Re: Prevent auto scan in wireless devices

[Dan Williams <dcbw redhat com>]

On Tue, 2009-01-13 at 17:45 -0300, Aloisio Almeida wrote:
> Hi all,
> 
> I noticed that wireless devices are always scanning, and this is very
> bad to power consuption in embedded systems.

For most parts these days, scanning does not consume much power, and if
you scan passively, it will consume even less because the TX blocks of
the chip don't need to be turned on.  Can you get some quantifiable
power measurements on whether scanning (for both active and passive)
*really* make a large difference here?  Chips are good enough these days
that scanning doesn't really draw more power than having the chip awake.

> I would like to create a way to prevent automatic scan and just
> perform it when some cliente ask for it.
> Is it possible to do this? I mean, does it "brake" in some way the nm structure?

Is there some reason other than power saving that you'd like to do this?
There's quite a few reasons to make scanning periodic anyway.  The
largest is that if the connection drops, your AP roaming latency is
*huge* because you have to do a completely new scan to find out what AP
you want to go to next, whereas if there's been a recent scan, you can
just select one based on that list.  That's a problem even in embedded
environments, even within the same SSID.  If you don't tell the chip to
scan, you can't necessarily find other APs in the same SSID, thus your
roaming performance may break.

> Actually, I already did this patch to 0.6.6 version, but zero lines
> applied in new code :) Now i would like to create the patch and submit
> to upstream.
> 
> The basic idea is just make can_scan function (src/nm-device-wifi.c)
> return FALSE due to some user configurations or run flags
> (--no-bg-scan). In this case, "performScan" dbus method and
> "ScanPerformed" dbus signal must be created to allow clients to ask
> for a scan and to notice that the scan has been performed.
> 
> I'm attaching the 0.6.6 patch, as I said before the idea is the same.

I'd rather figure out if the functionality is really needed, ie if the
power savings are really worth it first.

Dan