And here is a hint for the controller, the controller 8bitdo SN30 Pro G Classic Gamepad must be connected to Xinput via Bluetooth or it won't work:

Mode:
Hold Y+START button POWER ON as SWITCH PRO mode.
Hold A+START button POWER ON as MAC mode.
Hold B+START button POWER ON as Android mode.
Hold X+START button POWER ON as Xinput mode.

A. LED 1 blinking: D-input mode
B. LED 2 blinking: X-input mode (Xbox 360 mode)
C. LED 3 blinking: macOS mode
D. LED Rotating: Switch mode or pairing mode

@Rutjes Hey
I had the sam issue with my arcade china controllers. I rotated the joystick 180 ( up to down ) and now it works. ( Only on the Pi 3+)
My new problem is, on retropie x86 ( on ubuntu), the reicast controller configurations doesn't work. When i launch the game no button works.
Could you fix your problems with your cabinet?

Nobody has any ideas?

Go here and scroll down to the bottom.

https://retropie.org.uk/docs/Dreamcast/

@aryetis
I'm seeing this in the console output:
evdev: Found 'Xbox Gamepad (userspace driver)' at '/dev/input/event17'
evdev: reading mapping file: 'controller_xboxdrv.cfg'

so from what I can tell it's using evdev and loading my gamepad at /dev/input/event17 which is the xbox gamepad defined in xboxdrv

@dankcushions thanks, I had the last retropie, but for some reason the reicast was previous version, now im playing with 2 player MVSC2, soul calibur, Sonic, etc with my friends in my family property (its like a farm, far away from everything but for vacations its awesome) with the latest version, updated and works like charm, thanks guys , you're really awesome

@burtthaman there is also a recaist button mapping utility in retropie setup. I have mapped several different controllers that way and have not had any issues with it.

@MWGemini
Thanks for the help. I'll try with a USB wired gamepad when I get back home later this week and see if it works.

First there's this. https://retropie.org.uk/forum/topic/3/read-this-first

Second, if you want your 8bitdo controllers to work with reicast like they do in the other emulators, have you tried setting them up in Rgui while in Reicast?

0_1484621263413_IMG_20170116_213046.jpg

@Sockett77
Please post the contents of your emu.cfg file.

Should be located in either:
/opt/retropie/configs/dreamcast/emu.cfg
or
/home/pi/.reicast/emu.cfg

@Concat
The 2nd one is closer to the correct format but not sure about the button mapping.

Just run evtest to get the correct mapping.

This thread might be helpful:
https://retropie.org.uk/forum/topic/5006/reicast-stuck-on-time-date