Ashley Whetter
RHEL6 and CentOS6 systems still use an old version of udev (147). It wasn't until udev 148 (Yep. 1 version off!) that the input class system changed from "ID_CLASS" to "ID_INPUT_{JOYSTICK,KEYBOARD,MOUSE,etc}" (http://lwn.net/Articles/364728/). Because SDL2 looks for the ID_INPUT_X field this means that it never detects any input devices on RHEL6 systems.
I've attached a patch which fixes the problem. If no input devices are detected with "ID_INPUT_X" then SDL will fallback to looking for the old style "ID_CLASS" udev field instead.
Because of the "big change" between udev versions I doubt it'll ever get upgraded on RHEL6, but because RHEL7 is on the way I don't know if this patch is worth merging. Hopefully it'll help anyone out that's having this problem though.
7ae52b1730