This is a short guide on how to set up the Mayflash gamecube adapter on Linux to work with Dolphin emulator and Steam.
For this guide, I am on Arch Linux and using the 4-port version of the Mayflash gamecube adapter, which I bought on Amazon for around $20.
Paste the following into /etc/udev/rules.d/51-gcadapter.rules:
SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="057e", ATTRS{idProduct}=="0337", MODE="0666"
Then, install wii-u-gc-adapter. (There is a package in the AUR)
After installing, run
sudo modprobe uinput
Now, you can run the adapter with
wii-u-gc-adapter
Make sure the Mayflash adapter is set to "Wii U" setting. In Dolphin, go to
Controller Setings, set the controller to "GameCube Adapter for Wii U". Your
controller should now work in Dolphin.
Make sure the Mayflash adapter is set to "PC" setting. If you haven't already, run wii-u-gc-adapter
In Steam, enter Big Picture mode, and then go to Settings -> Controller Settings.
You should see 4 "unregistered" controllers listed under "Detected
Controllers". Select the first one, and set your preferred keys.
You may need to plug the controller into the 4th port of the adapter when
setting up the keys (I have no idea why). Note that you will need to leave some
keys unmapped since gamecube controllers don't have enough buttons.
If you now see "Xbox One Controller" listed under "Detected Controllers" in Controller Settings, then you're good to go!
Plugging in more controllers should work just fine. I have tested with two controllers: an original Nintendo Gamecube controller and a 3rd party (Gamestop brand) controller
If you're having trouble getting the Mayflash adapter or controllers to be recognized, unplugging and replugging in the adapter seems to fix that.