forgot one other file needed. This one is amdgpu-kernel-module.nix, referenced in the configuration.nix as well. This is the part that packages the amdgpu module separately from the kernel:

{
  pkgs,
  lib,
  kernel ? pkgs.linuxPackages_latest.kernel,
}:

pkgs.stdenv.mkDerivation {
  pname = "amdgpu-kernel-module";
  inherit (kernel)
    src
    version
    postPatch
    nativeBuildInputs
    ;

  kernel_dev = kernel.dev;
  kernelVersion = kernel.modDirVersion;

  modulePath = "drivers/gpu/drm/amd/amdgpu";

  buildPhase = ''
    BUILT_KERNEL=$kernel_dev/lib/modules/$kernelVersion/build

    cp $BUILT_KERNEL/Module.symvers .
    cp $BUILT_KERNEL/.config        .
    cp $kernel_dev/vmlinux          .

    make "-j$NIX_BUILD_CORES" modules_prepare
    make "-j$NIX_BUILD_CORES" M=$modulePath modules
  '';

  installPhase = ''
    make \
      INSTALL_MOD_PATH="$out" \
      XZ="xz -T$NIX_BUILD_CORES" \
      M="$modulePath" \
      modules_install
  '';

  meta = {
    description = "AMD GPU kernel module";
    license = lib.licenses.gpl3;
  };
}

In reply to @niko:nyanbinary.rs
6.12 works

Hello!
I can't get full sleep to work on my FW 13 with an Ultra 5 125H.
The tool from https://github.com/intel/S0ixSelftestTool says (after a few adjustments to make it work on NixOS):

S0ix substates residency delta value: S0i2.0 2196

S0ix substates residency delta value: S0i2.1 13249322

S0ix substates residency delta value: S0i2.2 0

Your system only get shallower S0ix substate residency: S0i2.1 308804753

(full output available at https://paste.sr.ht/~albertlarsan68/0ce10116393d54b43cce870322d16c8ecd3639e8)

Has anyone the same problem?

The fan does stop and the power led does blink normally during the test. The only expansion cards connected are a pair of USB-C with one for the power input and the other for a YubiKey.