So far we have only created /dev/modem based on the DATA5_CNTL
channel available from the modem. The DATAX_CNTL channels allow
sending QMI messages to the modem. This is sufficient for oFono
to work.
Unfortunately, ModemManager currently does not support starting
calls through the QMI interface. Instead, it uses serial AT commands
that can be alternatively used on all USB-based Qualcomm modems.
It turns out that we can also send serial AT commands through
the RPMSG interface: the DATAX channels (without _CNTL) all respond
to serial AT commands. We set it up at /dev/modem-at, configure
ModemManager accordingly and then we are able to start calls. Yay!
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