Date: Mon, 31 Jan 2011 16:54:20 +0100
From: Johannes Stezenbach <js@sig21.net>
To: Ivo Van Doorn <ivdoorn@gmail.com>
Cc: Johannes Berg <johannes@sipsolutions.net>,
	linux-wireless@vger.kernel.org, users@rt2x00.serialmonkey.com
Subject: Re: [rt2x00-users] [RFC] rt2x00: Add autowakeup timer for
 receiving beacons while in powersave mode
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On Mon, Jan 31, 2011 at 04:38:16PM +0100, Ivo Van Doorn wrote:
> >>
> >> TODO: Split out mac80211 changes into a separate patch. We also need
> >> to know if we need this check in mac80211 or in the driver. Personally I think the
> >> check belongs in mac80211, but at this time that work has been deferred to the drivers.
> >> However this means that a lot of logic is needed in the driver to check if the correct IE
> >> is available, and then check the value, while mac80211 will obtains that exact IE anyway
> >> during RX processing anyway...
> >
> > Hmm. I've always said that there's no efficient way to do this in
> > mac80211. If you do it close to the beacon, unexpected latency means
> > that we miss -- this can be severe if something else is happening in the
> > system, especially with USB devices. If you do it early before the
> > beacon, then you don't save much power. As a consequence, I don't really
> > like this in mac80211.
> >
> > However, it seems you only added "stay awake after beacon" code to
> > mac80211 that checks the TIM. Assuming the device actually stays awake
> > for multicast traffic by itself that seems like an option, though I
> > wouldn't wait for the next beacon before going to sleep again --
> > wouldn't that happen with the timer anyway?
> 
> The timer is needed to automatically wake the device up in time to receive
> the beacon. When the beacon is received all the driver needs to know if the
> device should go back to sleep again for the next beacon period.
> 
> This is a similar behavior as carl9170 driver, which currently checks for
> pending Mcast/Bcast frames in the RX path just before the frame is send
> to mac80211.

FWIW, this bc/mc check is also on the mac80211 TODO list:
http://wireless.kernel.org/en/developers/todo-list#power_saving


Johannes