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Re: [PATCH v4 2/5] mac80211: use chanctx reservation for AP CSA

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On 7 May 2014 11:07, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
> On Wed, 2014-05-07 at 11:05 +0200, Michal Kazior wrote:
>
>> > I don't think that works for all drivers - only drivers that actually
>> > generate and tx each beacon - but other drivers just update a template.
>> > We actually have some pending patches to make that work correctly for
>> > the CSA counters, but I'm not really sure what you want to happen in the
>> > case that one is switching while the other hasn't yet? Should it stop
>> > beaconing? Seems like clients would give up the switch then, no?
>>
>> Ah, I completely forgot about the template-based approach.
>>
>> But I don't think any beacons should be sent after cs_count=1 on the
>> old channel either way. Thus, I would expect template-based drivers to
>> stop beaconing after cs_count=1. cs_count=0 is a separate case as it
>> means "i can disappear at any time now" and is a special case, not
>> what happens after reaching cs_count=1 if I remember correctly.
>
> I'm not really sure how this would work though - drivers that have
> firmware generating beacons don't generally seem to be in a position to
> just stop that aspect and continue operating otherwise, with active
> clients and all that. It seems to me this might be really difficult to
> implement.

Yeah. This seems like a problem for template drivers. But on the other
hand if they are capable of handling CSA IE counter they should
understand CSA, shouldn't they?


>> Clients should expect next beacon to appear on the new channel after
>> seeing beacon with cs_count=1. In that case regular cqm for clients
>> should apply.
>>
>> Real-world multi-vif CSA will probably be off by up to 1 or 2 beacon
>> intervals between vifs which should be disruptive for clients I guess.
>
> "not be disruptive"?

Yeah. Typo.


> Remind me why we're doing this - didn't you originally have something in
> mind that was forcing them all to be synchronized with the switch?

No, not really.

With per-vif CSA requests you can end up submitting requests between
vif TBTTs, no? Having an atomic "switch vif1,vif2,vif3 to chanX" could
help - although currently ieee80211_beacon_get() isn't really
synchronized well so you still hit the problem.

And how do you expect to deal with multi-vif with APs with different
beacon intervals? Those inherently can't be tightly synchronized. The
same goes for AP+STA if you start AP first (with the other way around
you could, in theory, match STA's AP TBTT with your own).


Michał
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