Support for Fibocom MC610 with non-standard EPS bearer handling

[Giacinto Cifelli]

On Mon, Mar 24, 2025 at 3:08 PM Yannick Serafini <yannick.lanz at wifx.net> wrote:
>
> Hi there,

Hi,

>
> 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,

I'm surprised that there is actually a modem following the 27.007:
"The feature "initial PDP context" may be supported and is a
manufacturer specific option. For this option, the context with
<cid>=0 (context number 0) is defined upon startup and does not need
to be created with the +CGDCONT command. The initial PDP context has
particular manufacturer specific default settings disassociated with
any other default settings of +CGDCONT." (paragraph 10.1.0 of the
V16.12.0 (2022-06)).

> 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.

I haven't seen any other device in MM with this CID=0, so on this you
are right: it shall be added.
But there are functions to set the attach APN, which - however - are
not called by default, if memory serves.

>
> 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)
>

this goes together, yes, but in general it is only needed for private
APNs (as well as setting the attach APN to some actual value and not
to an empty string that lets the network do the job)

> 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.
>

this is not good.  it means that you have to go to CFUN=1, and while
the modem attempts to attach you need to change the APN/user/pwd...

> 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.

what is the network interface?  MBIM/ECM/NCM/...?

>
> Thank you,
> Yannick Serafini (Lanz)
> Wifx SA

Giacinto