Enforcing a touchscreen mapping in GNOME

Touchscreens are quite prevalent by now but one of the not-so-hidden secrets is that they're actually two devices: the monitor and the actual touch input device. Surprisingly, users want the touch input device to work on the underlying monitor which means your desktop environment needs to somehow figure out which of the monitors belongs to which touch input device. Often these two devices come from two different vendors, so mutter needs to use ... */me holds torch under face* .... HEURISTICS! :scary face:

Those heuristics are actually quite simple: same vendor/product ID? same dimensions? is one of the monitors a built-in one? [1] But unfortunately in some cases those heuristics don't produce the correct result. In particular external touchscreens seem to be getting more common again and plugging those into a (non-touch) laptop means you usually get that external screen mapped to the internal display.

Luckily mutter does have a configuration to it though it is not exposed in the GNOME Settings (yet). But you, my $age $jedirank, can access this via a commandline interface to at least work around the immediate issue. But first: we need to know the monitor details and you need to know about gsettings relocatable schemas.

Finding the right monitor information is relatively trivial: look at $HOME/.config/monitors.xml and get your monitor's vendor, product and serial from there. e.g. in my case this is:

  <monitors version="2">
   <configuration>
    <logicalmonitor>
      <x>0</x>
      <y>0</y>
      <scale>1</scale>
      <monitor>
        <monitorspec>
          <connector>DP-2</connector>
          <vendor>DEL</vendor>              <--- this one
          <product>DELL S2722QC</product>   <--- this one
          <serial>59PKLD3</serial>          <--- and this one
        </monitorspec>
        <mode>
          <width>3840</width>
          <height>2160</height>
          <rate>59.997</rate>
        </mode>
      </monitor>
    </logicalmonitor>
    <logicalmonitor>
      <x>928</x>
      <y>2160</y>
      <scale>1</scale>
      <primary>yes</primary>
      <monitor>
        <monitorspec>
          <connector>eDP-1</connector>
          <vendor>IVO</vendor>
          <product>0x057d</product>
          <serial>0x00000000</serial>
        </monitorspec>
        <mode>
          <width>1920</width>
          <height>1080</height>
          <rate>60.010</rate>
        </mode>
      </monitor>
    </logicalmonitor>
  </configuration>
</monitors>
  

Well, so we know the monitor details we want. Note there are two monitors listed here, in this case I want to map the touchscreen to the external Dell monitor. Let's move on to gsettings.

gsettings is of course the configuration storage wrapper GNOME uses (and the CLI tool with the same name). GSettings follow a specific schema, i.e. a description of a schema name and possible keys and values for each key. You can list all those, set them, look up the available values, etc.:

    $ gsettings list-recursively
    ... lots of output ...
    $ gsettings set org.gnome.desktop.peripherals.touchpad click-method 'areas'
    $ gsettings range org.gnome.desktop.peripherals.touchpad click-method
    enum
    'default'
    'none'
    'areas'
    'fingers'
  

Now, schemas work fine as-is as long as there is only one instance. Where the same schema is used for different devices (like touchscreens) we use a so-called "relocatable schema" and that requires also specifying a path - and this is where it gets tricky. I'm not aware of any functionality to get the specific path for a relocatable schema so often it's down to reading the source. In the case of touchscreens, the path includes the USB vendor and product ID (in lowercase), e.g. in my case the path is:

  /org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/

In your case you can get the touchscreen details from lsusb, libinput record, /proc/bus/input/devices, etc. Once you have it, gsettings takes a schema:path argument like this:

  $ gsettings list-recursively org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/
  org.gnome.desktop.peripherals.touchscreen output ['', '', '']

Looks like the touchscreen is bound to no monitor. Let's bind it with the data from above:

 
   $ gsettings set org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/ output "['DEL', 'DELL S2722QC', '59PKLD3']"

Note the quotes so your shell doesn't misinterpret things.

And that's it. Now I have my internal touchscreen mapped to my external monitor which makes no sense at all but shows that you can map a touchscreen to any screen if you want to.

[1] Probably the one that most commonly takes effect since it's the vast vast majority of devices