Key Layout Files

Key layout files (.kl files) are responsible for mapping Linux key codes and axis codes to Android key codes and axis codes and specifying associated policy flags.

Device-specific key layout files are required for all internal (built-in) input devices that have keys, including special keys such as volume, power and headset media keys.

Device-specific key layout files are optional for other input devices but they are recommended for special-purpose keyboards and joysticks.

If no device-specific key layout file is available, then the system will choose a default instead.

Location

Key layout 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/keylayout/Vendor_XXXX_Product_XXXX_Version_XXXX.kl
  • /system/usr/keylayout/Vendor_XXXX_Product_XXXX.kl
  • /system/usr/keylayout/DEVICE_NAME.kl
  • /data/system/devices/keylayout/Vendor_XXXX_Product_XXXX_Version_XXXX.kl
  • /data/system/devices/keylayout/Vendor_XXXX_Product_XXXX.kl
  • /data/system/devices/keylayout/DEVICE_NAME.kl
  • /system/usr/keylayout/Generic.kl
  • /data/system/devices/keylayout/Generic.kl

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 ''.

Generic Key Layout File

The system provides a special built-in generic key layout file called Generic.kl. This key layout is intended to support a variety of standard external keyboards and joysticks.

Do not modify the generic key layout!

Syntax

A key layout file is a plain text file consisting of key or axis declarations and flags.

Key Declarations

Key declarations each consist of the keyword key followed by a Linux key code number and an Android key code name, or the keyword `usage` followed by a HID usage and an Android key code name. The HID usage is represented as a 32-bit integer, where the high 16-bits represent the HID usage page and the low 16-bits represent the HID usage ID. Either of these declarations can then be followed by an optional set of whitespace delimited policy flags.


key 1     ESCAPE
key 114   VOLUME_DOWN       WAKE
key 16    Q                 VIRTUAL     WAKE
key usage 0x0c006F          BRIGHTNESS_UP

The following policy flags are recognized:

  • WAKE: The key should wake the device when it is asleep. For historical reasons, this flag behaves in the same manner as WAKE_DROPPED below.
  • WAKE_DROPPED: The key should wake the device when it is asleep but the key itself should be dropped when the wake-up occurs. In a sense, the key's action was to wake the device, but the key itself is not processed.
  • SHIFT: The key should be interpreted as if the SHIFT key were also pressed.
  • CAPS_LOCK: The key should be interpreted as if the CAPS LOCK key were also pressed.
  • ALT: The key should be interpreted as if the ALT key were also pressed.
  • ALT_GR: The key should be interpreted as if the RIGHT ALT key were also pressed.
  • FUNCTION: The key should be interpreted as if the FUNCTION key were also pressed.
  • VIRTUAL: The key is a virtual soft key (capacitive button) that is adjacent to the main touch screen. This causes special debouncing logic to be enabled, see below.
  • MENU: Deprecated. Do not use.
  • LAUNCHER: Deprecated. Do not use.

Axis Declarations

Axis declarations each consist of the keyword axis followed by a Linux axis code number, and qualifiers that control the behavior of the axis including at least one Android axis code name.

Basic Axes

A basic axis simply maps a Linux axis code to an Android axis code name.

The following declaration maps ABS_X (indicated by 0x00) to AXIS_X (indicated by X).

axis 0x00 X

In the above example, if the value of ABS_X is 5 then AXIS_X will be set to 5.

Split Axes

A split axis maps a Linux axis code to two Android axis code names, such that values less than or greater than a threshold are split across two different axes when mapped. This mapping is useful when a single physical axis reported by the device encodes two different mutually exclusive logical axes.

The following declaration maps values of the ABS_Y axis (indicated by 0x01) to AXIS_GAS when less than 0x7f or to AXIS_BRAKE when greater than 0x7f.

axis 0x01 split 0x7f GAS BRAKE

In the above example, if the value of ABS_Y is 0x7d then AXIS_GAS is set to 2 (0x7f - 0x7d) and AXIS_BRAKE is set to 0. Conversely, if the value of ABS_Y is 0x83 then AXIS_GAS is set to 0 and AXIS_BRAKE is set to 4 (0x83 - 0x7f). Finally, if the value of ABS_Y equals the split value of 0x7f then both AXIS_GAS and AXIS_BRAKE are set to 0.

Inverted Axes

An inverted axis inverts the sign of the axis value.

The following declaration maps ABS_RZ (indicated by 0x05) to AXIS_BRAKE (indicated by BRAKE), and inverts the output by negating it.

axis 0x05 invert BRAKE

In the above example, if the value of ABS_RZ is 2 then AXIS_BRAKE is set to -2.

Center Flat Position Option

The Linux input protocol provides a way for input device drivers to specify the center flat position of joystick axes but not all of them do and some of them provide incorrect values.

The center flat position is the neutral position of the axis, such as when a directional pad is in the very middle of its range and the user is not touching it.

To resolve this issue, an axis declaration may be followed by a flat option that specifies the value of the center flat position for the axis.

axis 0x03 Z flat 4096

In the above example, the center flat position is set to 4096.

Comments

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

# A comment!

Blank lines are ignored.

