Date: Sun, 5 May 2013 22:06:14 +0200 (CEST)
From: Thommy Jakobsson <thommyj@gmail.com>
To: =?ISO-8859-15?Q?Michael_B=FCsch?= <m@bues.ch>
cc: =?ISO-8859-2?Q?Rafa=B3_Mi=B3ecki?= <zajec5@gmail.com>,
	linville@tuxdriver.com, linux-wireless@vger.kernel.org,
	b43-dev@lists.infradead.org, piotras@gmail.com,
	Larry.Finger@lwfinger.net
Subject: Re: [PATCH] B43: Handle DMA RX descriptor underrun
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On Sun, 5 May 2013, Michael B?sch wrote:

> On Sun, 5 May 2013 21:50:33 +0200
> Rafa? Mi?ecki <zajec5@gmail.com> wrote:
> 
> > 2013/5/5 Michael B?sch <m@bues.ch>:
> > > On Sun, 5 May 2013 18:31:20 +0200
> > > Rafa? Mi?ecki <zajec5@gmail.com> wrote:
> > >
> > >> Still worth considering is my previous e-mail. Why writing (for
> > >> example) 1 to RXSTOPINDEX doesn't stop firmware from using slot 1?
> > >
> > > What makes you think this register does not work?
> > 
> > Take a look at this:
> > 
> > [  327.224976] [DBG] old current:5      new current:6
> > [  327.224982] [DBG] reading slot 5
> > [  327.224997] [DBG] writing stop slot 6
> > 
> > In above ring->slot was 5, but IRQ was generated, and we read new
> > "current" using get_current_rxslot. It appeared to be 6. So we read
> > packet from slot 5 and then called
> > ops->set_current_rxslot(ring, 6);
> > AFAIU hardware shouldn't use slot 6, right? But take a look at what
> > happens next:
> > 
> > [  327.319582] [DBG] old current:6      new current:7
> > [  327.319590] [DBG] reading slot 6
> > [  327.319619] [DBG] writing stop slot 7
> > 
> > Hardware generated IRQ and we get_current_rxslot returned 7. It means
> > we're allowed to read slots up to 7 (excluding). It other words it
> > means firmware used slot 6... but 100ms earlier we forbid firmware to
> > use slot 6!
> 
> I'd rather say that this is a race condition between your testing code
> and the firmware.
If I understood Rafael this is the normal rx-code (with some 
printouts). When you use set_current_rxslot you dont forbid the hardware 
to use slot 6, you say that the last slot that you can use is 5.So the 
firmware actually checks if we step from 5 to 6, not if the slot is 6.

//Thommy