@shaolinmatt

Did you follow the directions on the Wiki/Docs here
https://retropie.org.uk/docs/Dreamcast/

You can check the md5sum or CRC32 of your BIOS files dc_boot.bin and dc_flash.bin to verify you have the correct ones.
https://retropie.org.uk/docs/Dreamcast/#bios

If you go into the RetroPie-Setup Script you can goto Configuration / tools then to reicast - Dreamcast emulator Reicast then Configure input devices for Reicast

The location for your controller mapping config file is /opt/retropie/configs/dreamcast/mappings/

You will also need to verify that in /home/pi/.reicast/emu.cfgthat both evdev_mapping_1 and evdev_mapping_2 are pointing to the matching controller mapping configuration file(s).

Example

evdev_mapping_1 = /opt/retropie/configs/dreamcast/mappings/controller_Xbox360WirelessReceiver(XBOX).cfg evdev_mapping_2 = /opt/retropie/configs/dreamcast/mappings/controller_Xbox360WirelessReceiver(XBOX).cfg

Also, make sure that evdev_device_id_1 and evdev_device_id_2 are set to the corresponding controller's /dev/input/event* number (run evtest to find out which event number your controller is using).

Example

[input] evdev_device_id_1 = 0 evdev_device_id_2 = 1