nixos/nvidia: assert open option is manually set on drivers >= 560

This requirement was introduced in
NixOS#337289 as a way to make sure users
"explicitly pick which version of the driver they want since nvidia
recommends the open one, but that is incompatible with older drivers".
This is reasonable, however the user isn't informed in any real way
aside from the upcoming release notes

This has caused a
[good](NixOS#337289 (comment))
[amount](NixOS#337289 (comment))
[of](NixOS#338196)
[confusion](NixOS/nixos-hardware#1092) amongst
users. By introducing this assertion and using a new `useOpenModules`
local variable, we can have the same behavior but display a proper error
message to hopefully clear things up until we can safely make this a
default

nixos/modules/hardware/video/nvidia.nix

@@ -10,13 +10,15 @@ let
 
   cfg = config.hardware.nvidia;
 
+  useOpenModules = if cfg.open == null then false else cfg.open;
+
   pCfg = cfg.prime;
   syncCfg = pCfg.sync;
   offloadCfg = pCfg.offload;
   reverseSyncCfg = pCfg.reverseSync;
   primeEnabled = syncCfg.enable || reverseSyncCfg.enable || offloadCfg.enable;
   busIDType = lib.types.strMatching "([[:print:]]+[\:\@][0-9]{1,3}\:[0-9]{1,2}\:[0-9])?";
-  ibtSupport = cfg.open || (nvidia_x11.ibtSupport or false);
+  ibtSupport = useOpenModules || (nvidia_x11.ibtSupport or false);
   settingsFormat = pkgs.formats.keyValue { };
 in
 {
@@ -255,11 +257,12 @@ in
       };
 
       open = lib.mkOption {
+        default = if lib.versionOlder nvidia_x11.version "560" then false else null;
         example = true;
         description = "Whether to enable the open source NVIDIA kernel module.";
-        type = lib.types.bool;
+        type = lib.types.nullOr lib.types.bool;
         defaultText = lib.literalExpression ''
-          lib.mkIf (lib.versionOlder config.hardware.nvidia.package.version "560") false
+          if lib.versionOlder config.hardware.nvidia.package.version "560" then false else null
         '';
       };
 
@@ -287,6 +290,10 @@ in
               assertion = !(nvidiaEnabled && cfg.datacenter.enable);
               message = "You cannot configure both X11 and Data Center drivers at the same time.";
             }
+            {
+              assertion = cfg.open != null;
+              message = "`hardware.nvidia.open` must be set when using driver versions >= 560.";
+            }
           ];
           boot = {
             blacklistedKernelModules = [
@@ -319,8 +326,7 @@ in
           };
           environment.systemPackages = [ nvidia_x11.bin ];
 
-          hardware.nvidia.open = lib.mkIf (lib.versionOlder nvidia_x11.version "560") (lib.mkDefault false);
-          hardware.nvidia.gsp.enable = lib.mkDefault (cfg.open || lib.versionAtLeast nvidia_x11.version "555");
+          hardware.nvidia.gsp.enable = lib.mkDefault (useOpenModules || lib.versionAtLeast nvidia_x11.version "555");
         })
 
         # X11
@@ -384,12 +390,12 @@ in
             }
 
             {
-              assertion = cfg.open -> (cfg.package ? open);
+              assertion = useOpenModules -> (cfg.package ? open);
               message = "This version of NVIDIA driver does not provide a corresponding opensource kernel driver.";
             }
 
             {
-              assertion = cfg.open -> cfg.gsp.enable;
+              assertion = useOpenModules -> cfg.gsp.enable;
               message = "The GSP cannot be disabled when using the opensource kernel driver.";
             }
 
@@ -592,7 +598,7 @@ in
               "L+ /run/nvidia-docker/extras/bin/nvidia-persistenced - - - - ${nvidia_x11.persistenced}/origBin/nvidia-persistenced";
 
           boot = {
-            extraModulePackages = if cfg.open then [ nvidia_x11.open ] else [ nvidia_x11.bin ];
+            extraModulePackages = if useOpenModules then [ nvidia_x11.open ] else [ nvidia_x11.bin ];
             # nvidia-uvm is required by CUDA applications.
             kernelModules =
               lib.optionals config.services.xserver.enable [
@@ -603,14 +609,14 @@ in
               # With the open driver, nvidia-uvm does not automatically load as
               # a softdep of the nvidia module, so we explicitly load it for now.
               # See https://github.com/NixOS/nixpkgs/issues/334180
-              ++ lib.optionals (config.services.xserver.enable && cfg.open) [ "nvidia_uvm" ];
+              ++ lib.optionals (config.services.xserver.enable && useOpenModules) [ "nvidia_uvm" ];
 
             # If requested enable modesetting via kernel parameters.
             kernelParams =
               lib.optional (offloadCfg.enable || cfg.modesetting.enable) "nvidia-drm.modeset=1"
               ++ lib.optional ((offloadCfg.enable || cfg.modesetting.enable) && lib.versionAtLeast nvidia_x11.version "545") "nvidia-drm.fbdev=1"
               ++ lib.optional cfg.powerManagement.enable "nvidia.NVreg_PreserveVideoMemoryAllocations=1"
-              ++ lib.optional cfg.open "nvidia.NVreg_OpenRmEnableUnsupportedGpus=1"
+              ++ lib.optional useOpenModules "nvidia.NVreg_OpenRmEnableUnsupportedGpus=1"
               ++ lib.optional (config.boot.kernelPackages.kernel.kernelAtLeast "6.2" && !ibtSupport) "ibt=off";
 
             # enable finegrained power management