Add ootb RPi SD "support"

[blaggacao]

Example of #72 — since this is potentially out of scope for nixflk, I'm leaving it as draft for others to discover how I solved it. Maybe some aspets can inform nixflk, though (like renaming niximg to nixISO, or such).

[blaggacao]

latest kernel support got merged in master, so the patch + overlay is soon no more necesary

[blaggacao]

Ok, so:

$ nix develop
$ flk sd

fails with (what the heck?)

/nix/store/fy819ys8fyzm02bjvzfvr7qiihr4allh-flk-env/bin/nix: line 2: 104701 Segmentation fault      (core dumped) /nix/store/6b7s5f2vrlixmlyr9bi1gkrgkq6xqgmw-nix-2.4pre20201205_a5d85d0/bin/nix --option experimental-features "nix-command flakes ca-references" "$@"

But on a similar structured repo, it fails with:

Package ‘raspberrypi-firmware-1.20200601’ in /nix/store/vc3v85pjs6v04wn5w03z1b0mkl6r0p5q-source/pkgs/os-specific/linux/firmware/raspberrypi/default.nix:20 is not supported on ‘x86_64-linux’, refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.

(use '--show-trace' to show detailed location information)

I'm concluding that cross compilation is not supported to date in nixflk.

[nrdxp]

I haven't messed with cross compilation in general in nix. So while I wouldn't mind including support for it in nixflk, we'd need someone more experience to implement it.

[mohe2015]

EDIT: Sorry this doesn't cross compile, it uses qemu emulation which may not be what you want.

@blaggacao The radical but easiest way would likely to be: https://gist.github.com/mohe2015/3baaf764264f7bb9630bbe18dbdb6d8b/revisions (sorry for the not perfect diff, I just used my local version). This changes the build path to e.g. build with nix build .#nixosConfigurations.aarch64-linux.nixSD.config.system.build.toplevel. This doesn't really cross-compile but uses qemu binfmt emulation but I don't think this is even possible otherwise.

Edit: You then can't use the normal nixos-rebulid because it searches for the wrong path. No liability for any damages because I don't know myself what I'm doing but:

sudo nix build --profile /nix/var/nix/profiles/system .#nixosConfigurations.x86_64-linux.NixOS.config.system.build.toplevel
sudo /nix/var/nix/profiles/system/bin/switch-to-configuration switch

should work.

nix build .#nixosConfigurations.aarch64-linux.nixSD.config.system.build.sdImage

should build the aarch64 image.

To get the old way to work again a merge of both methods would probably be necessary.

[nrdxp]

This seems to be supplanted by your #105, can I close?