need to know two things:

# The target ISA (''Instruction Set Architecture'')
# If you need to cross-compile

The target ISA is the ISA of the ''target board''. This would be defined by the CPU used on the board.

You will need to cross-compile if you are not building on the same ''architecture'' as the target board. For example, if you are building for an <code>AArch64</code> ''target board'' on a standard <code>x86_64</code> laptop, you would need to cross-compile.

With those facts known, you can build the appropriate attribute from a Nixpkgs checkout.

The general format is:<syntaxhighlight lang="shell-session">
$ nix-build -A [pkgsCross.$cross_name.]$packageName
$ # Where the part in [brackets] is needed for cross-compilation.
</syntaxhighlight>
{| class="wikitable"
|+
<code>pkgsCross</code> attribute name cheat sheet
!Architecture
!Package Set
|-
|<code>aarch64</code>
|<code>aarch64-multiplatform</code>
|-
|<code>armv7l</code>
|<code>armv7l-hf-multiplatform</code>
|-
|<code>armv6l</code>
|<code>raspberryPi</code> ''[sic]''
|}

==== Example build ====
The following examples use the U-Boot package for a ''LibreTech CC ‘LePotato’'', which uses an ''S905X'' SoC. This SoC uses an <code>AArch64</code> ISA.

To build the package with cross-compilation, you need to prefix with the correct cross-compilation package set:<syntaxhighlight lang="shell-session">
$ # Example running on x86_64.
$ uname -m 
x86_64
$ nix-build -A pkgsCross.aarch64-multiplatform.ubootLibreTechCC
</syntaxhighlight>If, instead, you are building natively (same architecture) you can refer to the package attribute name directly:<syntaxhighlight lang="shell-session">
$ # Example output for an AArch64 system.
$ uname -m 
aarch64
$ nix-build -A ubootLibreTechCC
</syntaxhighlight>

=== Installing U-Boot ===
You will need to consult either the [https://docs.u-boot.org/ Official U-Boot documentation], or a board-specific page to find out how to use the produced output.

The large variety of ecosystems supported by U-Boot makes it impossible to document succinctly.

==== SBC and related projects in NixOS ====

* [[NixOS on ARM]]
* [[NixOS on RISCV/VisionFive 2|NixOS on RISCV]]

== Using NixOS with U-Boot ==
{{Note|This section assumes U-Boot is installed already.}}
With U-Boot, NixOS currently still assumes use of [https://docs.u-boot.org/en/v2024.04/develop/distro.html U-Boot's Generic Distro Configuration Concept] as the mechanism for discoverable boot. The preferred scheme is ''extlinux-compatible''. This is how the kernel <code>Image</code>, <code>initrd</code>, dtb, and Kernel command-line arguments are provided with complete support for the lifecycles of generations.

Note that U-Boot is moving to [https://docs.u-boot.org/en/v2024.04/develop/bootstd.html ''U-Boot Standard Boot'']. How it affects the assumptions from NixOS is still not documented.

Booting through UEFI with U-Boot should work. Booting with UEFI is tentatively supported, with intent of being the only fully supported boot method in the future. See [[NixOS on ARM/UEFI]], which describes part of the drawbacks.

=== Quick overview about the Generic Distro Configuration support ===
U-Boot is scripted to scan all attached storage devices and partitions, and look for a file named <code>/extlinux/extlinux.conf</code> or <code>/boot/extlinux/extlinux.conf</code>. This will will be generated by NixOS when configured with {{Nixos:option|boot.loader.generic-extlinux-compatible.enable}}.

The partition also needs to '''have its "bootable" flag set'''. This is true for ''both MBR and GPT partitioning schemes''.

== Vendor U-Boot ==

The term ''vendor U-Boot'' means the provided pre-built U-Boot binaries and the source as well. It generally implies use of an older fork of U-Boot from a ''BSP'' (''Board Support Package'').

Vendor U-Boot may or may not work to boot and use NixOS.

The answer is actually really complicated, and depends not only about U-Boot, but the intended Linux kernel to be booted, and the boot method in use.

The <code>buildUBoot</code> implementation in Nixpkgs may or may not work to build a vendor U-Boot, and this usage is unsupported.

A few vendors ship good pre-built modern U-Boot implementations. Booting NixOS with those ''should work''. If it does not, things may need to be fixed on any of the end. But also, this depends on the kernel, and boot method in use.

== Other U-Boot distributions ==
There exists other pre-built U-Boot distributions. As long as they follow the mainline U-Boot semantics, and are close enough in age to the current releases, they should work to boot NixOS.

=== Tow-Boot ===
{{Aside|Tow-Boot is not a NixOS project.<br />Though it is built with Nix expressions, and made by authors involved with the NixOS project.}}
Stock NixOS should be assumed to work with Tow-Boot, always depending on the boot method and kernel in use.

[https://tow-boot.org/ Tow-Boot] is currently used as a reference as the only distro-agnostic U-Boot build, for support purposes.

== See also ==

* [[NixOS on ARM/UEFI]]
* [https://source.denx.de/u-boot/u-boot Official U-Boot source repo]
* [https://docs.u-boot.org/ Official U-Boot documentation]