Changing kernel config isn't changing the resulting configfile

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I’m still hoping to get nixos running well on the Inovato Quadra and still unable to see any network devices after it boots.

Armbian has an image for this device that works well; looking at the kernel config they are using (and a patch that was being used prior to official armbian support), I think that CONFIG_AC200_PHY and CONFIG_MFD_AC200 may be important settings to enable.

Unfortunately, when I try to build a kernel with these settings… nothing changes. Looking at the resulting kernel configfile, it’s not reflecting my changes.

I tried modifying CONFIG_FS_ENCRYPTION (which is yes by default) just to see if anything would happen, but it doesn’t:

kernelPackages = pkgs.linuxPackagesFor (
  pkgs.linux_6_12.override {
    extraStructuredConfig =
      let
        inherit (lib.kernel) module no yes;
      in
      {
        AC200_PHY = yes;
        MFD_AC200 = yes;
        FS_ENCRYPTION = lib.mkForce no;
      };
  }
);

$ nix build .#packages.x86_64-linux.quadra.config.boot.kernelPackages.kernel.configfile
$ grep AC200 ./result
$ echo $?
1
$
$
$ grep FS_ENCRYPTION ./result
CONFIG_FS_ENCRYPTION=y
CONFIG_FS_ENCRYPTION_ALGS=y
# CONFIG_FS_ENCRYPTION_INLINE_CRYPT is not set

What am I doing wrong?