ModemManager auto reconnect

[Dan Williams]

On Tue, 2016-02-09 at 11:27 +1100, Brendan Simon (eTRIX) wrote:
> On 9/02/2016 9:08 AM, Dan Williams wrote:
> > On Tue, 2016-02-09 at 07:31 +1100, Brendan Simon (eTRIX) wrote:
> > > I have a number or remotely deployed arm embedded linux systems
> > > that
> > > uses a 3G modem via USB to post http information once per second.
> > >  The
> > > modem goes down from time to time.  One reason is we assert the
> > > power
> > > reset line on the modem once a day (as I've been told that the
> > > modems
> > > need to be reset periodically, otherwise functionality degrades
> > > or
> > > eventually lost).  The other reasons I am not sure of (service
> > > provider
> > > initiated?  bugs in the application code?
> > > 
> > > The modem can be off for many hours before eventually
> > > reconnecting
> > > (possibly by watchdog reboot) which is not acceptable for our
> > > real
> > > -time
> > > monitoring application.
> > > 
> > > Does MM automatically try to re-establish a connection if it sees
> > > the
> > > connection go down, or the modem is reset?
> > > 
> > > Are there recommended or suggested MM settings such that MM will
> > > continually try to reconnect if the modem goes down then back up?
> > This is left up to the connection manager that controls
> > ModemManager
> > and tells it to connect.  So whatever you're using to tell
> > ModemManager
> > to start the data connection, that's the thing that would monitor
> > for
> > the modem being disconnected and periodically restart the data
> > connection.
> 
> I created a NetworkManager broadband connection in a Debian virtual
> machine with a modem connected (dev kit).  I then copied the file
> that
> was created
> (/etc/NetworkManager/system-connections/telstra-mobile-broadband-1)
> into
> my build system which will populate my target rootfs for the embedded
> system.
> 
> I found that the modem was not coming up at boot, but would come up
> if I
> used the `nmtui-connect` utility to activate the connection.  A
> systemd
> service was created to bring up the modem at boot time (a one-shot
> script, which brings the modem down then up then calls `nmcli d
> connect
> cdc-wdm0`).
> 
> The scirpt works but I'm not sure if it's the `right way` to get the
> modem up automatically.  One interesting thing which may be a clue is
> that the modem connection does not appear in the `nmtui-edit`
> utility,
> but does appear in the `nmtui-connect` utility.  Is this expected ? 
> Also the LAN connection shows up in both utilities but there a
> connection file does not exist in /etc/NetworkManager/system
> -connections/
> 
> I presume NetworkManager is starting and managing the connection,
> yes? 
> How do I get it to auto-reconnect when the connection goes down or is
> not available?  We are looking at running a custom script invoked via
> cron every minute, but that seems a bit hacky.

What version of NM?  There have been improvements to the autoconnect
logic for WWAN recently in NM, especially if the SIM has a PIN enabled.
 Aside from bugs, we would expect NM to periodically retry the
connection if it fails.

Dan