nixos/modules: assertion for required PM files

nixos/modules/hardware/video/nvidia.nix

@@ -179,28 +179,41 @@ in
           You cannot configure both an Intel iGPU and an AMD APU. Pick the one corresponding to your processor.
         '';
       }
+
       {
         assertion = primeEnabled -> pCfg.nvidiaBusId != "" && (pCfg.intelBusId != "" || pCfg.amdgpuBusId != "");
         message = ''
           When NVIDIA PRIME is enabled, the GPU bus IDs must configured.
         '';
       }
+
       {
         assertion = offloadCfg.enable -> versionAtLeast nvidia_x11.version "435.21";
         message = "NVIDIA PRIME render offload is currently only supported on versions >= 435.21.";
       }
+
       {
         assertion = !(syncCfg.enable && offloadCfg.enable);
         message = "Only one NVIDIA PRIME solution may be used at a time.";
       }
+
       {
         assertion = !(syncCfg.enable && cfg.powerManagement.finegrained);
         message = "Sync precludes powering down the NVIDIA GPU.";
       }
+
       {
         assertion = cfg.powerManagement.enable -> offloadCfg.enable;
         message = "Fine-grained power management requires offload to be enabled.";
       }
+
+      {
+        assertion = cfg.powerManagement.enable -> (
+          builtins.pathExists (cfg.package.out + "/bin/nvidia-sleep.sh") &&
+          builtins.pathExists (cfg.package.out + "/lib/systemd/system-sleep/nvidia")
+        );
+        message = "Required files for driver based power management don't exist.";
+      }
     ];
 
     # If Optimus/PRIME is enabled, we: