Re: Problems with modems/Example config for QMI-based modem,

[Dan Williams <dcbw redhat com>]

On Wed, 2018-02-28 at 18:40 +0100, Jakob Hasse wrote:
> Hello,
>
> We want to integrate different modems into our embedded system. 
> Recently, our Linux-distributor decided to introduce NetworkManager. 
> When we switch to NetworkManager, network connectivity via 3G/2G is 
> never reliable: the connection either succeeds only sporadically or
> not 
> at all. This happens on three types of modems from two different 
> vendors. When we set up the modems manually with pppd or with a QMI
> test 
> program, everything works fine an reliably, also automatically after
> boot.
> Could you give me advice what steps to do to produce debug/log output
> to 
> then come back here and have someone look at it?

You want to increase the NetworkManager and ModemManager logging level:

nmcli g log level debug
mmcli -G debug

and then plug your modem in.  This will dump debug logging to whatever
system log service your linux distro uses.  For systemd-based distros,
you want:

journalctl -b -u NetworkManager
journalctl -b -u ModemManager

other distros will use /var/log/messages, /var/log/daemon.log, or
wherever else syslog 'daemon' facility output goes.

> Furthermore, it would be really nice to have an example
> configuration 
> file for /etc/NetworkManager/system-connections/ to configure a
> modem 
> with QMI connection. All the examples I had so far (seem to) only 
> establish the connection with pppd.

There should be no difference between a QMI or MBIM based modem and a
PPP based one.  The only options you really need are the APN and
possibly the username and password in the [gsm] section.

Since your email address is ".ch" I'm going to assume you don't care
about CDMA/EVDO and thus you only need to use [gsm].  The simplest
connection would be something like:

[connection]
id=T-Mobile Internet
uuid=0083942e-b260-4b73-a1fd-860f3e6ca2c2
type=gsm

[gsm]
apn=epc.tmobile.com
number=*99#

Again, there should be no difference between QMI and PPP-based modems
from a NetworkManager perspective.  ModemManager abstracts that away
for us.

> Another problem is that it often happens that we use NetworkManager
> on 
> devices which changes some settings and afterwards we can't even use
> the 
> old methods to establish a connection. This is really scary since it 
> basically renders our devices unusable so far. Is it possible to
> tell 
> NetworkManager to put the device in the original state again after
> using it?

What kind of changes happen here?  Are they things that are done
externally, or done by NetworkManager?

> I also have a question to ModemManager but don't know whether I'm
> right 
> with it here:
> We want to send AT commands to the modems while they serve an
> internet 
> connection. Normally, this is possible since the Modems all have 
> multiple serial interfaces. But when using ModemManager, it locks
> all 
> serial interfaces of the modems and we can't use them anymore. I
> know 
> that ModemManager can send AT commands with "mmcli --command" but
> only 
> in debug mode, so apparently not for production use. Is it possible
> to 
> send commands another way with ModemManager or prevent it from
> locking 
> all AT interfaces?

Yeah, this request comes up periodically, and MM should probably just
not open ports it doesn't need.  With PPP-based modems MM needs at
least two AT ports (one for the PPP connection and a second to use for
status while connected).  I don't think MM will care about the AT ports
when it's using QMI/MBIM, but if you can get debug logs from
ModemManager we can verify that.

Dan