netdev - Re: b44: regression in 2.6.22 (resend)

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Date:	Tue, 29 May 2007 22:45:22 +0200
From:	Michael Buesch <mb@...sch.de>
To:	"Gary Zambrano" <zambrano@...adcom.com>
Cc:	"Maximilian Engelhardt" <maxi@...monizer.de>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"linux-wireless" <linux-wireless@...r.kernel.org>,
	"Stephen Hemminger" <shemminger@...ux-foundation.org>,
	"Arnaldo Carvalho de Melo" <acme@...stprotocols.net>,
	"Jeff Garzik" <jgarzik@...ox.com>, netdev@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: b44: regression in 2.6.22 (resend)

On Tuesday 29 May 2007 16:14:35 Gary Zambrano wrote:
> On Mon, 2007-05-28 at 16:55 +0200, Michael Buesch wrote:
> > On Monday 28 May 2007 16:12:12 Maximilian Engelhardt wrote:
> > > On Monday 28 May 2007, Michael Buesch wrote:
> > > > Can you also test the following patch?
> > > > I think there's a bug in b44 that is doesn't properly discard
> > > > shared IRQs, so it might possibly generate a NAPI storm, dunno.
> > > > Worth a try.
> > > >
> > > > Index: linux-2.6.22-rc3/drivers/net/b44.c
> > > > ===================================================================
> > > > --- linux-2.6.22-rc3.orig/drivers/net/b44.c	2007-05-27 23:01:44.000000000
> > > > +0200 +++ linux-2.6.22-rc3/drivers/net/b44.c	2007-05-28 12:48:27.000000000
> > > > +0200 @@ -911,6 +911,8 @@ static irqreturn_t b44_interrupt(int irq
> > > >  	spin_lock(&bp->lock);
> > > >
> > > >  	istat = br32(bp, B44_ISTAT);
> > > > +	if (istat == 0xFFFFFFFF)
> > > > +		goto out; /* Shared IRQ not for us */
> > > >  	imask = br32(bp, B44_IMASK);
> > > >
> > > >  	/* The interrupt mask register controls which interrupt bits
> > > > @@ -942,6 +944,7 @@ irq_ack:
> > > >  		bw32(bp, B44_ISTAT, istat);
> > > >  		br32(bp, B44_ISTAT);
> > > >  	}
> > > > +out:
> > > >  	spin_unlock(&bp->lock);
> > > >  	return IRQ_RETVAL(handled);
> > > >  }
> > > 
> > > I did try this patch on a affected kernel, but I didn't notice any big 
> > > difference. Perhaps the kernel is a bit less slow during the test, but It's 
> > > hard to tell.
> > 
> > Ok, but anyway. I think this is a bug and needs to be fixed this way. Gary?
> > 
> 
> Thanks Michael.
> No, I don't think this is a bug and it does not need to be fixed.

Are you sure? I'm not so sure, because
1) On bcm43xx the reverse engineers told us that the card
   returns 0xFFFFFFFF for no-irq-pending. Since b44 and bcm43xx
   are very similiar in IRQ and DMA I just thought it would
   be the case there, too. Just guessing.
2) PCMCIA cards usually return all-ones if you try to read a
   register of a card that's been removed. So it's good
   practice to check for this and bail out early in the IRQ
   path. Do PCMCIA cards (PC-card, not neccessarily a real
   16bit PCMCIA card) for b44 exist?

-- 
Greetings Michael.
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