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

Please see the udev manpage (man udev) for general documentation about 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 like simple mice, keyboards, lid switches, etc. are supported by the general "evdev" X.org input driver, so /lib/udev/rules.d/65-xorg-evdev.rules sets ENV{x11_driver}="evdev" by default.

Later rules (i. e. with a prefix higher than 65) can change the property to use a different driver. 66-xorg-synaptics.rules is a prominent example for using the "synaptics" driver for touchpads and setting ENV{x11_driver}="synaptics" for devices which have ID_INPUT_TOUCHPAD.

Keyboard layout

In Ubuntu, the system default keyboard layout is configured in /etc/default/console-setup, in particular the XKB* fields. 64-xorg-xkb.rules greps those variables out of there and attaches them to all devices which have ID_INPUT_KEY, so that they end up as udev properties like shown above (XKBLAYOUT=de).

Machine specific quirks

66-xorg-synaptics.rules also shows how to define machine specific options and quirks:

•  ATTR{[dmi/id]product_name}=="Inspiron 1011|Inspiron 1012", \
     ENV{x11_options.JumpyCursorThreshold}="90", \
     ENV{x11_options.AreaBottomEdge}="4100"

[dmi/id]product_name means that the value is read from /sys/class/dmi/id/product_name, which has all the DMI data.

Note that this rule only gets executed for devices which have ID_INPUT_TOUCHPAD, which is tested earlier on in the file. If you want to define your own rule file (e. g. /etc/udev/rules.d/90-xorg-local.rules), you should explicitly add this test, and of course you don't need to check the machine name:

•  ENV{ID_INPUT_TOUCHPAD}=="1", ENV{x11_options.AreaBottomEdge}="5000"