ModemManager can't open serial port after previously opening it successfully

[Matthew Starr]

> -----Original Message-----
> From: Aleksander Morgado [mailto:aleksander at aleksander.es]
> Sent: Thursday, July 19, 2018 2:16 PM
> 
> Hey,
> 
> On Thu, Jul 19, 2018 at 3:47 PM, Matthew Starr <mstarr at hedonline.com>
> wrote:
> > I am running ModemManager 1.8.0 and NetworkManager 1.8.0 on an
> embedded device with the Linux OS built by buildroot.  I am using a u-blox
> TOBY-R200 cell modem connected over USB with virtual serial interfaces
> /dev/ttyACM0 through /dev/ttyACM5.
> >
> > Sometimes while connected to the cellular network for a longer period of
> time (sometimes around 200 minutes, other times I have reached over 800
> minutes), ModemManager will report a modem hangup that was not
> initiated by user action or loss of signal since this is a stationary device that is
> being tested.  Usually ModemManager will reconnect with NetworkManager
> prompting for the associated cell profile to be reconnected.  My issue is that
> sometimes no matter how many times I try or auto reconnects are tried,
> ModemManager cannot reestablish a data connection.  ModemManager just
> keeps reporting that it can't open /dev/ttyACM0 which is was using just fine
> before.  The interesting thing is that if I restart ModemManager, it is able to
> open the /dev/ttyACM0 port right away and make a data connection again,
> so it doesn't look to be an issue with the modem causing serial port issues.
> This looks to be some sort of serial port handling issue in ModemManager,
> maybe just related to u-blox but I am not sure.  I looked at the state of the
> /dev/ttyACM0 device using lsof and it is not reporting that any other process
> has that device file open or locked.
> >
> 
> These seem relevant, yes:
> 
> > Jul 19 12:52:31 canect2 daemon.warn ModemManager[302]: <warn>
> Couldn't load Operator Code: 'Cannot run sequence: 'Could not open serial
> device ttyACM0: it has been forced close''
> > Jul 19 12:52:31 canect2 daemon.warn ModemManager[302]: <warn>
> Couldn't load Operator Name: 'Cannot run sequence: 'Could not open serial
> device ttyACM0: it has been forced close''
> ...
> 
> Any chance you can get a debug output of MM while reproducing this issue?

I will try to run with debug turned on, but it does not happen all the time.  I will send an update when and if I can get it to happen with debug turned on.

Best regards,
Matthew Starr