Faster resume probing

[Dylan Van Assche]

Hi everyone,

I have been working on the Quectel EG25-G modem in the Pine64
PinePhone. This modem is connected over USB with the host CPU and
communicates using AT commands and QMI. ModemManager works fine on the
PinePhone when the phone is awake, however, ModemManager's probing
approach when resuming is rather slow if we have an incoming call. Most
of the time, the modem rings several times already before ModemManager
starts to notice this when waking up from sleep (this is not the case
when the device is awake when a call is received). Therefore, I already
digged into ModemManager and tried to improve this, as discussed in
[1].

Thanks to the logs of ModemManager, I could identify several places
where we lose some time during the resume sequence of a suspend/resume
cycle:

1. ModemManager waits a fixed amount of time to make sure that all
ports of a modem show up using udev, losing ~1.5s
2. The EG25-G modem contains ~20 carrier configs which are read one-by-
one by ModemManager during the initialization phase which takes ~1.5s
3. Order of service initialization: the voice interface for handling
calls is initialized at the very end, while location, time, etc. are
initialized first. ~500ms
4. Many other small things, each saving several 100ms.

In total, it takes ~6.5s from waking up until the incoming call is
announced on DBus.

The resume sequence can probably be optimized in other places as well,
but my knowledge of the ModemManager code is rather limited. Because of
this, I would like to have some input from everyone on this mailing
list about this problem. All suggestions are welcome!

Kind regards,
Dylan Van Assche


[1] 
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/194