Re: The meaning of Enable in MM/NM world of mobile broadband?

[Dan Williams <dcbw redhat com>]

On Mon, 2011-08-01 at 09:26 +0100, Andrew Bird (Sphere Systems) wrote:
> Hi all,
> 	I'm trying to understand the meaning of 'Enable Mobile Broadband' from 
> the NM applet menu. I have a few questions:
> 
> Is the NM applet Enable item just a direct control of the WWAN devices's radio 
> function?

Not necessarily.  Enable means "power up the modem and initialize it so
that it is able to be connected at some point in the future."  Before
the modem is enabled, it may not be powered up, it may not be registered
with a network, it may not send/receive SMS, etc.  Basically, before
anything can be done with the modem beyond detecting that the modem
exists and whether the modem needs a PIN, the modem must be enabled.

> 	a) if so, what happens at startup, is the modem's radio functionality 
> checked and the Enable checkitem state set accordingly, or is the modem's 
> radio forced into some default/remembered NM/MM derived state?

ModemManager starts all modems in disabled state, and NetworkManager
should not be automatically enabling modems; that's left to the user
because at the moment, most people won't be using their modems full-time
and probably don't want them to suck down power when they aren't
actually being used.  It's up to the connection manager (NM or something
else) to determine whether or not to enable the device, and right now NM
requires the user to explicitly make that decision.  If you're using an
embedded platform, whatever tells MM to connect probably wants to enable
the modem when that thing starts.

> 	b) if so, what happens if we have two modems (perhaps one is embedded and 
> the other external) and their radios are in different states, or need to be?

MM allows both modems to have different state.  With NetworkManager
however, because there isn't a separate rfkill for each WWAN device,
there's a single composite state just like there is for WiFi.  If any
modem is disabled, the checkbox (and NM's "WwanEnabled" state) will be
FALSE.  If the user checks the box (or something sets WwanEnabled to
TRUE) NM will attempt to call MM's Enable() method for all devices.

I'm somewhat unclear on use-cases where multiple modems would be
connected to a system under normal conditions where both wouldn't be
able to be enabled at the same time.  Thoughts?

> 	c) if so, does MM prefer to use +CFUN=0  or +CFUN=4 to Disable the modem?

That's left to the modem plugin.  CFUN=0 often has the unfortunate
side-effect of actually powering off the device so it's not used on many
devices.  CFUN=4 is unfortunately unimplemented on a wide range of
devices, but if it is available we should be using it.  Patches for that
would be accepted; we can check CFUN=? on startup and then store the
returned list of available modes and use that to determine whether to
set CFUN=4 on disable.

> 	d) if not, what function does Enable on the applet perform?

Not sure what you're asking here...

> 	e) if not, is there another way to turn off the radio, perhaps an Aircraft 
> Mode?

That is current calling Enable(FALSE) for the modem on the MM DBus
interface, which is supposed to place the modem into low-power mode.
RFKILL/Aircraft mode isn't what we want to do here, because that is
easily confused with the physical button that many laptops have, which
often just disconnects the device from the USB bus completely.  Which
means MM can't see it anymore because it doesn't exist.  So I think just
having the device disabled via the equivalent of CFUN=4 is sufficient,
and leave actual airplane mode/rfkill to the BIOS, if any.

Dan

> 
> Thanks,
> 
> 
> Andrew
> 
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