libei's docs for ei_device_button_button () reads: "Button codes must match the defines in linux/input-event-codes.h"
I shouldn't have looked.
There are over 100 different BTN_ definitions. The hell am I supposed to know what these do. Probably gonna have to sniff uinput to figure any of this out. I don't wanna grep the kernel tree tonight please don't make me...
And yet, I am screaming at my trackball. Why couldn't it have more buttons!? Where's my physical device that grants me incredible buttons like GEAR_DOWN, PINKIE, and TOOL_AIRBRUSH !?