input: document gamepad API and add extra keycodes

Until today all gamepad input drivers report their data differently. It is
nearly impossible to write applications for more than one device in a
generic way. Therefore, this patch introduces a uniform gamepad API which
will be used for all new drivers.

Instead of mapping buttons by their labels, we now map them by position.
This allows applications to work with any gamepad regardless of the labels
on the buttons. Furthermore, we standardize the ABS_* codes for analog
triggers and sticks.

For D-Pads the long overdue BTN_DPAD_* codes are introduced. They should
be fairly obvious how to use. To avoid confusion, the action buttons now
have BTN_EAST/SOUTH/WEST/NORTH aliases.
Reported-by: Todd Showalter <todd@electronjump.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/input/gamepad.txt

+                            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>