Subject: Re: Power saving on mac80211
From: Johannes Berg <johannes@sipsolutions.net>
To: Kalle Valo <kalle.valo@nokia.com>
Cc: "Luis R. Rodriguez" <mcgrof@gmail.com>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	Zhu Yi <yi.zhu@intel.com>
In-Reply-To: <87y70l59p4.fsf@nokia.com> (sfid-20081019_092443_483129_2042AF58)
References: <43e72e890810171810t1c65642g998db601cfe2a390@mail.gmail.com>
	 <87bpxh6t1n.fsf@nokia.com>  <87y70l59p4.fsf@nokia.com>
	 (sfid-20081019_092443_483129_2042AF58)
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Date: Sun, 19 Oct 2008 12:26:46 +0200
Message-Id: <1224412006.6324.45.camel@johannes.berg> (sfid-20081019_122716_043792_5F4E48FD)
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On Sun, 2008-10-19 at 10:24 +0300, Kalle Valo wrote:

> > Nokia is very interested about client side PSM. Currently it's only
> > on/off type, but I would like to see a dynamic version of this there
> > the PSM will be enabled when the data path has been idle long enough
> > (eg. 200 ms). I will be sending patches later.
>=20
> Oh, I forgot about beacon filtering. stlc45xx firmware (but no support
> in driver yet) supports beacon filtering in PSM mode, meaning that the
> beacons are not delivered to the host (not even DTIM beacons). This
> improves power consumption because the host CPU is not woken up all
> the time.
>=20
> I'm sure that stlc4560 isn't the only chip supporting this feature
> because it's quite essential in low power devices. I have been
> planning to implement support to mac80211 in the near future. Though
> don't know yet what is needed, I haven't investigated this at all.

I thought about it recently, it's fairly easy.

As a preliminary step, I'd change mac80211 to not trigger the timer
continuously, but rather reset it ever time we get a beacon from the AP,
so that the timer only fires when we haven't seen a beacon for too long.
This helps with powersave already because then we only wake up for
beacons, not for the timer too.

Then add a new flag to mac80211 hw description that says "driver will
watch beacons". This means that the code above that rearms the timer for
the future 'nothing from AP' check is skipped.

Also add a way for mac80211 to tell the driver after how many beacons
(or calculate the timeout in ms, it's convertible) it should tell
mac80211 about the situation. Finally, to make it possible to have the
driver tell mac80211, extend the MLME callback (ieee80211_notify_mac)

johannes

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