If your USB gamepad is not supported - you can report the keycodes to me that each button on your pad generates. I will also need the 'name' of the pad (and I mean - the 'name' that pops up in RetroArch Android) - that way I can put in autodetection. The manual explains this in more detail. (NOT THE CORES MANUAL - but the 'RetroArch Android 'manual).
The direction button on USB gamepad works, but other button not.
Here's my USB gamepad name and key values from RetroArch Android.
HID[DragonRise.Inc DragonRise USB Gam.
The direction key : src 16777232
square: pad 0: 188 ac=0 src=1281
triangle: pad 0: 189 ac=0 src=1281
cross: pad 0: 190 ac=0 src=1281
circle: pad 0: 191 ac=0 src=1281
L1: pad 0: 192 ac=0 src=1281
R1: pad 0: 193 ac=0 src=1281
L2: pad 0: 194 ac=0 src=1281
R2: pad 0: 195 ac=0 src=1281
SELECT: pad 0: 196 ac=0 src=1281
START: pad 0: 197 ac=0 src=1281
There are houndreds of types of USB gamepads in the world, you can not list them all. So I think the best way is not presetting the key, but mappering it by users themselves.