ModemManager randomly uses AT instead of QMI interface on Quectel EG25G

[Riku Voipio]

Hi,

I have a problem where rarely, but randomly ModemManager choosest to make
the the connection over the AT rather than the QMI interface. A failed one
looks on the log like:

-snip-
<info>  ModemManager (version 1.16.10) starting in system bus...
systemd[1]: Started Modem Manager.
[/dev/cdc-wdm0] Opening device with flags 'version-info, proxy'...
[/dev/cdc-wdm0] created endpoint
cannot connect to proxy: Could not connect: Connection refused
spawning new qmi-proxy (try 1)...
cannot connect to proxy: Could not connect: Connection refused
spawning new qmi-proxy (try 2)...
[/dev/cdc-wdm0] Checking version info (45 retries)...
[/dev/cdc-wdm0] QMI Device supports 27 services:
[/dev/cdc-wdm0]    ctl (1.5)
[/dev/cdc-wdm0]    wds (1.67)
[/dev/cdc-wdm0]    dms (1.0)
[/dev/cdc-wdm0]    nas (1.25)
[/dev/cdc-wdm0]    qos (1.12)
[/dev/cdc-wdm0]    wms (1.10)
[/dev/cdc-wdm0]    auth (1.3)
[/dev/cdc-wdm0]    at (1.2)
[/dev/cdc-wdm0]    voice (2.1)
[/dev/cdc-wdm0]    cat2 (2.24)
[/dev/cdc-wdm0]    uim (1.46)
[/dev/cdc-wdm0]    pbm (1.4)
[/dev/cdc-wdm0]    test (1.0)
[/dev/cdc-wdm0]    loc (2.0)
[/dev/cdc-wdm0]    sar (1.0)
[/dev/cdc-wdm0]    csd (1.0)
[/dev/cdc-wdm0]    ts (1.0)
[/dev/cdc-wdm0]    tmd (1.0)
[/dev/cdc-wdm0]    wda (1.16)
[/dev/cdc-wdm0]    csvt (1.1)
[/dev/cdc-wdm0]    coex (1.0)
[/dev/cdc-wdm0]    pdc (1.0)
[/dev/cdc-wdm0]    rfrpe (1.0)
[/dev/cdc-wdm0]    dsd (1.0)
[/dev/cdc-wdm0]    unknown [0x2d] (1.0)
[/dev/cdc-wdm0]    unknown [0x30] (1.0)
[/dev/cdc-wdm0]    unknown [0x36] (1.0)
<info>  [device HomeModem] creating modem with plugin 'quectel' and '1' ports
<warn>  [base-manager] couldn't create modem for device 'HomeModem': Failed to find a net port in the QMI modem
<info>  [device HomeModem] creating modem with plugin 'quectel' and '5' ports
-snip-

The successful connections instead look like:

-snip-
<info>  [device HomeModem] creating modem with plugin 'quectel' and '6' ports
<info>  [base-manager] modem for device 'HomeModem' successfully created
-snip-

Any hints why this happens and howto prevent it? Due to random nature
it's hard to catch with debug on.

Cheers,
Riku