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

[Dan Williams]

On Wed, 2015-10-28 at 12:56 -0500, Dan Williams wrote:
> On Wed, 2015-10-28 at 17:04 +0100, José wrote:
> > Hi, I am working with a Sierra MC7710, and this is a output example from
> > ModemManager:
> 
> ...
> 
> > 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.
> 
> In any case, I just tried setting various modes with my UML290 using
> qmicli, and it allows setting various combinations:
> 
> cdma-1x|cdma-1xevdo
> gsm
> umts
> lte
> gsm|umts
> gsm|umts|lte
> cdma-1x|cdma-1xevdo|lte
> gsm|umts|lte|cdma-1x|cdma-1xevdo
> 
> The UML290 does not support these combinations:
> 
> gsm|lte
> umts|lte
> gsm|umts|cdma-1x|cdma-1xevdo
> gsm|lte|cdma-1x|cdma-1xevdo
> gsm|cdma-1x|cdma-1xevdo
> 
> But with a Huawei E392 (which has no CDMA/EVDO), I *can* set umts|lte
> and gsm|lte.
> 
> As a workaround, you can poke the device with:
> 
> sudo qmicli -p -d /dev/cdc-wdm0
> --nas-set-system-selection-preference="<mode>"
> 
> where <mode> is a combination like what I've listed above.
> 
> -----

I hit send too early...

The problem is that we can't ask the modem what mode combinations it
supports, which is probably why we hardcoded it in the first place.  I
suppose we should update the list to allow more combinations:

> 
>       *     (5) If current capabilities [GSM/UMTS + LTE]:
> +     *       [2G only]
> +     *       [3G only]
> +     *       [4G only]
> +     *       [2G + 3G]
> +     *       [2G + 4G]
> +     *       [3G + 4G]
>       *       [2G + 3G + 4G]
>       *
>       *     (6) If current capabilities [CDMA/EVDO + LTE]:
> +     *       [4G only]
> +     *       [2G + 3G]
>       *       [2G + 3G + 4G]
>       *
>       *     (7) If current capabilities [GSM/UMTS + CDMA/EVDO + LTE]:
> -     *       [2G + 3G + 4G]
> +     *       [4G only]
> +     *       [2G + 3G + 4G] (all 3GPP and 3GPP technologies)

Here, even though modems may allow 2G/3G/2G+3G for a specific 3GPP
family, ModemManager cannot express that, and thus we cannot allow 2G+3G
for these multi-mode devices.

Dan

>       */
> 
> 
> Dan
> 
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