Hi Alex,

 >
> It looks as though the issue is that the Cinterion parts don't support
> signal strength via +CIND ?

Yes, we look for "signal" indicator inside cind_support_cb.  If it isn't 
found, then we raise an error.  This tells the hw integrator to address 
the issue.  CIND logic is provided as a fallback / default since many 
manufacturers support this indicator for HFP.

>
> Yet in at_signal_strength() it looks to me as though if this is the case
> the code will fall back happily to using +CSQ

signal_strength (and hence +CSQ) is used to bootstrap the signal 
strength value.  This driver method is only called at very specific 
times.  The core does not poll signal_strength.  It is expected that the 
driver will send signal strength value to the core periodically, by 
whatever means is optimal for the hardware.  Most modems use a custom 
unsolicited notification or CIND to provide information about signal 
strength automatically.  I suspect Cinterion has a similar extension.

>
> That being the case I'm not sure why cind_support_cb() should be
> reporting an error and removing the device when it should be able to
> fallback and continue?

The consequence of the above is that it can't.  If no vendor extension 
is available for unsolicited notifications and signal strength is not 
provided via CIND, then the netreg atom driver can either poll signal 
strength manually or simply not provide any signal strength updates. 
For the latter, it must enable such behavior explicitly.  Hence why your 
patch providing OFONO_VENDOR_CINTERION logic makes this work properly.

Hope that made sense.

Regards,
-Denis