The evdev backend

This backend allows using Linux evdev devices such as mouses, keyboards, gamepads and joysticks as input and output devices. All buttons and axes available to the Linux system are mappable. Output is provided by the uinput kernel module, which allows creation of virtual input devices. This functionality may require elevated privileges (such as special group membership or root access).

Global configuration

Option Example value Default value Description
detect on off Output channel specifications for any events coming in on configured instances to help with configuration.

Instance configuration

Option Example value Default value Description
device /dev/input/event1 none evdev device to use as input device
input Xbox Wireless none Presentation name of evdev device to use as input (prefix-matched)
output My Input Device none Output device presentation name. Setting this option enables the instance for output
exclusive 1 0 Prevent other processes from using the device
id 0x1 0x2 0x3 none Set output device bus identification (Vendor, Product and Version), optional
axis.AXISNAME 34300 0 65536 255 4095 none Specify absolute axis details (see below) for output. This is required for any absolute axis to be output.
relaxis.AXISNAME 65534 32767 none Specify relative axis details (extent and optional initial value) for output and input (see below).

The absolute axis details configuration (e.g. axis.ABS_X) is required for any absolute axis on output-enabled instances. The configuration value contains, space-separated, the following values:

  • value: The value to assume for the axis until an event is received
  • minimum: The axis minimum value
  • maximum: The axis maximum value
  • fuzz: A value used for filtering the input stream
  • flat: An offset, below which all deviations will be ignored
  • resolution: Axis resolution in units per millimeter (or units per radian for rotational axes)

If an axis is not used for output, this configuration can be omitted.

For real devices, all of these parameters for every axis can be found by running evtest on the device.

To use the input from relative axes in absolute-value based protocols, the backend needs a reference frame to convert the relative movements to absolute values. To invert the mapping of the relative axis, specify the max value as a negative number, for example:

relaxis.REL_X = -1024 512

If relative axes are used without specifying their extents, the channel will generate normalized values of 0, 0.5 and 1 for any input less than, equal to and greater than 0, respectively. As for output, only the values -1, 0 and 1 are generated for the same interval.

Channel specification

A channel is specified by its event type and event code, separated by .. For a complete list of event types and codes see the kernel documentation. The most interesting event types are

  • EV_KEY for keys and buttons
  • EV_ABS for absolute axes (such as Joysticks)
  • EV_REL for relative axes (such as Mouses)

The evtest tool is useful to gather information on devices active on the local system, including names, types, codes and configuration supported by these devices.

Example mapping:

ev1.EV_KEY.KEY_A > ev1.EV_ABS.ABS_X

Note that to map an absolute axis on an output-enabled instance, additional information such as the axis minimum and maximum are required. These must be specified in the instance configuration. When only mapping the instance as a channel input, this is not required.

Known bugs / problems

Creating an evdev output device requires elevated privileges, namely, write access to the system’s /dev/uinput. Usually, this is granted for users in the input group and the root user.

Input devices may synchronize logically connected event types (for example, X and Y axes) via EV_SYN-type events. The MIDIMonster also generates these events after processing channel events, but may not keep the original event grouping.

EV_KEY key-down events are sent for normalized channel values over 0.9.

Extended event type values such as EV_LED, EV_SND, etc are recognized in the MIDIMonster configuration file but may or may not work with the internal channel mapping and normalization code.