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+ Linux Gamepad API
+----------------------------------------------------------------------------
+
+1. Intro
+~~~~~~~~
+Linux provides many different input drivers for gamepad hardware. To avoid
+having user-space deal with different button-mappings for each gamepad, this
+document defines how gamepads are supposed to report their data.
+
+2. Geometry
+~~~~~~~~~~~
+As "gamepad" we define devices which roughly look like this:
+
+ ____________________________ __
+ / [__ZL__] [__ZR__] \ |
+ / [__ TL __] [__ TR __] \ | Front Triggers
+ __/________________________________\__ __|
+ / _ \ |
+ / /\ __ (N) \ |
+ / || __ |MO| __ _ _ \ | Main Pad
+ | <===DP===> |SE| |ST| (W) -|- (E) | |
+ \ || ___ ___ _ / |
+ /\ \/ / \ / \ (S) /\ __|
+ / \________ | LS | ____ | RS | ________/ \ |
+ | / \ \___/ / \ \___/ / \ | | Control Sticks
+ | / \_____/ \_____/ \ | __|
+ | / \ |
+ \_____/ \_____/
+
+ |________|______| |______|___________|
+ D-Pad Left Right Action Pad
+ Stick Stick
+
+ |_____________|
+ Menu Pad
+
+Most gamepads have the following features:
+ - Action-Pad
+ 4 buttons in diamonds-shape (on the right side). The buttons are
+ differently labeled on most devices so we define them as NORTH,
+ SOUTH, WEST and EAST.
+ - D-Pad (Direction-pad)
+ 4 buttons (on the left side) that point up, down, left and right.
+ - Menu-Pad
+ Different constellations, but most-times 2 buttons: SELECT - START
+ Furthermore, many gamepads have a fancy branded button that is used as
+ special system-button. It often looks different to the other buttons and
+ is used to pop up system-menus or system-settings.
+ - Analog-Sticks
+ Analog-sticks provide freely moveable sticks to control directions. Not
+ all devices have both or any, but they are present at most times.
+ Analog-sticks may also provide a digital button if you press them.
+ - Triggers
+ Triggers are located on the upper-side of the pad in vertical direction.
+ Not all devices provide them, but the upper buttons are normally named
+ Left- and Right-Triggers, the lower buttons Z-Left and Z-Right.
+ - Rumble
+ Many devices provide force-feedback features. But are mostly just
+ simple rumble motors.
+
+3. Detection
+~~~~~~~~~~~~
+All gamepads that follow the protocol described here map BTN_GAMEPAD. This is
+an alias for BTN_SOUTH/BTN_A. It can be used to identify a gamepad as such.
+However, not all gamepads provide all features, so you need to test for all
+features that you need, first. How each feature is mapped is described below.
+
+Legacy drivers often don't comply to these rules. As we cannot change them
+for backwards-compatibility reasons, you need to provide fixup mappings in
+user-space yourself. Some of them might also provide module-options that
+change the mappings so you can adivce users to set these.
+
+All new gamepads are supposed to comply with this mapping. Please report any
+bugs, if they don't.
+
+There are a lot of less-featured/less-powerful devices out there, which re-use
+the buttons from this protocol. However, they try to do this in a compatible
+fashion. For example, the "Nintendo Wii Nunchuk" provides two trigger buttons
+and one analog stick. It reports them as if it were a gamepad with only one
+analog stick and two trigger buttons on the right side.
+But that means, that if you only support "real" gamepads, you must test
+devices for _all_ reported events that you need. Otherwise, you will also get
+devices that report a small subset of the events.
+
+No other devices, that do not look/feel like a gamepad, shall report these
+events.
+
+4. Events
+~~~~~~~~~
+Gamepads report the following events:
+
+Action-Pad:
+ Every gamepad device has at least 2 action buttons. This means, that every
+ device reports BTN_SOUTH (which BTN_GAMEPAD is an alias for). Regardless
+ of the labels on the buttons, the codes are sent according to the
+ physical position of the buttons.
+ Please note that 2- and 3-button pads are fairly rare and old. You might
+ want to filter gamepads that do not report all four.
+ 2-Button Pad:
+ If only 2 action-buttons are present, they are reported as BTN_SOUTH and
+ BTN_EAST. For vertical layouts, the upper button is BTN_EAST. For
+ horizontal layouts, the button more on the right is BTN_EAST.
+ 3-Button Pad:
+ If only 3 action-buttons are present, they are reported as (from left
+ to right): BTN_WEST, BTN_SOUTH, BTN_EAST
+ If the buttons are aligned perfectly vertically, they are reported as
+ (from top down): BTN_WEST, BTN_SOUTH, BTN_EAST
+ 4-Button Pad:
+ If all 4 action-buttons are present, they can be aligned in two
+ different formations. If diamond-shaped, they are reported as BTN_NORTH,
+ BTN_WEST, BTN_SOUTH, BTN_EAST according to their physical location.
+ If rectangular-shaped, the upper-left button is BTN_NORTH, lower-left
+ is BTN_WEST, lower-right is BTN_SOUTH and upper-right is BTN_EAST.
+
+D-Pad:
+ Every gamepad provides a D-Pad with four directions: Up, Down, Left, Right
+ Some of these are available as digital buttons, some as analog buttons. Some
+ may even report both. The kernel does not convert between these so
+ applications should support both and choose what is more appropriate if
+ both are reported.
+ Digital buttons are reported as:
+ BTN_DPAD_*
+ Analog buttons are reported as:
+ ABS_HAT0X and ABS_HAT0Y
+
+Analog-Sticks:
+ The left analog-stick is reported as ABS_X, ABS_Y. The right analog stick is
+ reported as ABS_RX, ABS_RY. Zero, one or two sticks may be present.
+ If analog-sticks provide digital buttons, they are mapped accordingly as
+ BTN_THUMBL (first/left) and BTN_THUMBR (second/right).
+
+Triggers:
+ Trigger buttons can be available as digital or analog buttons or both. User-
+ space must correctly deal with any situation and choose the most appropriate
+ mode.
+ Upper trigger buttons are reported as BTN_TR or ABS_HAT1X (right) and BTN_TL
+ or ABS_HAT1Y (left). Lower trigger buttons are reported as BTN_TR2 or
+ ABS_HAT2X (right/ZR) and BTN_TL2 or ABS_HAT2Y (left/ZL).
+ If only one trigger-button combination is present (upper+lower), they are
+ reported as "right" triggers (BTN_TR/ABS_HAT1X).
+
+Menu-Pad:
+ Menu buttons are always digital and are mapped according to their location
+ instead of their labels. That is:
+ 1-button Pad: Mapped as BTN_START
+ 2-button Pad: Left button mapped as BTN_SELECT, right button mapped as
+ BTN_START
+ Many pads also have a third button which is branded or has a special symbol
+ and meaning. Such buttons are mapped as BTN_MODE. Examples are the Nintendo
+ "HOME" button, the XBox "X"-button or Sony "P" button.
+
+Rumble:
+ Rumble is adverticed as FF_RUMBLE.
+
+----------------------------------------------------------------------------
+ Written 2013 by David Herrmann <dh.herrmann@gmail.com>

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