When I first started the game my SN30 gamepad was not recognized, but my X-Box 360 controller worked fine. Turns out, I just needed to add an entry to the .local/share/SteamWorld Dig 2/gamepads.cfg file:

03000000c82d00001290000011010000,8Bitdo SN30 GamePad,platform:Linux,a:b0,b:b1,x:b3,y:b4,back:b10,start:b11,leftshoulder:b6,rightshoulder:b7,leftx:a0,lefty: a1,

Hope this helps someone.

Thanks, this was a life saver!

It didn't quite work for me. Here's what I had to do:

1. Run the game (from start.sh) with the SN30 connected
2. In the output I got a message that said:

INFO: Connected unrecognized joystick 05000000c82d00006228000000010000 (8Bitdo SN30 GamePad)

Note that this is not the same long number as in user torham's answer.

3. Substitute this serial code (or whatever it is) into the config string torham provided and add it to the gamepads.cfg file as described above.

I don't know whether the serial number varies from individual controller to controller, or whether I just have a slightly different model from torham. Hope this helps!