It's used with SDL mappings.
here is an example of one I use.
Input.add_joy_mapping("03000000ad1b000000f9000000010000,360 Afterglow,a:b0,b:b1,x:b2,y:b3,back:b6,guide:b8,start:b7,leftstick:b9,rightstick:b10,leftshoulder:b4,rightshoulder:b5,dpup:h0.1,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,leftx:a0,lefty:a1,rightx:a3,righty:a4,lefttrigger:a2,righttrigger:a5,platform:Linux,", true)
To get the string I used this tool. http://generalarcade.com/gamepadtool/