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+The input 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.
+A single hardware event generates multiple input events. Each input event
+contains the new value of a single data item. A special event type, EV_SYN, is
+used to separate input events into packets of input data changes occurring at
+the same moment in time. In the following, the term "event" refers to a single
+input event encompassing a type, code, and value.
+The input protocol is a stateful protocol. Events are emitted only when values
+of event codes have changed. However, the state is maintained within the Linux
+input subsystem; drivers do not need to maintain the state and may attempt to
+emit unchanged values without harm. Userspace may obtain the current state of
+event code values using the EVIOCG* ioctls defined in linux/input.h. The event
+reports supported by a device are also provided by sysfs in
+class/input/event*/device/capabilities/, and the properties of a device are
+provided in class/input/event*/device/properties.
+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 state changes of keyboards, buttons, or other key-like
+ devices.
+* EV_REL:
+ - Used to describe relative axis value changes, e.g. moving the mouse 5 units
+ to the left.
+* EV_ABS:
+ - Used to describe absolute axis value changes, e.g. describing the
+ coordinates of a touch on a touchscreen.
+* EV_MSC:
+ - Used to describe miscellaneous input data 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.
+ - Used to receive force feedback device status.
+Codes define the precise type of event.
+EV_SYN event values are undefined. Their usage is defined only by when they are
+sent in the evdev event stream.
+ - Used to synchronize and separate events into packets of input data changes
+ occurring at the same moment 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.
+ - TBD
+ - Used to synchronize and separate touch events. See the
+ multi-touch-protocol.txt document for more information.
+ - Used to indicate buffer overrun in the evdev client's event queue.
+ Client should ignore all events up to and including next SYN_REPORT
+ event and query the device (using EVIOCG* ioctls) to obtain its
+ current state.
+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 released, an event is
+emitted with value 0. Some hardware send events when a key is repeated. These
+events have a value of 2. 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>:
+ - 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.
+ BTN_TOUCH is used for touch contact. While an input tool is determined to be
+ within meaningful physical contact, the value of this property must be set
+ to 1. Meaningful physical contact may mean any contact, or it may mean
+ contact conditioned by an implementation defined property. For example, a
+ touchpad may set the value to 1 only when the touch pressure rises above a
+ certain value. BTN_TOUCH may be combined with BTN_TOOL_<name> codes. For
+ example, a pen tablet may set BTN_TOOL_PEN to 1 and BTN_TOUCH to 0 while the
+ pen is hovering over but not touching the tablet surface.
+Note: For appropriate function of the legacy mousedev emulation driver,
+BTN_TOUCH must be the first evdev code emitted in a synchronization frame.
+Note: Historically a touch device with BTN_TOOL_FINGER and BTN_TOUCH was
+interpreted as a touchpad by userspace, while a similar device without
+BTN_TOOL_FINGER was interpreted as a touchscreen. For backwards compatibility
+with current userspace it is recommended to follow this distinction. In the
+future, this distinction will be deprecated and the device properties ioctl
+EVIOCGPROP, defined in linux/input.h, will be used to convey the device type.
+ - 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 all BTN_TOOL_<name> codes and the BTN_TOUCH code are orthogonal in
+ purpose. A trackpad event generated by finger touches should generate events
+ for one code from each group. At most only one of these BTN_TOOL_<name>
+ codes should have a value of 1 during any synchronization frame.
+Note: Historically some drivers emitted multiple of the finger count codes with
+a value of 1 in the same synchronization frame. This usage is deprecated.
+Note: In multitouch drivers, the input_mt_report_finger_count() function should
+be used to emit these codes. Please see multi-touch-protocol.txt for details.
+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:
+ - These codes are used for vertical and horizontal scroll wheels,
+ respectively.
+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:
+ - Used to describe the distance of a tool from an interaction surface. This
+ event should only be emitted while the tool is hovering, meaning in close
+ proximity of the device and while the value of the BTN_TOUCH code is 0. 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 events describe stateful binary switches. For example, the SW_LID code is
+used to denote when a laptop lid is closed.
+Upon binding to a device or resuming from suspend, a driver must report
+the current switch state. This ensures that the device, kernel, and userspace
+state is in sync.
+Upon resume, if the switch state is the same as before suspend, then the input
+subsystem will filter out the duplicate switch state reports. The driver does
+not need to keep the state of the switch at any time.
+EV_MSC events are used for input and output events that do not fall under other
+EV_LED events are used for input and output to set and query the state of
+various LEDs on devices.
+EV_REP events are used for specifying autorepeating events.
+EV_SND events are used for sending sound commands to simple sound output
+EV_FF events are used to initialize a force feedback capable device and to cause
+such device to feedback.
+EV_PWR events are a special type of event used specifically for power
+mangement. Its usage is not well defined. To be addressed later.
+The guidelines below ensure proper single-touch and multi-finger functionality.
+For multi-touch functionality, see the multi-touch-protocol.txt document for
+more information.
+REL_{X,Y} must be reported when the mouse moves. BTN_LEFT must be used to report
+the primary button press. BTN_{MIDDLE,RIGHT,4,5,etc.} should be used to report
+further buttons of the device. REL_WHEEL and REL_HWHEEL should be used to report
+scroll wheel events where available.
+ABS_{X,Y} must be reported with the location of the touch. BTN_TOUCH must be
+used to report when a touch is active on the screen.
+BTN_{MOUSE,LEFT,MIDDLE,RIGHT} must not be reported as the result of touch
+contact. BTN_TOOL_<name> events should be reported where possible.
+Legacy trackpads that only provide relative position information must report
+events like mice described above.
+Trackpads that provide absolute touch position must report ABS_{X,Y} for the
+location of the touch. BTN_TOUCH should be used to report when a touch is active
+on the trackpad. Where multi-finger support is available, BTN_TOOL_<name> should
+be used to report the number of touches active on the trackpad.
+BTN_TOOL_<name> events must be reported when a stylus or other tool is active on
+the tablet. ABS_{X,Y} must be reported with the location of the tool. BTN_TOUCH
+should be used to report when the tool is in contact with the tablet.
+BTN_{STYLUS,STYLUS2} should be used to report buttons on the tool itself. Any
+button may be used for buttons on the tablet except BTN_{MOUSE,LEFT}.
+BTN_{0,1,2,etc} are good generic codes for unlabeled buttons. Do not use
+meaningful buttons, like BTN_FORWARD, unless the button is labeled for that
+purpose on the device.