GPRS support for Ofono

[Denis Kenzior]

Hi Remi,

> > The core only reports these attributes, it doesn't actually use them for
> > anything.  When attached you can simply send a status=registered, LAC,
> > CellId and Tech set to -1.  We can see about removing these a bit later.
>
> I would do that, but there is no apparent way to return the attached state
> without making a statement on roaming as well. IOW, this is unimplementable
> with ISI.

Don't worry about this for now.  The roaming of CGREG is never checked, only 
CREG roaming state is checked.  You can safely return status=registered here.

>
> > > That is different. If you are outside of coverage, or in a non-GPRS
> > > area, you can turn auto-attach on. That will make some operators happy
> > > but it wont't attach just yet.
> >
> > Sorry I don't see a practical difference.  You set 'Powered' to TRUE,
> > gprs atom issues a 'set_attached' with TRUE.  Once you have attached the
> > driver sends out a ofono_gprs_status_notify.  Whether this happens in 1
> > second or 2 hours doesn't really matter.
>
> Again and again, auto-attach and attach are different things. Setting
> attach to false will forcefully deactivate all existing contexts and detach
> the terminal from GPRS. Setting auto-attach to false will cause the
> terminal to detach after all pending contexts are deactivated, but won't
> cause them to be detached immediately (AT+CGATT does that).
>
> Conversely, enabling auto-attach will cause the terminal to attach once it
> has GPRS coverage, whereas false will have it wait for an explicit attach
> request, or implicitly for a context activation. This is dkifferent from
> attaching which is an immediate action and will fail if GPRS is unavailable
> (like AT+CGATT).
>

Again, I don't see a point of exposing this to the user.  Worst case we can 
always add intelligence / heuristics to oFono to attempt re-attaching 
periodically.  We have to solve this for all modems, since there are no such 
'auto-attach' semantics in 27.007.

One strategy you can use today is to just activate auto-attach when the driver 
is asked to attach and force a detach when the driver is asked to detach.  
This is essentially the behavior of most AT command modems.

> Furthermore, when attaching, the terminal is supposed to state its intent
> to activate a context immediately or not. If it does, then some
> optimizations are possible in 3G case.

I have two suggestions here:
1. Lets make it work first, we can then make it work better.
2. Can you describe these optimizations and suggest how the API/behavior can 
be changed to take advantage of them?

Regards,
-Denis