diff --git a/deploy/nixos/hardware/raspberry-pi-4.nix b/deploy/nixos/hardware/raspberry-pi-4.nix
index a30d22c..8fc469b 100644
--- a/deploy/nixos/hardware/raspberry-pi-4.nix
+++ b/deploy/nixos/hardware/raspberry-pi-4.nix
@@ -59,11 +59,27 @@
   # the kernel falls back to the device tree's stdout-path, i.e. this UART.
   boot.kernelParams = [ "console=tty0" ];
 
-  # Kiosk display: vc4/v3d kernel modesetting via the firmware KMS overlay.
-  # This is the Pi 4's equivalent of the Pi 5's default KMS path — it also
-  # sets services.xserver.videoDrivers to modesetting (fbdev fallback), so we
-  # don't set that here. Electron renders through it as on the UP Board.
-  hardware.raspberry-pi."4".fkms-3d.enable = true;
+  # Kiosk display: mainline full KMS, NOT nixos-hardware's fkms-3d.
+  #
+  # fkms-3d applies the rpi4-cma-overlay and rpi4-vc4-fkms-v3d-overlay device
+  # tree overlays. Those target nodes that exist in the Raspberry Pi VENDOR
+  # kernel's DTBs and not in mainline's, so with the mainline kernel above the
+  # overlay step fails outright:
+  #
+  #   Applying overlay rpi4-vc4-fkms-v3d-overlay
+  #   libfdt.FdtException: pylibfdt error -1: FDT_ERR_NOTFOUND
+  #
+  # It is also unnecessary. "fkms" is FIRMWARE KMS, the older route where the
+  # VideoCore firmware owns the display and Linux drives it at arm's length.
+  # Mainline does full KMS instead, and mainline's own bcm2711-rpi-4-b.dtb
+  # already describes the hardware — it carries brcm,bcm2711-vc5 and
+  # brcm,2711-v3d nodes, checked with dtc. The vc4 and v3d drivers bind to
+  # those directly with no overlay involved.
+  #
+  # fkms-3d used to set services.xserver.videoDrivers as a side effect. Nothing
+  # needs to replace it: the shared configuration.nix already declares
+  # modesetting, which is the correct driver for full KMS and what the x86
+  # machines use. Setting it again here only produced a duplicate entry.
 
   hardware.enableRedistributableFirmware = true;