MC7455 modem stops responding over QMI and serial

[Aleksander Morgado]

Hey,

> I have a few MC4755 modems connected using the qmi_wwan driver (on linux
> 4.14.171) managed by modemmanager. After some time a few of these modems
> have completely stopped responding to any commands over QMI, not showing
> up in modemmanager. The AT command serial port (/dev/ttyUSB2) and the
> NMEA serial port (/dev/ttyUSB1) are both missing. The only port that is
> available is /dev/ttyUSB0, which seems to be for diagnostic monitoring.
> Nothing is sent over this port, however.
>
> This issue persists after a cold reboot. The qmi_wwan driver doesn’t
> show anything strange in dmesg log. What could be causing this issue?
> Thanks in advance.

The modules you have are probably having runtime firmware failures,
rebooting themselves, and after X reboots the device goes into a
"maintenance mode" with that single ttyUSB0 you're seeing. I assume
this may happen if for example the runtime firmware is considered
corrupted (so it doesn't mind how many times you try to boot it, it
will always fail to boot). If you have Sierra support, it would be
good to contact them so that they guide you on how to debug the actual
issues.

Do you, by any chance, have some logic in your system to "hard reset"
the modem cutting power to the device abruptly, or something like
that? Cutting the power abruptly while the modem is running may lead
to internal filesystem corruptions that could render the device
unusable until it's re-flashed.


--
Aleksander
https://aleksander.es