linux-kernel - Re: pci-gart_64.c: could we get better explanation?

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Message-ID: <20080625222543.GD1844@elf.ucw.cz>
Date:	Thu, 26 Jun 2008 00:25:43 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Dave Jones <davej@...emonkey.org.uk>,
	Andi Kleen <andi@...stfloor.org>,
	Trivial patch monkey <trivial@...nel.org>
Subject: Re: pci-gart_64.c: could we get better explanation?

On Tue 2008-05-27 05:51:39, Benjamin Herrenschmidt wrote:
> 
> On Mon, 2008-05-26 at 20:47 +0200, Pavel Machek wrote:
> > Hi!
> > 
> > +++ b/arch/x86/kernel/pci-gart_64.c
> > @@ -774,6 +778,7 @@ #endif
> >  
> >         /*
> >          * Try to workaround a bug (thanks to BenH)
> >          * Set unmapped entries to a scratch page instead of 0.
> >          * Any prefetches that hit unmapped entries won't get an bus abort
> > 
> > Could someone explain? Which bug is being worked around? CPUs
> > prefetching over NULL?
> 
> P2P bridge prefetching on DMA reads.

---

Add better explanation to pci-gart.

Signed-off-by: Pavel Machek <pavel@...e.cz>

diff --git a/arch/x86/kernel/pci-gart_64.c b/arch/x86/kernel/pci-gart_64.c
index 838a1d3..e72e583 100644
--- a/arch/x86/kernel/pci-gart_64.c
+++ b/arch/x86/kernel/pci-gart_64.c
@@ -800,10 +800,10 @@ #endif
 	wbinvd();
 
 	/*
-	 * Try to workaround a bug (thanks to BenH)
+	 * Try to workaround a bug (thanks to BenH):
 	 * Set unmapped entries to a scratch page instead of 0.
 	 * Any prefetches that hit unmapped entries won't get an bus abort
-	 * then.
+	 * then. (P2P bridge may be prefetching on DMA reads).
 	 */
 	scratch = get_zeroed_page(GFP_KERNEL);
 	if (!scratch)

-- 
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