How to recover from serial device being force closed.

[Jessy Exum]

Hello ModemManager devs,

I have encountered an issue while running ModemManager (version 1.18.4) on
a Cinterion PLAS8 modem. *Sometimes, the serial device used for AT
communication with the modem is force closed (due to G_IO_HUP) without any
obvious reason.*

Despite the modem being unreachable over the dead connection, the modem
remains listed in mmcli (without being marked as having an error), and any
ModemManager command that would send an AT command to the modem fails with
an error similar to:

GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Connected: Cannot
run sequence: 'Could not open serial device ttyACM1: it has been
forced close'


When I first saw this issue, I thought it was the modem acting up (it would
not be the first time), but I found that restarting ModemManager recovers
the modem. This tells me that the modem was still functioning, and
ModemManager was unable (or never tried) to re-open the channel. I have not
had this error occur on a device with ModemManager debugging enabled, so I
can not be sure if reconnection is attempted (I will try to find a way to
manually trigger this issue so I can get better debugging).

If I could reset the modem, that should cause it to be re-initiated in
ModemManger and likely fix the issue. But as far as I know, there is no way
to have ModemManager re-enumerate a modem without removing the device
(physically, through a modem reset, etc). Unfortunately, I am also unable
to recover the modem through ModemManager because a modem reset requires
the ability to send AT commands to the modem. Currently, the only solutions
I have found to recover the modem is to physically power cycle the modem
(which is not always possible), or to stop and restart ModemManager
(inconvenient if there are other modems). I may also be able to use an
external script to detect ModemManager getting into this state, and
directly send a reset command to the modem over the AT channel (without
ModemManager), but I would prefer if ModemManager was able to recover this
on its own.

The error occurs rarely/randomly enough that I am not interested in chasing
down why this happens (it could be the modem being flakey, electrical
interference, whatever). I am more interested in being able to gracefully
recover from this situation, so *I want to know what ModemManager should be
doing so I can try to help fix it.*

Any suggestions or information would be greatly appreciated.

Thanks for all the hard work in this useful tool,
Jessy Diamond Exum
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