From: Helmut Schaa <helmut.schaa@googlemail.com>
To: Johannes Berg <johannes@sipsolutions.net>
Subject: Re: Scan while TX/RX'ing a lot of data
Date: Wed, 20 May 2009 15:43:11 +0200
Cc: "Luis R. Rodriguez" <mcgrof@gmail.com>,
	Dan Williams <dcbw@redhat.com>,
	"linux-wireless" <linux-wireless@vger.kernel.org>,
	Aeolus.Yang@atheros.com,
	Senthil Balasubramanian <senthilkumar@atheros.com>,
	Gaurav.Jauhar@atheros.com, David Miller <davem@davemloft.net>,
	Jouni Malinen <j@w1.fi>
References: <43e72e890905141052o1f072bc5m4bc5922327617f8b@mail.gmail.com> <200905201341.47185.helmut.schaa@gmail.com> <1242819888.19216.6.camel@johannes.local>
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Am Mittwoch, 20. Mai 2009 schrieb Johannes Berg:
> On Wed, 2009-05-20 at 13:41 +0200, Helmut Schaa wrote:
> > Am Dienstag, 19. Mai 2009 schrieb Johannes Berg:
> > > On Mon, 2009-05-18 at 15:43 +0200, Helmut Schaa wrote:
> > 
> > > > One solution would be to force all drivers to report the tx status. Another
> > > > one would be to just wait until the device's queue is empty. At that point
> > > > we know that all pending data frames were sent out and additionally the
> > > > nullfunc frame was also sent out, so we can safely switch to the next
> > > > to-be-scanned channel.
> > > 
> > > Some drivers now flush queues at channel switch time, but there's no way
> > > to guarantee that the software queues are actually empty and the
> > > nullfunc frame isn't queued up behind other traffic. Also, waiting for
> > > any driver to report a status will lead to problems because the driver
> > > might just have dropped the frame for various reasons like being out of
> > > memory.
> > 
> > Hmm, the problem is basically (from a mac80211 perspective) that the
> > mdev tx queue could still contain data frames when the nullfunc frame
> > gets queued, right? And we do not assure that these frames are sent
> > out before the channel switch.
> > 
> > Hence, would it be possible to:
> > 1) Stop all sub_if tx queues (afterwards no new data frames should
> >    appear in the mdev tx queue)
> > 2) Queue the nullfunc frame
> > 3) Flush the mdev's tx queue
> > 4) Switch the channel
> > ?
> 
> I don't think that is sufficient,

Maybe not sufficient, but it's at least an improvement.

> unless the driver also flushes the 
> hardware queue at channel switch time.

Yes, but that would be the drivers responsibility and could be
fixed in a second step.

> Which we may want to make explicit with a new callback or so?

And make it mandatory?

Helmut