jellyfin: fix Arc A380 VAAPI transcode (ASPM power gating)

Arc A380 GPU (07:00.0) becomes unreachable (MMIO returns 0xFFFFFFFF)
when PCIe ASPM powersupersave puts it into L1.1/L1.2 substates.
Both i915 and xe drivers hit the same hardware failure.

Fix: disable runtime PM for the GPU in power-tune, run after powertop
so the override sticks. Use i915 driver (xe has iHD buffer mapping
failures on this GPU/kernel 6.12 combination).

configuration.nix

@@ -120,15 +120,12 @@
     };
   };
 
-  # Arc A380 (56a5) is not in xe's default probe list on kernel 6.12.
-  # Without force_probe, xe refuses to bind and i915 claims the device
-  # instead -- producing broken MMIO (forcewake 0xFFFFFFFF) and crashing
-  # every VAAPI transcode.
-  hardware.intelgpu.driver = "xe";
-  boot.kernelParams = [
-    "xe.force_probe=56a5"
-    "i915.force_probe=!56a5"
-  ];
+  # Intel Arc A380 (DG2, 56a5) uses the i915 driver on kernel 6.12.
+  # The xe driver's iHD media driver integration has buffer mapping
+  # failures on this GPU/kernel combination. i915 works correctly for
+  # VAAPI transcode as long as ASPM deep states are disabled for the
+  # GPU (see modules/power.nix).
+  hardware.intelgpu.driver = "i915";
 
   # Per-service 2MB hugepage budget calculated in service-configs.nix.
   boot.kernel.sysctl."vm.nr_hugepages" = service_configs.hugepages_2m.total_pages;

modules/power.nix

@@ -65,10 +65,14 @@
   # ASPM powersupersave: deepest PCIe link power states (L1.1/L1.2). The
   # pcie_aspm=force boot param enables ASPM, but the runtime policy defaults
   # to "default" which only uses L0s. powersupersave adds L1 substates for
-  # all downstream devices (NVMe, AHCI, NIC).
+  # all downstream devices (NVMe, AHCI, NIC). The Intel Arc A380 GPU is
+  # excluded -- it hangs on L1 substate re-entry (MMIO returns 0xFFFFFFFF).
   systemd.services.power-tune = {
     description = "Apply power-saving sysfs knobs (EPP, ASPM policy)";
-    after = [ "multi-user.target" ];
+    after = [
+      "multi-user.target"
+      "powertop.service"
+    ];
     wantedBy = [ "multi-user.target" ];
     serviceConfig = {
       Type = "oneshot";
@@ -85,6 +89,13 @@
             # PCIe ASPM policy
             aspm=/sys/module/pcie_aspm/parameters/policy
             [ -f "$aspm" ] && echo powersupersave > "$aspm"
+
+            # Intel Arc A380 (07:00.0) hangs when PCIe link enters L1
+            # substates or runtime PM suspends it. Force the device to stay
+            # active. This runs after powertop --auto-tune (which sets
+            # power/control=auto on every device) so the override sticks.
+            gpu=/sys/bus/pci/devices/0000:07:00.0/power/control
+            [ -f "$gpu" ] && echo on > "$gpu"
           '';
         }
       );