Overview

The Android input subsystem nominally consists of an event pipeline that traverses multiple layers of the system.

Input Pipeline

At the lowest layer, the physical input device produces signals that describe state changes such as key presses and touch contact points. The device firmware encodes and transmits these signals in some way such as by sending USB HID reports to the system or by producing interrupts on an I2C bus.

The signals are then decoded by a device driver in the Linux kernel. The Linux kernel provides drivers for many standard peripherals, particularly those that adhere to the HID protocol. However, an OEM must often provide custom drivers for embedded devices that are tightly integrated into the system at a low-level, such as touch screens.

The input device drivers are responsible for translating device-specific signals into a standard input event format, by way of the Linux input protocol. The Linux input protocol defines a standard set of event types and codes in the linux/input.h kernel header file. In this way, components outside the kernel do not need to care about the details such as physical scan codes, HID usages, I2C messages, GPIO pins, and the like.

Next, the Android EventHub component reads input events from the kernel by opening the evdev driver associated with each input device. The Android InputReader component then decodes the input events according to the device class and produces a stream of Android input events. As part of this process, the Linux input protocol event codes are translated into Android event codes according to the input device configuration, keyboard layout files, and various mapping tables.

Finally, the InputReader sends input events to the InputDispatcher which forwards them to the appropriate window.

Control Points

There are several stages in the input pipeline which effect control over the behavior of the input device.

Driver and Firmware Configuration

Input device drivers frequently configure the behavior of the input device by setting parameters in registers or even uploading the firmware itself. This is particularly the case for embedded devices such as touch screens where a large part of the calibration process involves tuning these parameters or fixing the firmware to provide the desired accuracy and responsiveness and to suppress noise.

Driver configuration options are often specified as module parameters in the kernel board support package (BSP) so that the same driver can support multiple different hardware implementations.

This documentation does attempt to describe driver or firmware configuration, but it does offer guidance as to device calibration in general.

Board Configuration Properties

The kernel board support package (BSP) may export board configuration properties via SysFS that are used by the Android InputReader component, such as the placement of virtual keys on a touch screen.

Refer to the device class sections for details about how different devices use board configuration properties.

Resource Overlays

A few input behaviors are configured by way of resource overlays in config.xml such as the operation of lid switch.

Here are a few examples:

  • config_lidKeyboardAccessibility: Specifies the effect of the lid switch on whether the hardware keyboard is accessible or hidden.

  • config_lidNavigationAccessibility: Specifies the effect of the lid switch on whether the trackpad is accessible or hidden.

  • config_longPressOnPowerBehavior: Specifies what should happen when the user holds down the power button.

  • config_lidOpenRotation: Specifies the effect of the lid switch on screen orientation.

Refer to the documentation within frameworks/base/core/res/res/values/config.xml for details about each configuration option.

Key Maps

Key maps are used by the Android EventHub and InputReader components to configure the mapping from Linux event codes to Android event codes for keys, joystick buttons and joystick axes. The mapping may be device or language dependent.

Refer to the device class sections for details about how different devices use key maps.

Input Device Configuration Files

Input device configuration files are used by the Android EventHub and InputReader components to configure special device characteristics such as how touch size information is reported.

Refer to the device class sections for details about how different devices use input device configuration maps.

Understanding HID Usages and Event Codes

There are often several different identifiers used to refer to any given key on a keyboard, button on a game controller, joystick axis or other control. The relationships between these identifiers are not always the same: they are dependent on a set of mapping tables, some of which are fixed, and some which vary based on characteristics of the device, the device driver, the current locale, the system configuration, user preferences and other factors.

Physical Scan Code

A physical scan code is a device-specific identifier that is associated with each key, button or other control. Because physical scan codes often vary from one device to another, the firmware or device driver is responsible for mapping them to standard identifiers such as HID Usages or Linux key codes.

