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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>