[RFC] QMAP data aggregation support in ModemManager

[Aleksander Morgado]

Hey all!

I've been working on updating ModemManager to allow setting up QMAP
data aggregated links transparently, either with qmi_wwan
add_mux/del_mux or with qmi_wwan+rmnet (pass_through=Y).

See https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/447

For the qmi_wwan+rmnet logic this MR in libqmi to disable the MAP
checksum seems to be required, I don't fully understand why yet:
https://gitlab.freedesktop.org/mobile-broadband/libqmi/-/merge_requests/215

For the qmi_wwan add_mux/del_mux logic, it works quite well without
any additional thing required, with the limitation of max 4 links for
now, because we cannot create new links once the master interface is
already up (so we always precreate 4 link interfaces upon the first
connection attempt). The MTU of the master link is set to the max data
aggregation size reported by the modem in this case.

What do you think of these changes? For now, I've made it the default
to "multiplex=request" when setting up a connection, so that you can
directly use e.g. NetworkManager to bring up the connection without
any single change in NetworkManager itself. That may change later on,
and we may leave the default as "multiplex=none" instead; comments
welcome on that as well.

Anyone wants to give all this a try? Comments?

-- 
Aleksander
https://aleksander.es