Offhand this looks reasonable. It's a bit of groundwork for mips32, but AFAIK not enough to even try to evaluate a mips build just yet.

I ask because other doubles have entries there, and because when I try to use these configs I get something like:

$ nix build -f . --arg crossSystem '{config="mips-unknown-linux-gnu";}' bash
error: while evaluating the attribute 'stdenv' of the derivation 'bash-4.4-p12-mips-unknown-linux-gnu' at /home/will/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:148:11:
while evaluating the attribute 'defaultNativeBuildInputs' of the derivation 'stdenv' at /home/will/nixpkgs/pkgs/stdenv/generic/default.nix:72:14:
while evaluating the attribute 'gcc' at /home/will/nixpkgs/pkgs/top-level/all-packages.nix:5798:3:
while evaluating 'lowPrio' at /home/will/nixpkgs/lib/meta.nix:52:13, called from /home/will/nixpkgs/pkgs/top-level/all-packages.nix:5958:10:
while evaluating 'addMetaAttrs' at /home/will/nixpkgs/lib/meta.nix:15:28, called from /home/will/nixpkgs/lib/meta.nix:52:18:
while evaluating 'wrapCC' at /home/will/nixpkgs/pkgs/top-level/all-packages.nix:6713:12, called from /home/will/nixpkgs/pkgs/top-level/all-packages.nix:5958:19:
while evaluating 'wrapCCWith' at /home/will/nixpkgs/pkgs/top-level/all-packages.nix:6697:16, called from /home/will/nixpkgs/pkgs/top-level/all-packages.nix:6713:16:
while evaluating 'callPackageWith' at /home/will/nixpkgs/lib/customisation.nix:113:35, called from /home/will/nixpkgs/pkgs/top-level/all-packages.nix:6698:7:
while evaluating 'makeOverridable' at /home/will/nixpkgs/lib/customisation.nix:72:24, called from /home/will/nixpkgs/lib/customisation.nix:117:8:
while evaluating anonymous function at /home/will/nixpkgs/pkgs/build-support/cc-wrapper/default.nix:8:1, called from /home/will/nixpkgs/lib/customisation.nix:74:12:
while evaluating the attribute 'configureFlags' of the derivation 'glibc-2.26-131-mips-unknown-linux-gnu' at /home/will/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:148:11:
while evaluating the attribute 'ARCH' of the derivation 'linux-headers-4.4.10-mips-unknown-linux-gnu' at /home/will/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:148:11:
while evaluating the attribute 'platform.kernelArch' at /home/will/nixpkgs/lib/systems/default.nix:25:7:
attribute 'kernelArch' missing, at /home/will/nixpkgs/pkgs/os-specific/linux/kernel-headers/default.nix:17:12

Are more similar changes coming? O:)

Can you look at lib/systems/examples.nix? I think the one that is there should be mips32el. Try

nix-store -r /nix/store/jlfhcadzpjyls8y22v3l9f86j86lisxz-mips64el-unknown-linux-gnu-binutils-2.28
nix-store -r /nix/store/pcm4j6k546fprai1fghqyn93faa3mdgc-hello-2.10-mips64el-unknown-linux-gnu
/nix/store/jlfhcadzpjyls8y22v3l9f86j86lisxz-mips64el-unknown-linux-gnu-binutils-2.28/bin/mips64el-unknown-linux-gnu-readelf -a /nix/store/pcm4j6k546fprai1fghqyn93faa3mdgc-hello-2.10-mips64el-unknown-linux-gnu/bin/hello
/nix/store/jlfhcadzpjyls8y22v3l9f86j86lisxz-mips64el-unknown-linux-gnu-binutils-2.28/bin/mips64el-unknown-linux-gnu-ld --help

I see "32s" everywhere.

According to google, the Fuloong MiniPC is based on a Loongson 2F, which wikipedia says is a MIPS64. That doesn't stop it from running 32 bit code (MIPS64 specifies a backward compatibility mode) and it does look rather like whoever set it up for nixpkgs is expecting to do just that. https://en.wikipedia.org/wiki/Loongson#Loongson_2F

For nixwrt so far I've been defining the systems inline where I needed it - e.g. https://github.com/telent/nixwrt/blob/nixwrt/nixwrt/default.nix#L18 but maybe that's a completely back to front way of doing it. I am happy to add an example system and platform for "generic mips-based router" to this PR if you think that makes it more generally useful to other people?

I do, because it saves me sorting out what a reasonable configuration might be if I wanted to quickly test targeting mips32 O:).

But that's not a great reason-- I think ideally a suitable "basic" configuration (setting kernelArch) would be automagically inferred from the basic triple?

Anyway don't think this should block on account of any of this, I was just eager to give it a go :).

I added some more changes, fixing what is there finally. I was going to point https://hydra.nixos.org/jobset/nixpkgs/ericson2314-cross-trunk at your branch to help you finish things off, but hydra is down ATM. Ping me to do that later if I forget.

Not that you forgot but just FYI hydra is back up! \o/ @Ericson2314

Heheh I did. I just did an eval now https://hydra.nixos.org/build/70240205 . It at least evals, though we are getting that same weird "lto isn't a supported language" error in GCC. Whatever, this is still progress so I'm merging.

@telent https://hydra.nixos.org/jobset/nixpkgs/cross-trunk will soon include the evaling, but broken fuloong builds. Feel free to replace that with a more relevant/modern mips platform. If you could maintain MIPS jobs if your choosing, that what would be great!

This merge broke stdenv on aarch64. https://hydra.nixos.org/build/70246649

Even changing non-cross stdenv was probably not intended, but I haven't read this.

Indeed, not intended at all.