GPRS support for Ofono

[Rémi Denis-Courmont]

> 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.

> > 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). 

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.

Rémi