Re: [PATCH v2 0/3] mac80211 suspend/resume

[Dan Williams <dcbw@redhat.com>]

On Tue, 2009-01-06 at 18:12 +0100, Marcel Holtmann wrote:
> Hi Dan,
> 
> > > > > > Running pm-suspend from pm-utils directly also triggers the problem, 
> > > > > > so that would seem to excuse gnome-power-manager at least.
> > > > > 
> > > > > What's the status of this? Should I look into things a bit?
> > > > 
> > > > Well, I guess I should have noticed this a lot earlier, but anyway the 
> > > > problem was pm-utils on Fedora 10:
> > > > 
> > > > /usr/lib/pm-utils/sleep.d/55NetworkManager:
> > > > 
> > > >     suspend_nm()
> > > >     {
> > > >         # Tell NetworkManager to shut down networking
> > > >         dbus-send --system                         \
> > > >             --dest=org.freedesktop.NetworkManager  \
> > > >             /org/freedesktop/NetworkManager        \
> > > >             org.freedesktop.NetworkManager.sleep
> > > >     }
> > > > 
> > > > I really don't think this is necessary (g-p-m will also do it if you 
> > > > set the proper gconf setting.)
> > > 
> > > this should not be needed at all. I have systems running wpa_supplicant
> > > and not any of the pm-utils scripts messing with it. During suspend and
> > > later resume it indicates normally just only a new handshake with the AP
> > > or a disconnect if the AP got out of range.
> > > 
> > > I think Network Manager is perfectly capable of handling state changes
> > > from wpa_supplicant. I really do think that this hack only exists of
> > > some broken drivers from really old kernels or for the 0.6 version of
> > > Network Manager. Remember that Ubuntu's suspend/resume solution used to
> > > be to unload all networking drivers on suspend.
> > 
> > You still want to tell NM to go to sleep so it doesn't see the
> > disconnection from the supplicant (triggered by the driver because it
> > was going to sleep), and thus try to reconnect, or try a different AP.
> > Ideally NM would simply listen for signals from some power service such
> > that we wouldn't have to have this hack, but there isn't a global power
> > service yet on the system bus.
> > 
> > Furthermore, it's nice to know if we've gone to sleep or not so that we
> > can do some optimizations on wakeup to find APs and reconnect faster.
> 
> actually the fastest way to re-connect is to just let wpa_supplicant do
> it and then you don't even have to go through DHCP again if the lease
> time is still valid. This works great if the AP is still in range.
> 
> What is your downside with the letting wpa_supplicant send you a
> disconnect when the AP is out of range after resume?

Depends on the driver what the resume behavior is with the supplicant.
But you want to alert *something* that the system is now going to sleep,
so that it can clean up state before doing so.

The problem is that you have absolutely no idea how long the sleep will
be.  It's at least 2 minutes with S2D, because that's how long it takes
to write your state out to disk.  It's less with S2R obviously, but when
you resume, there's no guarantee that you'll be in the same place.  You
cannot assume that you will be.

If you keep trying to reconnect to the same AP, many times you simply
won't be there and you'll spend the 10 or 20 seconds reconnecting to an
AP miles away, time that could have been spent scanning for the AP
that's *really* where you are.  Many people I know don't often suspend
at the same location they resume, thus trying to reconnect hurts
reconnection latency.

The ideal way to handle all this is to be *aware* of suspend/resume in
the supplicant (or NM), and on resume, do a quick probe-scan or two to
find out if the old AP is still around.  If it's not, do a full scan to
find all APs, and then pick the best one, which is probably not the one
you were associated with before.  Then you actually start the auth/assoc
process with an AP you know exists.

So my point is that *something* needs to be aware of the suspend/resume
at a userlevel, whether it's NM or the supplicant doesn't really matter
as long as you can do what I describe above on resume.

Dan