Force 2G/3G/4G modes (when they are not reported as supported)

[Aleksander Morgado]

On Wed, Oct 28, 2015 at 6:59 PM, Dan Williams <dcbw at redhat.com> wrote:
>> > As you can see, there is only one supported mode (allowed: 2g, 3g, 4g;
>> > preferred: none) which (as far as I know) offers me no contl about
>> > which network technology is being used. When using this modem with
>> > other SIM cards or other protocols (this modem also supports QMI,
>> > direct IP and serial) the supported modes varies.
>> >
>> > My understanding is that the supported modes are provided by the modem
>> > to ModemManager. Which variables affect the supported modes reported?
>> >
>> > Is there a way to force ModemManager to use only 2g/3g/4g despite of
>> > the supported modes reported by the modem?
>>
>> I don't recall why it was hardcoded in MM specifically to disallow mode
>> switching on LTE-capable devices.  Aleksander would probably know more.

ModemManager has 2 settings controlling this behavior: capabilities
and modes. When 4G is enabled in the MC7710 (capabilities
gsm-umts+lte) the only allowed setting is 2G+3G+4G (auto). If you want
to set it as 2G+3G (i.e. no 4G) you'll need to switch capabilities to
just gsm-umts (no lte). Once that is done, the modem will get rebooted
and you'll now see a new list of mode combinations, including "2G
only", "3G only", "2G+3G, 3G preferred" and "2G+3G, 2G preferred".
This behavior is actually very specific to how the MC7710 responds to
our commands (i..e how we compute the list of supported modes and
capabilities). Other modems like the MC7304 may not require the power
cycle to switch capabilities just to get in 2G+3G mode.

This may also give more info:
https://sigquit.wordpress.com/2013/06/03/changing-modes-and-capabilities-in-modemmanager/

As for why we hardcode the values. Well, those are the modes that QMI
supports. The only confusion here is why you didn't get the 2G+3G
modes when 4G was also enabled, which I said I believe is very
specific to the MC7710 if I'm not mistaken.

-- 
Aleksander
https://aleksander.es