How to cross-compile the SD Card images?

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Despite being commented out, rpizero1 in my flake is a cross compiled system (x86_64-linux → armv7l-linux): https://github.com/colemickens/nixcfg/blob/b9b033a12cc8aa140e847aa27c951f9eead314eb/flake.nix#L188

But the important parts are:

  1. a specific pinned nixpkgs. for me, whatever crosspkgs is in my flake inputs was a known good revision for (x86_64->aarch64 crossing). Your luck will vary with your cross, whatever rev you’re on, what exact packages you need, what the caches have, etc.

  2. that nixpkgs was imported with system = x86_64-linux: https://github.com/colemickens/nixcfg/blob/b9b033a12cc8aa140e847aa27c951f9eead314eb/flake.nix#L188

  3. and then later, in the machine config, https://github.com/colemickens/nixcfg/blob/b9b033a12cc8aa140e847aa27c951f9eead314eb/hosts/rpizero1/configuration.nix#L25:

      nixpkgs.crossSystem = lib.systems.examples.raspberryPi;

EDIT: Sorry, rpizero1 = (Raspberry Pi Zero W). So, my cross target was armv6, not armv7l, so again, see caveat about finding a known good nixpkgs where the core pkgs cross compile successfully. This can be confirmed by following systems.examples.raspberryPi to here: https://github.com/NixOS/nixpkgs/blob/1c65a509fbbc17a2853a657ea1391de0aab9e793/lib/systems/examples.nix#L37-L39

  raspberryPi = {
    config = "armv6l-unknown-linux-gnueabihf";
  } // platforms.raspberrypi;
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