External monitor not recognized (T480s, Intel Integrated)

Help
1

My external monitor is not show by xrandr nor the Display configuration of KDE and gets no signal from my Laptop.
I haven’t found forum or other help with this combination of symptoms and Graphic card (and also have not understood a lot of the forum answers I found with similar issues), so I hope someone can help. Maybe it’s a simple config.

Display connections tried:

  • VGA via Thunderbolt Docking Station
  • HDMI - Displayport - Cable directly at Laptop

The monitor works with almost the same nixos config on a L470 ThinkPad via VGA.

System:

  • ThinkPad T480s
  • NixOS unstable with KDE
  • Monitor: Dell U2412Mb
    Sections of configuration.nix that seem maybe relevant:
{ config, pkgs, ... }:
let
  ...
in
  {
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];

  # Bootloader.
  boot.loader.systemd-boot.enable = true;
  boot.loader.efi.canTouchEfiVariables = true;
  boot.kernel.sysctl = {
    "kernel.sysrq" = 1;  # 1 enables all kernel requests:
  };

  services = {  # non-user facing programs
    # Enable the X11 windowing system.
    # but for some reason it's still running wayland
    xserver.enable = true;

    # Enable the KDE Plasma Desktop Environment.
    displayManager.sddm.enable = true;
    desktopManager.plasma6.enable = true;

  };
  security.rtkit.enable = true;
  # Define a user account. Don't forget to set a password with ‘passwd’.
  users.users = {
    me = {
      isNormalUser = true;
      # "scanner" from https://unix.stackexchange.com/a/518688
      # adbusers kvm just for installing Annikas Pixel Graphene
      extraGroups = [ "networkmanager" "wheel" "scanner" "adbusers" "kvm" ];
     };
   };

  # Allow unfree packages
  nixpkgs.config.allowUnfree = true;

  # List packages installed in system profile. To search, run:
  # $ nix search wget
  environment.systemPackages = with pkgs; [
    usbutils  # for lsusb
    pciutils  # for pciutils
  ];

  system.stateVersion = "23.11";
}

Output of lspci while Monitor is connected to HDMI port with HDMI-Displayport-Cable

00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers (rev 08)
00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07)
00:04.0 Signal processing controller: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor Thermal Subsystem (rev 08)
00:08.0 System peripheral: Intel Corporation Xeon E3-1200 v5/v6 / E3-1500 v5 / 6th/7th/8th Gen Core Processor Gaussian Mixture Model
00:14.0 USB controller: Intel Corporation Sunrise Point-LP USB 3.0 xHCI Controller (rev 21)
00:14.2 Signal processing controller: Intel Corporation Sunrise Point-LP Thermal subsystem (rev 21)
00:15.0 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #0 (rev 21)
00:16.0 Communication controller: Intel Corporation Sunrise Point-LP CSME HECI #1 (rev 21)
00:17.0 SATA controller: Intel Corporation Sunrise Point-LP SATA Controller [AHCI mode] (rev 21)
00:1c.0 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #1 (rev f1)
00:1c.4 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #5 (rev f1)
00:1c.6 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #7 (rev f1)
00:1f.0 ISA bridge: Intel Corporation Sunrise Point LPC/eSPI Controller (rev 21)
00:1f.2 Memory controller: Intel Corporation Sunrise Point-LP PMC (rev 21)
00:1f.3 Audio device: Intel Corporation Sunrise Point-LP HD Audio (rev 21)
00:1f.4 SMBus: Intel Corporation Sunrise Point-LP SMBus (rev 21)
00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (4) I219-LM (rev 21)
04:00.0 PCI bridge: Intel Corporation JHL6240 Thunderbolt 3 Bridge (Low Power) [Alpine Ridge LP 2016] (rev 01)
05:00.0 PCI bridge: Intel Corporation JHL6240 Thunderbolt 3 Bridge (Low Power) [Alpine Ridge LP 2016] (rev 01)
05:01.0 PCI bridge: Intel Corporation JHL6240 Thunderbolt 3 Bridge (Low Power) [Alpine Ridge LP 2016] (rev 01)
05:02.0 PCI bridge: Intel Corporation JHL6240 Thunderbolt 3 Bridge (Low Power) [Alpine Ridge LP 2016] (rev 01)
06:00.0 System peripheral: Intel Corporation JHL6240 Thunderbolt 3 NHI (Low Power) [Alpine Ridge LP 2016] (rev 01)
3c:00.0 USB controller: Intel Corporation JHL6240 Thunderbolt 3 USB 3.1 Controller (Low Power) [Alpine Ridge LP 2016] (rev 01)
3d:00.0 Network controller: Intel Corporation Wireless 8265 / 8275 (rev 78)

Monitor is not recogized by xrandr:

xrandr
Screen 0: minimum 16 x 16, current 1920 x 1080, maximum 32767 x 32767
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
   1920x1080     59.96*+
   1440x1080     59.99
   1400x1050     59.98
.... many more formats

and output of lshw:

$ sudo lshw -c display
  *-display
       product: i915drmfb
       physical id: 2
       bus info: pci@0000:00:02.0
       logical name: /dev/fb0
       version: 07
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm bus_master cap_list rom fb
       configuration: depth=32 driver=i915 latency=0 resolution=1920,1080
       resources: irq:149 memory:db000000-dbffffff memory:80000000-9fffffff ioport:e000(size=64) memory:c0000-dffff

After removing services.xserver.enable = true;, the following changed:

  • xrandr is not available
  • NixOS information:
% nix-info -m
 - system: `"x86_64-linux"`
 - host os: `Linux 6.12.35, NixOS, 25.11 (Xantusia), 25.11pre823481.3016b4b15d13`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.28.4`
 - channels(root): `"nixos"`
 - nixpkgs: `/nix/store/l9fdfiyxfnrsmahvdwqrc5lmqxkkv0l0-nixos/nixos`
  • The display configuration in the KDE settings does not show the rectangle where you can arrange your monitors:
    Screenshot_Display_configuration

Any explanation and ideas are appreciated!

Addition:

  • A monitor connected with HDMI - HDMI - cable was properly recognized and usable (unfortunately it’s not the hardware I have at home).
  • A Fedora 40 Xfce Live Stick System does not recognize the monitor at the HDMI - DisplayPort cable either.
    Addition II:
  • The HDMI - DisplayPort cable was working with my other NixOS Computer as a DisplayPort - HDMI cable (DisplayPort at the DockingStation, HDMI at another TV monitor). According to a comment to a question on askubuntu, this is the issue for the HDMI-DisplayPort setup but it leaves the Thunderbolt-VGA-VGA issue still open:

Are you aware that HDMI/Displayport adaptors/cables are directional (well, all the ones I know about anyway). They only work when plugged in the right way around. So you can buy either a HDMI to DP OR a DP to HDMI adaptor. You need of course HDMI to DP (HDMI source outputting to a DP monitor).