Building Nix packages for the Raspberry Pi 3 or newer requires building for an ARM 64 architecture, which Nix refers to as aarch64-linux.

To build aarch64-linux binaries we can:

1. Build natively on an aarch64-linux machine.
2. Cross compile for aarch64-linux.
3. Compile with an emulator.

The first option is the simplest. For example, a Raspberry Pi with Nix installed compiles the aarch64-linux binaries reasonably well. Typically, you need to build only a few packages from source and the rest may be downloaded from the Nix public binary cache. You can also use a more powerful ARM 64 computer for building, if you own or rent one.

I have little to say about the second option because I never managed to get it working. I also suspect that cross compiled packages are not in the public binary cache, so build times are longer than native builds.

In the rest of the article I explore the third option: compiling with an emulator, in particular with QEMU.

On NixOS

On NixOS, it’s trivial to use QEMU to build for different architectures. You need to adjust one parameter in your NixOS host configuration:

boot.binfmt.emulatedSystems = [ "aarch64-linux" ];

This snippet installs and configures QEMU and enables emulated builds for the Raspberry Pi. For example, to build hello from source, run:

nix build nixpkgs#legacyPackages.aarch64-linux.hello --no-substitute

For this demonstration I added the --no-substitute flag to disallow binary substitutes. In other words, this flag forces Nix to build all packages from source.

If the compilation with the emulated tool chain works, Nix writes an ARM 64-bit binary at ./result/bin/hello

Using GitHub Actions

Recently I discovered the setup-qemu-action from Docker which helps us to configure a hosted GitHub Action runner to build for the Raspberry Pi:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: docker/setup-qemu-action@v3                                     # ⑴
      - uses: cachix/install-nix-action@v23                                   # ⑵
        with:
          extra_nix_config: "extra-platforms = aarch64-linux"
      -run: |                                                                 # ⑶
        nix build nixpkgs#legacyPackages.aarch64-linux.hello --no-substitute

This GitHub Actions workflow starts an Ubuntu virtual machine and installs Nix:

1. Install the QEMU static binaries using a GitHub Action from Docker.
2. Install Nix using a GitHub Action from Cachix and configure it to allow building for aarch64-linux.
3. Build hello for aarch64-linux.

Summary

Using two GitHub Actions from Docker and Cachix I set up a workflow to build packages for the Raspberry Pi using freely available Ubuntu runners from GitHub.

In a related article I explain how I use this technique to build and deploy NixOS on Raspberry Pi.