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+The evdev protocol uses a map of types and codes to express input device values
+to userspace. This document describes the types and codes and how and when they
+may be used.
+
+Types:
+==========
+Types are groupings of codes under a logical input construct. Each type has a
+set of applicable codes to be used in generating events. See the Codes section
+for details on valid codes for each type.
+
+* EV_SYN:
+ - Used as markers to separate events. Events may be separated in time or in
+ space, such as with the multitouch protocol.
+* EV_KEY:
+ - Used to describe keyboard and other key-like input events.
+* EV_REL:
+ - Used to describe relative input events, e.g. moving the mouse 5 units to the
+ left.
+* EV_ABS:
+ - Used to describe absolute input events, e.g. describing the coordinates of a
+ touch on a touchscreen.
+* EV_MSC:
+ - Used to describe miscellaneous input events that do not fit into other
+ types.
+* EV_SW:
+ - Used to describe binary state input switches.
+* EV_LED:
+ - Used to turn LEDs on devices on and off.
+* EV_SND:
+ - Used to output sound to devices.
+* EV_REP:
+ - Used for autorepeating devices.
+* EV_FF:
+ - Used to send force feedback commands to an input device.
+* EV_PWR:
+ - A special type for power button and switch input.
+* EV_FF_STATUS:
+ - Used to receive force feedback device status.
+
+Codes:
+==========
+Codes define the precise type of event.
+
+EV_SYN Codes:
+----------
+EV_SYN event values are undefined. Their usage is
+defined only by when they are sent in the evdev event stream.
+
+* SYN_REPORT:
+ - Used to synchronize and separate events in time. For example, motion of a
+ mouse may set the REL_X and REL_Y values for one motion, then emit a
+ SYN_REPORT. The next motion will emit more REL_X and REL_Y values and send
+ another SYN_REPORT.
+* SYN_CONFIG:
+ - TBD
+* SYN_MT_REPORT:
+ - Used to synchronize and separate touch events. See the
+ multi-touch-protocol.txt document for more information.
+
+EV_KEY:
+----------
+EV_KEY events take the form KEY_<name> or BTN_<name>. For example, KEY_A is used
+to represent the 'A' key on a keyboard. When a key is depressed, an event with
+the key's code is emitted with value 1. When the key is depressed, an event is
+emitted with value 0. In general, KEY_<name> is used for keyboard keys, and
+BTN_<name> is used for other types of momentary switch events.
+
+A few EV_KEY codes have special meanings:
+
+* BTN_TOOL_<name>, BTN_TOUCH:
+ - These codes are used in conjunction with input trackpads, tablets, and
+ touchscreens. These devices may be used with fingers, pens, or other tools.
+ When an event occurs and a tool is used, the corresponding BTN_TOOL_<name>
+ code should be set to a value of 1. When the tool is no longer interacting
+ with the input device, the BTN_TOOL_<name> code should be reset to 0. All
+ trackpads, tablets, and touchscreens should use at least one BTN_TOOL_<name>
+ code when events are generated. For non-tablet devices, the tool is usually
+ BTN_TOUCH.
+
+* BTN_TOOL_FINGER, BTN_TOOL_DOUBLETAP, BTN_TOOL_TRIPLETAP, BTN_TOOL_QUADTAP:
+ - These codes denote one, two, three, and four finger interaction on a
+ trackpad or touchscreen. For example, if the user uses two fingers and moves
+ them on the touchpad in an effort to scroll content on screen,
+ BTN_TOOL_DOUBLETAP should be set to value 1 for the duration of the motion.
+ Note that these codes and the BTN_TOOL_<name> and BTN_TOUCH codes are
+ orthogonal in purpose. A trackpad event generated by finger touches should
+ generate events for one code from each group.
+
+* KEY_SUSPEND, KEY_POWER:
+ - These codes are reserved for the EV_PWR type.
+
+EV_REL:
+----------
+EV_REL events describe relative changes in a property. For example, a mouse may
+move to the left by a certain number of units, but its absolute position in
+space is unknown. If the absolute position is known, EV_ABS codes should be used
+instead of EV_REL codes.
+
+A few EV_REL codes have special meanings:
+
+* REL_WHEEL, REL_HWHEEL:
+ - These codes are used for vertical and horizontal scroll wheels,
+ respectively.
+
+EV_ABS:
+----------
+EV_ABS events describe absolute changes in a property. For example, a touchpad
+may emit coordinates for a touch location.
+
+A few EV_ABS codes have special meanings:
+
+* ABS_PRESSURE:
+ - Used to describe the pressure of a touch interaction on an input device.
+* ABS_DISTANCE:
+ - Used to describe the distance of a tool from an interaction surface. This
+ should only be used while the tool is in close proximity of the device. If
+ the input device may be used freely in three dimensions, consider ABS_Z
+ instead.
+* ABS_MT_<name>:
+ - Used to describe multitouch input events. Please see
+ multi-touch-protocol.txt for details.
+
+EV_SW:
+----------
+EV_SW events describe stateful binary switches. For example, the SW_LID code is
+used to denote when a laptop lid is closed.
+
+EV_MSC:
+----------
+EV_MSC events are used for input and output events that do not fall under other
+categories.
+
+EV_LED:
+----------
+EV_LED events are used for input and output to set and query the state of
+various LEDs on devices.
+
+EV_REP:
+----------
+EV_REP events are used for specifying autorepeating events.
+
+EV_SND:
+----------
+EV_SND events are used for sending sound commands to simple sound output
+devices.
+
+EV_FF:
+----------
+EV_FF events are used to initialize a force feedback capable device and to cause
+such device to feedback.
+
+EV_PWR:
+----------
+EV_PWR events are a special type of key event used specifically for monitoring
+power buttons and switches. The two codes in use are:
+
+* KEY_POWER:
+ - Used to denote a power button event.
+* KEY_SUSPEND:
+ - Used to denote a suspend button event.