Examples

Keyboard

# This is an example of a key layout file for a keyboard.

key 1     ESCAPE
key 2     1
key 3     2
key 4     3
key 5     4
key 6     5
key 7     6
key 8     7
key 9     8
key 10    9
key 11    0
key 12    MINUS
key 13    EQUALS
key 14    DEL

# etc...

System Controls

# This is an example of a key layout file for basic system controls, such as
# volume and power keys which are typically implemented as GPIO pins that
# the device decodes into key presses.

key 114   VOLUME_DOWN       WAKE
key 115   VOLUME_UP         WAKE
key 116   POWER             WAKE

Capacitive Buttons

# This is an example of a key layout file for a touch device with capacitive buttons.

key 139    MENU           VIRTUAL
key 102    HOME           VIRTUAL
key 158    BACK           VIRTUAL
key 217    SEARCH         VIRTUAL

Headset Jack Media Controls

# This is an example of a key layout file for headset mounted media controls.
# A typical headset jack interface might have special control wires or detect known
# resistive loads as corresponding to media functions or volume controls.
# This file assumes that the driver decodes these signals and reports media
# controls as key presses.

key 163   MEDIA_NEXT        WAKE
key 165   MEDIA_PREVIOUS    WAKE
key 226   HEADSETHOOK       WAKE

Joystick

# This is an example of a key layout file for a joystick.

# These are the buttons that the joystick supports, represented as keys.
key 304   BUTTON_A
key 305   BUTTON_B
key 307   BUTTON_X
key 308   BUTTON_Y
key 310   BUTTON_L1
key 311   BUTTON_R1
key 314   BUTTON_SELECT
key 315   BUTTON_START
key 316   BUTTON_MODE
key 317   BUTTON_THUMBL
key 318   BUTTON_THUMBR

# Left and right stick.
# The reported value for flat is 128 out of a range from -32767 to 32768, which is absurd.
# This confuses applications that rely on the flat value because the joystick actually
# settles in a flat range of +/- 4096 or so.  We override it here.
axis 0x00 X flat 4096
axis 0x01 Y flat 4096
axis 0x03 Z flat 4096
axis 0x04 RZ flat 4096

# Triggers.
axis 0x02 LTRIGGER
axis 0x05 RTRIGGER

# Hat.
axis 0x10 HAT_X
axis 0x11 HAT_Y

Wake Keys

Wake keys are special keys that wake the device from sleep, such as the power key.

By default, for internal keyboard devices, no key is a wake key. For external keyboard device, all keys are wake keys.

To make a key be a wake key, set the WAKE_DROPPED flag in the key layout file for the keyboard device.

Note that the WindowManagerPolicy component is responsible for implementing wake key behavior. Moreover, the key guard may prevent certain keys from functioning as wake keys. A good place to start understanding wake key behavior is PhoneWindowManager.interceptKeyBeforeQueueing.

Virtual Soft Keys

The input system provides special features for implementing virtual soft keys.

There are three cases:

  1. If the virtual soft keys are displayed graphically on the screen, as on the Galaxy Nexus, then they are implemented by the Navigation Bar component in the System UI package.

    Because graphical virtual soft keys are implemented at a high layer in the system, key layout files are not involved and the following information does not apply.

  2. If the virtual soft keys are implemented as an extended touchable region that is part of the main touch screen, as on the Nexus One, then the input system uses a virtual key map file to translate X / Y touch coordinates into Linux key codes, then uses the key layout file to translate Linux key codes into Android key codes.

    Refer to the section on Touch Devices for more details about virtual key map files.

    The key layout file for the touch screen input device must specify the appropriate key mapping and include the VIRTUAL flag for each key.

  3. If the virtual soft keys are implemented as capacitive buttons that are separate from the main touch screen, as on the Nexus S, then the kernel device driver or firmware is responsible for translating touches into Linux key codes which the input system then translates into Android key codes using the key layout file.

    The key layout file for the capacitive button input device must specify the appropriate key mapping and include the VIRTUAL flag for each key.

When virtual soft key are located within or in close physical proximity of the touch screen, it is easy for the user to accidentally press one of the buttons when touching near the bottom of the screen or when sliding a finger from top to bottom or from bottom to top on the screen.

To prevent this from happening, the input system applies a little debouncing such that virtual soft key presses are ignored for a brief period of time after the most recent touch on the touch screen. The delay is called the virtual key quiet time.

To enable virtual soft key debouncing, we must do two things.

First, we provide a key layout file for the touch screen or capacitive button input device with the VIRTUAL flag set for each key.

key 139    MENU           VIRTUAL
key 102    HOME           VIRTUAL
key 158    BACK           VIRTUAL
key 217    SEARCH         VIRTUAL

Then, we set the value of the virtual key quiet time in a resource overlay for the framework config.xml resource.

<!-- Specifies the amount of time to disable virtual keys after the screen is touched
     in order to filter out accidental virtual key presses due to swiping gestures
     or taps near the edge of the display.  May be 0 to disable the feature.
     It is recommended that this value be no more than 250 ms.
     This feature should be disabled for most devices. -->
<integer name="config_virtualKeyQuietTimeMillis">250</integer>

Validation

Make sure to validate your key layout files using the Validate Keymaps tool.