Re: Thoughts about mac80211 client PS implementation

[Kalle Valo <kalle.valo@nokia.com>]

Johannes Berg <johannes@sipsolutions.net> writes:

> On Wed, 2008-11-05 at 22:54 +0200, Kalle Valo wrote:
>
>> >> PS should be disabled while associated and running software scan, and
>> >> naturally re-enabled after the scan has finished. I assume hardware
>> >> scanning implementations are clever enough to disable PS when scanning
>> >> and we don't have to worry about that case.
>> >
>> > And on that too. And should there be a monitor flag that disables PS, so
>> > that we can "refcount" the PS bit in a way?
>> 
>> Sorry, I don't follow you here. What do you mean by a monitor flag?
>
> Well if you add a monitor interface you may want to turn off PS,

Ok, now I got it. I think your proposal makes sense.

> but I suppose you can just do that on the wlan0 interface you're
> associated on.

Yeah, but that's an extra hassle. In my opinion better to do it in
mac80211.

>> > Why would you queue up frames? To reduce the number of radio wakeups
>> > when TX traffic is low?
>> 
>> Just because I assume that invoke_tx_handlers() cannot sleep but
>> ieee80211_hw_config() sleeps. I didn't think of any other way to solve
>> this, but I haven't thought that much about this. What do you think?
>
> But wouldn't that mean the other way around, i.e. whenever we transmit
> we disable it and then start a timer that re-enables it? Ah, you're
> thinking because TX handlers cannot sleep and the config callback may
> sleep?

Yes, that's what I'm worrying. And I definitely want op_config() to
sleep, it makes my implementation in stlc45xx a lot easier :)

> Hmm. I suppose then we'd have to defer the actual disable PS mode to
> a work struct.

Yes, that's what I was thinking. But now that I have thought about
this more, I think queueing of the frames is not necessary. The first
frames can be sent while in Power Save mode (ie. PSM bit set in Frame
Control) and PS disable can happen later when the work struct is
scheduled. I don't think this being a problem, we just have to be
careful with race conditions.

> Maybe we don't want to disable PS for a single frame though

Actually I think we do. The reason why I'm interested in dynamic PS is
the receive latency (transmit latency minimal is in practise). For
example, let's think about DNS request. In the best scenario only one
frame is transmitted, and if we don't come out receiving the reply to
the dns request will take a long time. If DTIM is 3 and beacon
interval 100 ms, the RTT for the DNS request/reply would be 300 ms.
That's a long delay to a case where user has pressed a link in the
browser and the browser starts to load a web page.

> so how about this:
>
>  * When a frame is transmitted, store the current frame counter (do we
>    have one? we could add one) somewhere and arm a timer to fire N
>    milliseconds in the future.
>  * The timer checks that we have transmitted
>    more than M frames in the time since it was started, and if so queues
>    a work struct to disable PS.
>  * The work function also sets a flags somewhere "dynamic PS has
>    disabled PS" and arms another timer to fire after R millisecinds,
>    that timer will queue work to enable PS again
>  * the TX code, when the "dynamic PS has disabled PS" flag is set, mods
>    the re-enable timer to be R milliseconds in the future again
>
> That way, M frames within a period of N milliseconds will disable PS,
> and it'll still disabled until R milliseconds elapse without traffic,
> and no timers are fired unless they are needed.

I think this sounds good. As I said above, I'm not sure about checking
of M frames, but I'll implement it anyway and test it in practise.

Johannes, thanks a lot for the help, again :)

-- 
Kalle Valo