Support for Fibocom MC610 with non-standard EPS bearer handling

[Yannick Serafini]

Hi there,

I'm working on adding proper support for the Fibocom MC610 modem which has some unusual behavior regarding the initial EPS bearer configuration that's causing issues with ModemManager.

The key issue is that on this modem, the attachment bearer must use CID 0, which is:
1) Not documented in the modem's documentation
2) Not reported by AT+CGDCONT=? which only shows CIDs 1-7:
   +CGDCONT: (1-7),"IP",,,(0-3),(0-4)
   +CGDCONT: (1-7),"IPV6",,,(0-3),(0-4)
   +CGDCONT: (1-7),"IPV4V6",,,(0-3),(0-4)
   +CGDCONT: (1-7),"PPP",,,(0-3),(0-4)
   +CGDCONT: (1-7),"Non-IP",,,(0-3),(0-4)

I've confirmed with Fibocom support and through direct AT command testing that only CID 0 can be used for the initial EPS bearer.

With some SIM cards where the APN is "internet" (or perhaps other standard APNs), the modem automatically configures the attachment bearer:
AT+CGDCONT?
+CGDCONT: 0,"IP","internet","<IP>",0,0

However, with SIMs requiring a different APN or authentication, this automatic configuration fails and ModemManager needs to explicitly configure the bearer - which it currently can't do properly because of this CID 0 issue.

Similarly, for authentication settings, both AT+CGAUTH and AT+MGAUTH commands don't report CID 0 as valid:
+CGAUTH: (1..7),(0..2),(0..20),(0..20)
+MGAUTH: (1-7),(0-2),(64),(64)

I've tried overriding 'load_initial_eps_bearer_cid' to return 0, but the profile manager still fails because this CID is outside the range reported by AT+CGDCONT=?.

An additional complication: when the modem is in power-down mode (CFUN=4), AT+CGDCONT? returns just "OK" with no profile data. This breaks the 'handle_set_initial_eps_bearer_settings_step' function, which powers down the modem first and then tries to get the current profile list.

I've seen in the code that ModemManager has special handling for some modems' EPS bearer quirks, but I'm not sure where to start with these particular issues. Any guidance on the best approach would be greatly appreciated.

Thank you,
Yannick Serafini (Lanz)
Wifx SA