dotfiles/.claude/tasks/0003-kernel-and-hardware.md

spec, blocked-by

spec	blocked-by
laptop-mvi	0001-skeleton-and-building-host

What to build

Give neogaia the kernel and hardware enablement it needs to run well on the Dell XPS 13 9380: the CachyOS kernel pulled as a binary from chaotic-nyx (not compiled from source), Intel microcode, and the redistributable firmware for the QCA6174 wifi. Add a zram toggle Module and enable it here.

The kernel is selected through a small per-Host kernel mechanism so other Hosts can choose different kernels. The chaotic substituter and its trusted public key are added to the Nix settings so the kernel is fetched from the binary cache from the first build.

Acceptance criteria

• neogaia runs the CachyOS kernel selected via a per-Host kernel mechanism, sourced from chaotic-nyx.
• The chaotic substituter and trusted public key are in the Nix settings, so the kernel is fetched from cache rather than compiled.
• Intel microcode is enabled.
• Redistributable firmware is enabled so the QCA6174 wifi hardware is available.
• A zram toggle Module exists (following the Enable convention) and is enabled on neogaia.
• The neogaia toplevel still builds with all of the above.

Implementation Notes

• Per-Host kernel mechanism = native boot.kernelPackages. neogaia sets boot.kernelPackages = pkgs.linuxPackages_cachyos directly in its Host directory (hosts/neogaia/default.nix). No custom wrapper option was added: boot.kernelPackages is already a per-Host setting, so other Hosts pick their own kernel the same way. A string→package wrapper would have been premature abstraction with one Host and one kernel, so it was deliberately left out; the "mechanism" is the per-Host placement of the native option.
• Substituter/key live in the shared base, via the extra- options. They were added to system/default.nix (shared by every Host), not just neogaia, because the chaotic module is wired for all Hosts and the cache is general plumbing. nix.settings.extra-substituters / extra-trusted-public-keys are used rather than the replacing substituters / trusted-public-keys, so cache.nixos.org (and any other substituter) is only appended to, never dropped. chaotic's own module also provides these entries; the explicit declaration is belt-and-suspenders and keeps the built system's cache config visible and independent of that module.
• Dev-host build needed a daemon-level cache. Building the toplevel here first compiled the CachyOS kernel (and rustc bootstrap) from source, because the build daemon's /etc/nix/nix.conf had no nyx-cache substituter — the built system's nix.settings do not govern the daemon doing the build, and the dev user is a non-trusted client that cannot add substituters from the CLI. Adding extra-substituters/extra-trusted-public-keys for nyx-cache to /etc/nix/nix.conf (sudo) and restarting nix-daemon fixed it; the build then fetched the kernel (7.1.3) from the cache. Recorded as a gotcha in CLAUDE.md.
• zram Module kept minimal. modules/zram.nix is a pure toggle (zramSwap.enable = true under mkIf), following the modules/example.nix reference shape. This preserves task 0002's exact behaviour while moving the switch behind the Enable convention; the direct zramSwap.enable = true previously inline in the Host was removed in favour of modules.zram.enable = true.
• Why zram graduated to a Module but kernel/microcode/firmware stayed inline. zram is a reusable, cross-Host feature toggle (the spec calls for it as a Module), whereas the kernel choice, Intel microcode, and firmware are neogaia-specific hardware facts that belong to the Host itself.