Initial signal quality query on MBIM modems

[Ben Chan]

Hi Aleksander and Dan

ModemManager currently relies on unsolicited MBIM_CID_SIGNAL_STATE
notification to obtain signal quality updates, and it doesn't query the
initial signal quality. I've observed that some MBIM modems issue a
MBIM_CID_SIGNAL_STATE
notification only when there is a notable change in RSSI. The signal
quality may remain at 0 for quite some time. It's more noticeable when
simply restarting ModemManager after the modem has been initialized and
enabled once.

We could simply enable periodic signal quality polling on an MBIM modem,
but that's less ideal as it may unnecessarily wake the modem up from USB
selective suspend (unless we use a much longer polling period).

I'm thinking about letting ModemManager issue an initial
MBIM_CID_SIGNAL_STATE query when it enables MBIM_CID_SIGNAL_STATE
notification.  Does that sound reasonable to you. If so, I'll submit a
patch.

Thanks,
Ben
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