Make modemmanager select data port instead of control port on Sierra MC8700/8790

[Davy Herben <davy 21net com>]

Hi all,

I'm working on a system which provides internet access in mobile
environments through multiple connections (cellular, satellite,
wifi...). We use NetworkManager to manage the wifi and cell connections,
and our own software to monitor and log the connection statistics
(signal strength, SSID, cell id...).

All this runs on an ubuntu machine. We're in the process of upgrading
from an old 8.04 to 10.04. This bumps the NetworkManager from the 7.x
range to 8.0 (or 8.4 via a PPA).

The sierra cards expose a number of ttyUSB ports: a few app ports, a
control port (responds with useful data to AT commands), and a data port
(can be used to open a connection, but answers only OK/ERROR to AT
commands). 

In 7.x, I used udev rules to force NM to connect using the data port,
like this:

KERNELS=="ttyUSB7", DRIVERS=="sierra", ENV{ID_NM_MODEM_GSM}="1"

My software then connects to the control port to monitor the status,
also when the data connection is open.

With NM 8.0 / 8.4, the modemmanager detects the sierra modems, but
selects the control port instead of the data port. The connection is
successfully established, but I get no monitoring data as the control
port is locked.

I also considered reading the data I need from udev in my application,
but it looks like the data like signal strength are not reported for
these cards.

How would you guys handle this? Is there a way to force the ModemManager
to connect to the data port? Or (less ideally) to get the data I need
some other way?

Thanks,

Davy