Re: Prevent auto scan in wireless devices

[Aloisio Almeida <aloisio almeida openbossa org>]

On Wed, Jan 14, 2009 at 5:54 PM, Dan Williams <dcbw redhat com> wrote:
> On Wed, 2009-01-14 at 17:35 -0300, Aloisio Almeida wrote:
>>
>> The main question is why lead to user the responsibility to save power
>> if (in my point of view) nm can do it automatically. The "power
>
> It shouldn't be up to the user, the system should be optimized for
> maximum power saving already.  This means that if the wifi isn't being
> actively used, the system shouldn't be powering the card on.  That
> should be transparent to the user and requires no changes to
> NetworkManager.

Ok, so you're saying that the user or system should take care about
the power saving. In this case we should have a "power saving daemon"
turning on/off the wireless card following some policies. Some
requirements of this daemon:

1. In boot time, if the daemon want to keep my wireless card off, nm
shouldn't turn it on when launching.
2. To turn off automatically the wireless card after 2 minutes
disconnected (a example of power saving rule) the daemon should get
the "wireless connection disconnected" information by listen a nm
signal.

I mean, this daemon needs information that nm has and this daemon does
operations on devices that we suppose to leave nm manage. So, why
don't we make nm manage these things based on "power saving mode" set
by system/user configurations? Why should we leave 2 different
applications to control the wireless card power?

>> saving" mode could be activated by user, by default or by an event
>> came from power system management and it can prevent the system to
>> waste power. Think as a embedded system user, you want a device that
>> its default behavior is always save power.
>
> Again, is scanning while associated *actually* wasting power?

I've never tested this, my guess is: it doesn't

> If the user isn't using wifi, then the chip doesn't need to be
> turned on, and NetworkManager will ignore it and not scan.

As I said before, in my point of view, we should leave just NM handle
the power of wireless cards.

>> As I said before, "no scanning" was the first idea to save power, it's
>> not the main goal. I want a device that has a huge battery life. Turn
>> on features only when asked by user is a good way. NM ( the wireless
>> manager) can do this turning the wireless card off after some time
>> disconnected and turn it on when user ask for it by user.
>
> Yep.  You can certainly do this with a "turn wifi on" / "turn wifi off"
> sort of thing in the UI, which most embedded devices have anyway.  There
> are other ways to achieve the same functionality without having that
> sort of user choice either.
>
> But at the end of the day, if you want to save power, you'll want to
> turn off the wifi chipset when the user isn't going to use it.
>
>> I prefer to loose 2 or 3 seconds to get the first list of available
>> APs than loose battery life during the X minutes (or hours) scanning
>> and keeping the wireless card on. In desktops it doesn't matter, in
>> notebooks it maybe doesn't affect so much, but in a embedded system i
>> already noticed that it matters...
>
> Again, does it *really* save power to not scan when the wifi is already
> active?

Maybe my sentence was a little bit confusing, let me clarify. I am not
saying that "not scan when the wifi is already active saves power" I
am saying that when I don't want to connect to wireless network i also
don't want to waste power keeping my wireless card on.

And yeah, using Nokia ITs (N800, N810) running Mamona distribution
(with nm) is VERY easy to notice that with wireless card on, the
battery life goes down deeply. Maybe the driver and/or device is not
that good, this is a thing that I need to check. But anyway wireless
is a RF device, so it consume very much power for a embedded device.