Customizing display configuration

This section describes how to customize display configuration using:

kernel modes
EDID files

Example situations it can help you with:

display controllers (external hardware) not advertising EDID at all,
misbehaving graphics drivers,
loading custom display configuration before the Display Manager is running,

Forcing display modes

In case of very wrong monitor controller and/or video driver combination you can force the display to be enabled and skip some driver-side checks by adding video=<OUTPUT>:e to boot.kernelParams. This is exactly the case with amdgpu drivers

{
  # force enabled output to skip `amdgpu` checks
  hardware.display.outputs."DP-1".mode = "e";
  # completely disable output no matter what is connected to it
  hardware.display.outputs."VGA-2".mode = "d";

  /* equals
  boot.kernelParams = [ "video=DP-1:e" "video=VGA-2:d" ];
  */
}

Crafting custom EDID files

To make custom EDID binaries discoverable you should first create a derivation storing them at $out/lib/firmware/edid/ and secondly add that derivation to hardware.display.edid.packages NixOS option:

{
  hardware.display.edid.packages = [
    (pkgs.runCommand "edid-custom" {} ''
       mkdir -p $out/lib/firmware/edid
       base64 -d > "$out/lib/firmware/edid/custom1.bin" <<'EOF'
       <insert your base64 encoded EDID file here `base64 < /sys/class/drm/card0-.../edid`>
       EOF
       base64 -d > "$out/lib/firmware/edid/custom2.bin" <<'EOF'
       <insert your base64 encoded EDID file here `base64 < /sys/class/drm/card1-.../edid`>
       EOF
    '')
  ];
}

There are 2 options significantly easing preparation of EDID files:

hardware.display.edid.linuxhw
hardware.display.edid.modelines

Assigning EDID files to displays

To assign available custom EDID binaries to your monitor (video output) use hardware.display.outputs."<NAME>".edid option. Under the hood it adds drm.edid_firmware entry to boot.kernelParams NixOS option for each configured output:

{
  hardware.display.outputs."VGA-1".edid = "custom1.bin";
  hardware.display.outputs."VGA-2".edid = "custom2.bin";
  /* equals:
  boot.kernelParams = [ "drm.edid_firmware=VGA-1:edid/custom1.bin,VGA-2:edid/custom2.bin" ];
  */
}

Pulling files from linuxhw/EDID database

hardware.display.edid.linuxhw utilizes pkgs.linuxhw-edid-fetcher to extract EDID files from https://github.com/linuxhw/EDID based on simple string/regexp search identifying exact entries:

{
  hardware.display.edid.linuxhw."PG278Q_2014" = [ "PG278Q" "2014" ];

  /* equals:
  hardware.display.edid.packages = [
    (pkgs.linuxhw-edid-fetcher.override {
      displays = {
        "PG278Q_2014" = [ "PG278Q" "2014" ];
      };
    })
  ];
  */
}

Using XFree86 Modeline definitions

hardware.display.edid.modelines utilizes pkgs.edid-generator package allowing you to conveniently use XFree86 Modeline entries as EDID binaries:

{
  hardware.display.edid.modelines."PG278Q_60" = "    241.50   2560 2608 2640 2720   1440 1443 1448 1481   -hsync +vsync";
  hardware.display.edid.modelines."PG278Q_120" = "   497.75   2560 2608 2640 2720   1440 1443 1448 1525   +hsync -vsync";

  /* equals:
  hardware.display.edid.packages = [
    (pkgs.edid-generator.overrideAttrs {
      clean = true;
      modelines = ''
        Modeline "PG278Q_60"      241.50   2560 2608 2640 2720   1440 1443 1448 1481   -hsync +vsync
        Modeline "PG278Q_120"     497.75   2560 2608 2640 2720   1440 1443 1448 1525   +hsync -vsync
      '';
    })
  ];
  */
}

Complete example for Asus PG278Q

And finally this is a complete working example for a 2014 (first) batch of Asus PG278Q monitor with amdgpu drivers:

{
  hardware.display.edid.modelines."PG278Q_60" = "   241.50   2560 2608 2640 2720   1440 1443 1448 1481   -hsync +vsync";
  hardware.display.edid.modelines."PG278Q_120" = "  497.75   2560 2608 2640 2720   1440 1443 1448 1525   +hsync -vsync";

  hardware.display.outputs."DP-1".edid = "PG278Q_60.bin";
  hardware.display.outputs."DP-1".mode = "e";
}

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