fibocom: Minimum PDP context ID

[Aleksander Morgado]

Hey,

> I'm troubleshooting a problem with the Fibocom L610 LTE Cat-1 modem and
> ModemManager 1.18.6. The modem/SIM appears to connect PDP context 0
> automatically. The APN seems to be populated from the SIM. When trying
> to connect, ModemManager searches for the best context. If context 0
> matches our desired configuration (which it did in my case), it will try
> to reuse it. However, +CGACT will not report the status of context 0.
> Neither will +GTRNDIS allow to establish an ECM connection on context 0.
> This confuses ModemManager. Some SIMs seem to allow establishing an
> additional (identical) PDP context (e.g. with context ID 1), some don't.
> The log of the connection attempt follows down below.
>
> I'm wondering if mm_3gpp_profile_list_find_best should restrict the
> returned context ID to be within min_profile_id and max_profile_id. The
> modem reports a minimum of 1 so that would prevent ModemManager to use
> context 0.
>

If I'm not mistaken, in Intel chipset based modems, CID=0 is used to
handle the initial default EPS bearer; i.e. the bearer that gets
automatically setup when the modem gets registered in the network. If
+GTRNDIS isn't able to handle attempting a connection over that
bearer, we should definitely try to use CID>=1 instead. This is
probably something to handle in the Fibocom plugin itself.

-- 
Aleksander
https://aleksander.es