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Input Device Configuration Files

In this document

Input device configuration files (.idc files) contain device-specific configuration properties that affect the behavior of input devices.

Input device configuration files are typically not necessary for standard peripherals such as HID keyboards and mice since the default system behavior usually ensures that they will work out of the box. On the other hand, built-in embedded devices, particularly touch screens, almost always require input device configuration files to specify their behavior.

Rationale

Android automatically detects and configures most input device capabilities based on the event types and properties that are reported by the associated Linux kernel input device driver.

For example, if an input device supports the EV_REL event type and codes REL_X and REL_Y as well as the EV_KEY event type and BTN_MOUSE, then Android will classify the input device as a mouse. The default behavior for a mouse is to present an on-screen cursor which tracks the mouse's movements and simulates touches when the mouse is clicked. Although the mouse can be configured differently, the default behavior is usually sufficient for standard mouse peripherals.

Certain classes of input devices are more ambiguous. For example, multi-touch touch screens and touch pads both support the EV_ABS event type and codes ABS_MT_POSITION_X and ABS_MT_POSITION_Y at a minimum. However, the intended uses of these devices are quite different and cannot always be determined automatically. Also, additional information is required to make sense of the pressure and size information reported by touch devices. Hence touch devices, especially built-in touch screens, usually need IDC files.

Location

Input device configuration files are located by USB vendor, product (and optionally version) id or by input device name.

The following paths are consulted in order.

  • /system/usr/idc/Vendor_XXXX_Product_XXXX_Version_XXXX.idc
  • /system/usr/idc/Vendor_XXXX_Product_XXXX.idc
  • /system/usr/idc/DEVICE_NAME.idc
  • /data/system/devices/idc/Vendor_XXXX_Product_XXXX_Version_XXXX.idc
  • /data/system/devices/idc/Vendor_XXXX_Product_XXXX.idc
  • /data/system/devices/idc/DEVICE_NAME.idc

When constructing a file path that contains the device name, all characters in the device name other than '0'-'9', 'a'-'z', 'A'-'Z', '-' or '_' are replaced by '_'.

Syntax

An input device configuration file is a plain text file consisting of property assignments and comments.

Properties

Property assignments each consist of a property name, an =, a property value, and a new line. Like this:

property = value

Property names are non-empty literal text identifiers. They must not contain whitespace. Each components of the input system defines a set of properties that are used to configure its function.

Property values are non-empty string literals, integers or floating point numbers. They must not contain whitespace or the reserved characters \ or ".

Property names and values are case-sensitive.

Comments

Comment lines begin with '#' and continue to the end of the line. Like this:

# A comment!

Blank lines are ignored.

Example

# This is an example of an input device configuration file.
# It might be used to describe the characteristics of a built-in touch screen.

# This is an internal device, not an external peripheral attached to the USB
# or Bluetooth bus.
device.internal = 1

# The device should behave as a touch screen, which uses the same orientation
# as the built-in display.
touch.deviceType = touchScreen
touch.orientationAware = 1

# Additional calibration properties...
# etc...

Common Properties

The following properties are common to all input device classes.

Refer to the documentation of each input device class for information about the special properties used by each class.

device.internal

Definition: device.internal = 0 | 1

Specifies whether the input device is an internal built-in component as opposed to an externally attached (most likely removable) peripheral.

  • If the value is 0, the device is external.

  • If the value is 1, the device is internal.

  • If the value is not specified, the default value is 0 for all devices on the USB (BUS_USB) or Bluetooth (BUS_BLUETOOTH) bus, 1 otherwise.

This property determines default policy decisions regarding wake events.

Internal input devices generally do not wake the display from sleep unless explicitly configured to do so in the key layout file or in a hardcoded policy rule. This distinction prevents key presses and touches from spuriously waking up your phone when it is in your pocket. Usually there are only a small handful of wake keys defined.

Conversely, external input devices usually wake the device more aggressively because they are assumed to be turned off or not plugged in during transport. For example, pressing any key on an external keyboard is a good indicator that the user wants the device to wake up and respond.

It is important to ensure that the value of the device.internal property is set correctly for all internal input devices.

Validation

Make sure to validate your input device configuration files using the Validate Keymaps tool.