boot: kernel 7.x for FLM NPU firmware, raise amdgpu GTT to 56 GiB

modules/boot.nix

@@ -2,9 +2,23 @@
 { config, pkgs, lib, ... }:
 
 {
-  # Use latest kernel instead of LTS
-  # boot.kernelPackages = pkgs.linuxPackages_latest;
-  # Note: security.nix forces hardened kernel, so we use that.
+  # Kernel 7.x: the in-tree amdxdna driver on 6.18 is too old for FastFlowLM
+  # (flm validate: kernel_ok=false, and it loads NPU firmware 1.0.0.63 while
+  # FLM needs >=1.1.0.0 — the 7.x driver loads npu_7.sbin = 1.1.2.64 instead).
+  boot.kernelPackages = pkgs.linuxPackages_latest;
+
+  # amdgpu GTT (dynamically shared system RAM the iGPU can address) raised to
+  # 56 GiB. On kernel 6.18 `amdgpu.gttsize` is deprecated; TTM controls it in
+  # 4 KiB pages: 56 GiB / 4 KiB = 14680064. Pair with a shrunken BIOS UMA/VRAM
+  # carve-out (set to 1-2GB in UEFI; VRAM peak observed is ~2.5G and buffers
+  # spill to GTT) so ~60GB system RAM backs this pool — GTT is reclaimable,
+  # only consumed when the GPU actually touches it.
+  # Merges with boot.kernelParams in power.nix (list options are concatenated).
+  # Verify after reboot: cat /sys/class/drm/card*/device/mem_info_gtt_total
+  boot.kernelParams = [
+    "ttm.pages_limit=14680064"
+    "ttm.page_pool_size=14680064"
+  ];
 
   # Lanzaboote Secure Boot
   boot.loader.systemd-boot.enable = lib.mkForce false;