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

[Aleksander Morgado]

On Wed, Oct 28, 2015 at 10:45 PM, Dan Williams <dcbw at redhat.com> wrote:
> On Wed, 2015-10-28 at 20:06 +0100, Aleksander Morgado wrote:
>> 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.
>
> My read of the code indicates that we get the same behavior with USB
> devices like the UML290 and such too though, right?  Which don't have
> the same restriction as the MC77xx devices.  eg this code block in
> modem_load_supported_modes():
>
>     /* LTE and others, only allow to have all, no further preference */
>     else if (mm_iface_modem_is_3gpp_lte (_self)) {
>         /* 2G, 3G and 4G */
>         mode.allowed = (MM_MODEM_MODE_2G | MM_MODEM_MODE_3G |
> MM_MODEM_MODE_4G);
>         mode.preferred = MM_MODEM_MODE_NONE;
>         g_array_append_val (combinations, mode);
>     }
>
> obviously gets run for an LTE-capable modem, even ones that can change
> modes around unlike the 77xx devices.

You're right indeed. When LTE is involved we fix the supported modes.
Just checked it with my MC7304:
    Modes    |      supported: 'allowed: 2g, 3g, 4g; preferred: none'

Not sure why I though it was a MC7710 only thing... bad memory :/

-- 
Aleksander
https://aleksander.es