kernelPatches: ath driver: allow setting regulatory domain

Ports an OpenWRT patch for Atheros wireless drivers (ath*) which allows
the user to change the regulatory domain code to the one which actually
applies.

All Atheros devices have a regulatory domain burned into their EEPROM.
When using a device as AP, this domain is frequently overly restrictive
when compared to the regulation which applies in the country the device
actually operates in; often, this restriction disallows IR on all
channels making it impossible to use the device as an AP at all.

This commit introduces the NixOS config option
networking.wireless.athUserRegulatoryDomain which, if enabled, applies
the patch and sets the kernel config option ATH_USER_REGD.

The original OpenWRT patch targets Linux 5.8.

nixos/modules/hardware/network/ath-user-regd.nix

@@ -0,0 +1,31 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+let
+  kernelVersion = config.boot.kernelPackages.kernel.version;
+  linuxKernelMinVersion = "5.8";
+  kernelPatch = pkgs.kernelPatches.ath_regd_optional // {
+    extraConfig = ''
+      ATH_USER_REGD y
+    '';
+  };
+in
+{
+  options.networking.wireless.athUserRegulatoryDomain = mkOption {
+    default = false;
+    type = types.bool;
+    description = ''
+      If enabled, sets the ATH_USER_REGD kernel config switch to true to
+      disable the enforcement of EEPROM regulatory restrictions for ath
+      drivers. Requires at least Linux ${linuxKernelMinVersion}.
+    '';
+  };
+
+  config = mkIf config.networking.wireless.athUserRegulatoryDomain {
+    assertions = singleton {
+      assertion = lessThan 0 (builtins.compareVersions kernelVersion linuxKernelMinVersion);
+      message = "ATH_USER_REGD patch for kernels older than ${linuxKernelMinVersion} not ported yet!";
+    };
+    boot.kernelPatches = [ kernelPatch ];
+  };
+}

nixos/modules/module-list.nix

@@ -52,6 +52,7 @@
   ./hardware/ledger.nix
   ./hardware/logitech.nix
   ./hardware/mcelog.nix
+  ./hardware/network/ath-user-regd.nix
   ./hardware/network/b43.nix
   ./hardware/network/intel-2200bg.nix
   ./hardware/nitrokey.nix

pkgs/os-specific/linux/kernel/patches.nix

@@ -1,6 +1,19 @@
 { lib, fetchpatch, fetchurl }:
 
 {
+  ath_regd_optional = rec {
+    name = "ath_regd_optional";
+    patch = fetchpatch {
+      name = name + ".patch";
+      url = "https://github.com/openwrt/openwrt/raw/ed2015c38617ed6624471e77f27fbb0c58c8c660/package/kernel/mac80211/patches/ath/402-ath_regd_optional.patch";
+      sha256 = "1ssDXSweHhF+pMZyd6kSrzeW60eb6MO6tlf0il17RC0=";
+      postFetch = ''
+        sed -i 's/CPTCFG_/CONFIG_/g' $out
+        sed -i '/--- a\/local-symbols/,$d' $out
+      '';
+    };
+  };
+
   bridge_stp_helper =
     { name = "bridge-stp-helper";
       patch = ./bridge-stp-helper.patch;