Scan codes are mainly of interest for keyboards. Other devices typically communicate at a low-level using GPIO pins, I2C messages or other means. Consequently, the upper layers of the software stack rely on the device drivers to make sense of what is going on.

HID Usage

A HID usage is a standard identifier that is used to report the state of a control such as a keyboard key, joystick axis, mouse button, or touch contact point. Most USB and Bluetooth input devices conform to the HID specification, which enables the system to interface with them in a uniform manner.

The Android Framework relies on the Linux kernel HID drivers to translate HID usage codes into Linux key codes and other identifiers. Therefore HID usages are mainly of interest to peripheral manufacturers.

Linux Key Code

A Linux key code is a standard identifier for a key or button. Linux key codes are defined in the linux/input.h header file using constants that begin with the prefix KEY_ or BTN_. The Linux kernel input drivers are responsible for translating physical scan codes, HID usages and other device-specific signals into Linux key codes and delivering information about them as part of EV_KEY events.

The Android API sometimes refers to the Linux key code associated with a key as its "scan code". This is technically incorrect in but it helps to distinguish Linux key codes from Android key codes in the API.

Linux Relative or Absolute Axis Code

A Linux relative or absolute axis code is a standard identifier for reporting relative movements or absolute positions along an axis, such as the relative movements of a mouse along its X axis or the absolute position of a joystick along its X axis. Linux axis code are defined in the linux/input.h header file using constants that begin with the prefix REL_ or ABS_. The Linux kernel input drivers are responsible for translating HID usages and other device-specific signals into Linux axis codes and delivering information about them as part of EV_REL and EV_ABS events.

Linux Switch Code

A Linux switch code is a standard identifier for reporting the state of a switch on a device, such as a lid switch. Linux switch codes are defined in the linux/input.h header file using constants that begin with the prefix SW_. The Linux kernel input drivers report switch state changes as EV_SW events.

Android applications generally do not receive events from switches, but the system may use them interally to control various device-specific functions.

Android Key Code

An Android key code is a standard identifier defined in the Android API for indicating a particular key such as 'HOME'. Android key codes are defined by the android.view.KeyEvent class as constants that begin with the prefix KEYCODE_.

The key layout specifies how Linux key codes are mapped to Android key codes. Different key layouts may be used depending on the keyboard model, language, country, layout, or special functions.

Combinations of Android key codes are transformed into character codes using a device and locale specific key character map. For example, when the keys identified as KEYCODE_SHIFT and KEYCODE_A are both pressed together, the system looks up the combination in the key character map and finds the capital letter 'A', which is then inserted into the currently focused text widget.

Android Axis Code

An Android axis code is a standard identifier defined in the Android API for indicating a particular device axis. Android axis codes are defined by the android.view.MotionEvent class as constants that begin with the prefix AXIS_.

The key layout specifies how Linux Axis Codes are mapped to Android axis codes. Different key layouts may be used depending on the device model, language, country, layout, or special functions.

Android Meta State

An Android meta state is a standard identifier defined in the Android API for indicating which modifier keys are pressed. Android meta states are defined by the android.view.KeyEvent class as constants that begin with the prefix META_.

The current meta state is determined by the Android InputReader component which monitors when modifier keys such as KEYCODE_SHIFT_LEFT are pressed / released and sets / resets the appropriate meta state flag.

The relationship between modifier keys and meta states is hardcoded but the key layout can alter how the modifier keys themselves are mapped which in turns affects the meta states.

Android Button State

An Android button state is a standard identifier defined in the Android API for indicating which buttons (on a mouse or stylus) are pressed. Android button states are defined by the android.view.MotionEvent class as constants that begin with the prefix BUTTON_.

The current button state is determined by the Android InputReader component which monitors when buttons (on a mouse or stylus) are pressed / released and sets / resets appropriate button state flag.

The relationship between buttons and button states is hardcoded.

Further Reading

  1. Linux input event codes
  2. Linux multi-touch protocol
  3. Linux input drivers
  4. Linux force feedback
  5. HID information, including HID usage tables