The AT stuff is really problematic. Look at the hoops ISDN and software
modem drivers go through to emulate AT commands. I know even of a CDC-ACM
modem which can't deal with AT commands inline (that's within spec).
It seems to me we should have a modem API in kernel.

For devices which don't deal in AT commands probably but for devices
whose firmware provides an AT command interface over serial I would
disagree.