The Rock Band guitars have a photoresistor for precisely this purpose: the screen flashes and the guitar responds when the light hits it. It helps make the otherwise very painful calibration process transparent.

It would be so cool to get that to work in Linux. I know the instrument code is in hid-sony. Here are some open tabs I've got in case anyone's curious:

- https://pascal.giard.info/techreports/nguyen-daniel-autocali...

- https://www.niangames.com/articles/reverse-engineering-rockb...

- https://github.com/torvalds/linux/blob/master/drivers/hid/hi...