NixOS on ARM/UEFI

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Note: This page is written assuming AArch64. It is possible that following most of these instructions for armv7l would work just as well, but armv7l support in NixOS is not at a point where doing so will be nice.

This section of the NixOS on ARM documentation aims to document as much as possible about booting any ARM boards using UEFI. This will be written with a heavy bias about Single Board Computers (SBCs), as this is where booting is seen as complicated, cumbersome, when not described as impossible.

The Basics First

Target Support

Some things will not be specific to UEFI. For example, board support by the kernel used. This is written assuming that mainline Linux works enough on the target system so that you can install from the generic iso image.

Just as you could on x86_64 if your platform required it, you can build a customized iso image. Explaining this is out of scope for this article. The same pitfalls apply. For example, the generated configuration will not take into account configuring the customized kernel.

Initial Boot Firmware

Let's define what an Initial Boot Firmware is. It is a generic term I'm using to describe the first thing the CPU starts at boot time. On your typical x86_64 system, it would be what was previously called the BIOS. Now often diminutively called by the name EFI. This is what initializes enough of the hardware so that the operating system can start. Additionally, it often provides facilities for the user to do basic configuration, and manage boot options.

In the ARM with SBCs landscape, U-Boot is the de facto solution for the Initial Boot Firmware. Though U-Boot is confusingly, but rightly, often referred to as a Boot Loader. U-Boot plays double duties often. It is tasked with initializing the hardware, and often also used to handle loading and booting the operating system.

UEFI

The Unified Extensible Firmware Interface it not in itself a tangible thing. Wrongly abstracted, it is a specification used to provide an interface to describe a standard boot process, including an environment before the operating system starts, and protocols for operating systems.

There are multiple implementations of UEFI. Vendors like American Megatrends, Phoenix Technologies and Insyde Software may have produced the one on your personal x86_64 machine. TianoCore is the reference UEFI implementation, and Open Source. Luckily enough, U-Boot implements enough (and a bit more) of the UEFI spec.

SBBR? EBBR?

Other than letter salads, they are Server Base Boot Requirements and Embedded Base Boot Requirements. Two specifications for ARM. If your target is in compliance with either, booting with UEFI should already be supported. With the minimal UEFI support in U-Boot, targets that were not made to be EBBR compliant can be made compliant, or be close enough for what it matters.

UEFI, on my SBC???

Believe me or not, it's more likely that you can, if your SBC is well supported by mainline U-Boot. U-Boot provides enough UEFI to comply with EBBR, which in turn is enough to allow us to boot the AArch64 UEFI NixOS iso, and with almost no differences compared to the x86_64 guide, simply follow the installation instruction to boot into an installed system.

Getting an Initial Boot Firmware

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As an opinionated example, you can get started with Tow-Boot, a U-Boot distribution, which is intended to make the initial setup a bit easier by abstracting the platform differences so that they do not matter.

Any other UEFI complient Initial Boot Firmware can be used.

Getting the installer image

For the time being, the following jobs are recommended:

• nixos:trunk-combined:nixos.iso_minimal.aarch64-linux
• nixos:trunk-combined:nixos.iso_minimal_new_kernel.aarch64-linux

With the new kernel variant being preferred

This installer image should be written to a USB drive, like usual. In a pinch, it may also be written to an SD image, if your target's initial boot firmware does not need to be written to that same SD image. This UEFI iso is incompatible (by design) with shared firmware storage.

Installing

Following the usual installation steps for UEFI is almost enough. Here's what you need to be mindful about.

Sidenote: As the introduction stated, this guide assumes that the kernel in use fully supports your target board. If there are issues that comes from lack of hardware support, it is not a bug in this documentation.

Shared Firmware Storage

Note: This will make more sense when Getting an Initial Boot Firmware is finished...

If your Initial Boot Firmware lives on the target installation storage, e.g. written to an SD card and you install to the same SD card, you will need need to make sure that:

• You are not overwriting the firmware, if it is not protected by a partition.
• The partition table is not rewritten from scratch / zero.
• To not delete required existing firmware partitions.

Note: If your Initial Boot Firmware is not protected by a partition, consider choosing an alternative Initial Boot Firmware installation method or distribution that protects it.

Otherwise, you can do as you would usually, create an ESP partition, FAT32, to be mounted at /boot/, your preferred rootfs partition, swap if desired, etc.

Bootloader configuration

Know if your Initial Boot Firmware's UEFI implementation has writable EFI vars. This is not true for all UEFI implementations on ARM, but is something to be mindful about. If it does not, boot.loader.efi.canTouchEfiVariables has to be set to false.

{ /* configuration.nix */
  boot.loader.efi.canTouchEfiVariables = false;
}

Tip

Just like on x86_64 rEFInd installed to the fallback location (/EFI/BOOT/BOOTAA64.EFI) may be helpful.

For the time being, only GRUB2 has been verified to work. When using the systemd-boot bootloader on a test system, the kernel panics. Since EFI variables cannot be manipulated, using efiInstallAsRemovable handles installing GRUB2 to the default fallback location.

{ /* configuration.nix */
  boot.loader.grub.enable = true;
  boot.loader.grub.efiSupport = true;
  boot.loader.grub.efiInstallAsRemovable = true;
  boot.loader.grub.device = "nodev";
}

General Tips

Using the latest kernel is probably a good idea. Hardware support for ARM platforms is always improving, and using the latest kernel, rather than the "latest LTS", might be enough to break it or make it.

{ /* configuration.nix */
  boot.kernelPackages = pkgs.linuxPackages_latest;
}

Known Issues

Device Trees

As of right now, there is no consensus within Linux distros about the topic of managing device trees for the boot process with UEFI.

This current setup relies on the initial boot firmware providing an appropriate device tree for the kernel that will run.

With U-Boot, it is possible to make it load a device tree, for example a more up-to-date one, by placing the dtb folder from a kernel build output at the /dtb location in the ESP. U-Boot will automatically load a device tree according to heuristics, which should be the right one.

It is unknown how much of an actual issue this is in practice.