leds: add new LED_FUNCTION_PLAYER for player LEDs for game controllers.

Player LEDs are commonly found on game controllers from Nintendo and Sony
to indicate a player ID across a number of LEDs. For example, "Player 2"
might be indicated as "-x--" on a device with 4 LEDs where "x" means on.

This patch introduces LED_FUNCTION_PLAYER1-5 defines to properly indicate
player LEDs from the kernel. Until now there was no good standard, which
resulted in inconsistent behavior across xpad, hid-sony, hid-wiimote and
other drivers. Moving forward new drivers should use LED_FUNCTION_PLAYERx.

Note: management of Player IDs is left to user space, though a kernel
driver may pick a default value.

Signed-off-by: Roderick Colenbrander <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

Documentation/leds/well-known-leds.txt

···
 
 Notice there's a list of functions in include/dt-bindings/leds/common.h .
 
+* Gamepads and joysticks
+
+Game controllers may feature LEDs to indicate a player number. This is commonly
+used on game consoles in which multiple controllers can be connected to a system.
+The "player LEDs" are then programmed with a pattern to indicate a particular
+player. For example, a game controller with 4 LEDs, may be programmed with "x---"
+to indicate player 1, "-x--" to indicate player 2 etcetera where "x" means on.
+Input drivers can utilize the LED class to expose the individual player LEDs
+of a game controller using the function "player".
+Note: tracking and management of Player IDs is the responsibility of user space,
+though drivers may pick a default value.
+
+Good: "input*:*:player-{1,2,3,4,5}
+
 * Keyboards
   
 Good: "input*:*:capslock"

include/dt-bindings/leds/common.h

···
 #define LED_FUNCTION_MICMUTE "micmute"
 #define LED_FUNCTION_MUTE "mute"
 
+/* Used for player LEDs as found on game controllers from e.g. Nintendo, Sony. */
+#define LED_FUNCTION_PLAYER1 "player-1"
+#define LED_FUNCTION_PLAYER2 "player-2"
+#define LED_FUNCTION_PLAYER3 "player-3"
+#define LED_FUNCTION_PLAYER4 "player-4"
+#define LED_FUNCTION_PLAYER5 "player-5"
+
 /* Miscelleaus functions. Use functions above if you can. */
 #define LED_FUNCTION_ACTIVITY "activity"
 #define LED_FUNCTION_ALARM "alarm"