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Date: Wed, 7 May 2014 10:51:55 +0200
Message-ID: <CA+BoTQkuHOXS92WOtNfRAu01n66CTpOdwTVNHHoa7=SfaFE=-g@mail.gmail.com> (sfid-20140507_105203_277181_AE42ADE0)
Subject: Re: [PATCH v5] mac80211: implement multi-vif in-place reservations
From: Michal Kazior <michal.kazior@tieto.com>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: linux-wireless <linux-wireless@vger.kernel.org>,
	Luca Coelho <luca@coelho.fi>,
	Simon Wunderlich <sw@simonwunderlich.de>
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Sender: linux-wireless-owner@vger.kernel.org

On 7 May 2014 10:07, Johannes Berg <johannes@sipsolutions.net> wrote:
> On Wed, 2014-05-07 at 08:05 +0200, Michal Kazior wrote:
>
>> Hmm... Now that I think about the atomic swap - it actually becomes a
>> little bit of an issue in some cases.
>>
>> For one you might need to overcommit number of chanctx since swapping
>> requires both chanctx (old and new) to exist but that's the least of
>> the eproblem. If you have more than one interface you end up with
>> temporarily breaking interface combinations from driver point of view
>> while switching (first swap breaks it, last swap fixes it). Driver
>> won't know whether given swap is first/last unless we somehow pass it
>> through the switch_vif_chanctx(). IOW we actually need a "chanctx
>> transaction" (sort of a start-stop) that can batch up a couple of
>> chanctx switches for different vifs as an atomic op.
>
> Hmmm. Don't you already have that problem? Or you don't because you'd do
>
> for_each_affected_vif: unassign
> del chanctx [optional depending on reservation]
> add chanctx [ditto]
> for_each_affected_vif: assign
>
> right now?

Correct - this is how my patch deals with this problem. This was also
the reason why I split the new_chanctx() into alloc_chanctx() and
add_chanctx().


Michał