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I assume you’re using the same one I am (Mayflash SNES controller adapter for PC).It’s quite straight forward to get the usb adapter to work, it’s just plug and play. You should first check if the RPi sees the device with lsusb. It should be listed as GreenAsia. You should now be able to use the controllers to navigate in the emulation station menu. Then configure your controllers for RetroArch. I used this video as a guide:
Unfortunately I can only get one controller to work, there is no response from the second controller in RetroArch.