PPP on serial direct connection

[Mikko Salomäki <ms datarespons se>]

Hello,

I've successfully used NetworkManager (and ModemManager) for some time to manage various combinations of:
- ethernet
- wifi
- gsm

Truly networking that just works. Greatly appreciated!

During the on-going chip crisis then first the ethernet transceiver has been removed and now wifi is about to 
be removed for some configurations. In manufacturing a network connection is needed for some test logs and 
some bytes of data which now could be transmitted over gsm if it weren't for some modems not having frequency 
bands available in the factory's region. 
This has led to interest in using an available serial port for internet access through a gateway.

A simple test shows that using pppd may setup a connection but would require some additional management (i.e. 
routing) which is what NetworkManager is really good at.
NetworkManager can detect the interface (ppp0) created but lists it as disconnected. The ppp0 interface is in 
reality already connected by a pppd session (ping ok) and launching another pppd session through 
NetworkManager is probably not what we want(?).

To my understanding there is no suitable connection type and as serial ports are not added as devices (Can't 
find any mention of udev tricks to force the behaviour) and it probably is not possible to make 
NetworkManager to launch a pppd session on top of a serial port which is not a modem.

Could it be possible to be pointed in the right direction or is this totally out of scope for NetworkManager?

Regards,
Mikko