quoted

we have a GPS line discipline? What is that one doing? As far as I
know all GPS implementations are fully userspace.  

Hm, for some reason I thought that was what n_gsm.c was being used for,
but I could be wrong, I've never seen the hardware that uses that
code...

You probably have actually. It's the standard multiplexing protocol on
lots of 3G modems. It's an AT command that switches mode.

However it illustrates the kind of problems with trying to short circuit
the tty_port code. It's entirely possible for a modem to expose the GPS
on a channel of the multiplex on the USB connection to the modem which is
itself dynamic and can be hot unplugged.

Short circuiting the tty_port layer is not a trivial exercise.

Alan