Structure

Unless xorg.conf has manual configuration, X.org uses udev for detecting input devices, and udev properties for configuring them. This is what happens in detail:

1. A hardware input device is present at boot, or gets hotplugged.

2. The kernel detects this, and creates a new "input" device (e. g. /sys/class/input/input3 and an "event" interface for it (/sys/class/input/event3 and a device node /dev/input/event3 which you can use to talk to the input device, see /usr/include/linux/input.h).

3. udev picks up the "add" event and the new device. /lib/udev/rules.d/60-persistent-input.rules calls /lib/udev/input_id on it, which detects the class of the device (mouse, touchpad, etc.) and attaches some properties to it:

    $ /lib/udev/input_id /class/input/event4
    ID_INPUT=1
    ID_INPUT_KEY=1
    ID_INPUT_KEYBOARD=1
   
    $ /lib/udev/input_id /class/input/event14
    ID_INPUT=1
    ID_INPUT_MOUSE=1
    ID_INPUT_TOUCHPAD=1

4. udev applies additional rules shipped by the X.org packages (/lib/udev/rules.d/*xorg*), which assign an X11 driver and device specific properties like keyboard layout or touchpad quirks:

    P: /devices/platform/i8042/serio0/input/input4/event4
    E: DEVNAME=/dev/input/event4
    E: ID_INPUT=1
    E: ID_INPUT_KEY=1
    E: ID_INPUT_KEYBOARD=1
    E: XKBMODEL=pc105
    E: XKBLAYOUT=de
    E: XKBVARIANT=nodeadkeys
    E: x11_driver=evdev
   
    P: /devices/platform/i8042/serio1/input/input14/event14
    N: input/event14
    E: DEVNAME=/dev/input/event14
    E: ID_INPUT=1
    E: ID_INPUT_MOUSE=1
    E: ID_INPUT_TOUCHPAD=1
    E: x11_driver=synaptics
    E: x11_options.JumpyCursorThreshold=20

5. X.org picks up the add event or detects existing devices at startup, and adds it to its configuration:

    (II) config/udev: Adding input device "AT Translated Set 2 keyboard" (/dev/input/event4)
    (**) "AT Translated Set 2 keyboard": always reports core events
    (**) "AT Translated Set 2 keyboard": Device: "/dev/input/event4"
    (II) "AT Translated Set 2 keyboard": Found keys
    (II) "AT Translated Set 2 keyboard": Configuring as keyboard
    (II) XINPUT: Adding extended input device ""AT Translated Set 2 keyoard"" (type: KEYBOARD)
    (**) Option "xkb_rules" "evdev"
    (**) Option "xkb_model" "pc105"
    (**) Option "xkb_layout" "de"
    (**) Option "xkb_variant" "nodeadkeys"

Udev rules

Device classification

As shown above, udev itself already classifies the input devices with input_id. Existing flags are:

• ID_INPUT	All input devices have this flag.
  ID_INPUT_MOUSE	Touchscreens and tables have this flag as well, since by the type of events they can produce they act as a mouse.
  ID_INPUT_TABLET
  ID_INPUT_TOUCHSCREEN
  ID_INPUT_JOYSTICK
  ID_INPUT_KEY	Keyboards have this, but also things like lid switches or joystick which have just a few buttons
  ID_INPUT_KEYBOARD

Driver assignment

Most input devices