Fixed bug 5161 - Autodetect controller mappings based on the Linux Gamepad Specification

Jan Bujak

I wrote a new driver for my gamepad on Linux. I'd like SDL to support it out-of-box, as currently it just treats it as a generic joystick instead of a gamepad. From what I can see the only way to do that is to either 1) pick one of the already supported controllers' PID, VID and button layouts and have my driver send that (effectively lying that it's something else), or 2) submit a preconfigured, hardcoded mapping to SDL.

Both of those, in my opinion, are silly when we already have the Linux Gamepad Specification which standarizes this:

https://www.kernel.org/doc/html/v4.15/input/gamepad.html

Unfortunately SDL doesn't make use of it currently. So I've took it upon myself to add it; patch is in the attachments.

Basically what the patch does is that if SDL finds no built-it controller mappings for a given joystick it then asks the joystick backend to autodetect it, and that uses the relevant evdev bits to figure out which button/axis is which. (See the specs for more details.)

With this patch applied my own driver for my controller works out-of-box with SDL with no extra configuration and is correctly recognized as a gamepad; this is also going to be the case for any other driver which follows the Linux Gamepad Specification.

src/joystick/SDL_sysjoystick.h

@@ -136,6 +136,9 @@ typedef struct _SDL_JoystickDriver
     /* Function to perform any system-specific joystick related cleanup */
     void (*Quit)(void);
 
+    /* Function to get the autodetected controller mapping; returns false if there isn't any. */
+    SDL_bool (*GetGamepadMapping)(int device_index, SDL_GamepadMapping * out);
+
 } SDL_JoystickDriver;
 
 /* Windows and Mac OSX has a limit of MAX_DWORD / 1000, Linux kernel has a limit of 0xFFFF */