Light touch regulation#

Wed, 02 Oct 2019 23:34:42 +0000

Or less obliquely, how to configure the Thinkpad X1 Carbon (gen 4, if it matters) touchpad in NixOS to not respond to the lightest brush of a finger as if it were a click. An end to accidentally favouriting random tweets while browsing Twitter on Firefox. Or at least, I hope, the elimination of a significant source of such.

First off, the instructions at https://people.freedesktop.org/~whot/libinput-rtd/touchpad-pressure.html#touchpad-pressure-hwdb are very nearly all true, and you should read them because I am not going to recapitulate them. I say "almost" because I found that where it says list-quirks I had to say quirks list instead. Following that process led me to create a file /etc/libinput/local-overrides.quirks that reads as follows:

[Section touchpad pressure]
MatchUdevType=touchpad
MatchName=*SynPS/2 Synaptics TouchPad
MatchDMIModalias=dmi:*svnLENOVO:*:pvrThinkPadX1Carbon4th*
AttrPressureRange=45:42

Second, it's more complicated than that. In NixOS 19.03, the libinput binary doesn't look at that file, it looks at /nix/store/9lwm03xqd8pkbxc3hgq9iiginddiyha3-libinput-1.12.6/etc/libinput/local-overrides.quirks, which is owned by the libinput derivation and can't be changed. (This is probably a bug. I will report it in the morning). Because I am trying to stick to channels and not maintain more forks of nixpkgs than I need to, I decided to patch it with an overlay. Therefore

[dan@noetbook:~]$ cat /etc/nixos/overlays/libinput.nix
self : super: {
  libinput = super.libinput.overrideAttrs (o: {
    mesonFlags = o.mesonFlags ++ [
      "--sysconfdir=/etc"
    ];
  });
}

and

[dan@noetbook:~]$ grep -B1 -A9 overlays /etc/nixos/configuration.nix 
{
  nixpkgs.overlays = [ (import /etc/nixos/overlays/libinput.nix) ];
  environment.etc."libinput/local-overrides.quirks" = {
    text = ''
[Section touchpad pressure]
MatchUdevType=touchpad
MatchName=*SynPS/2 Synaptics TouchPad
MatchDMIModalias=dmi:*svnLENOVO:*:pvrThinkPadX1Carbon4th*
AttrPressureRange=45:42
'';
  };

and nixos-rebuild switch, and after some swearing caused mostly by accidentally forgetting to exit all the nix-shells I had accidentally got myself into, about 30 minutes rebuilding later, it worked.

Note that adding overlays in configuration.nix does not make them available to nix-env or nix run, so you probably also want to add libinput to environment.systemPackages if you want to test your quirks are getting picked up. Then you can do this:

[dan@noetbook:~]$ strace -e openat libinput quirks list /dev/input/event7 2>&1 |grep etc 
openat(AT_FDCWD, "/etc/libinput/local-overrides.quirks", O_RDONLY) = 3
[dan@noetbook:~]$ libinput quirks list /dev/input/event7
ModelSynapticsSerialTouchpad=1
AttrPressureRange=45:42
AttrThumbPressureThreshold=100

A great improvement / A+++ would recommend. Wish I'd done it three years ago.