linux-kernel - Re: [git head] Should X86_PAT really default to yes?

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Date:	Fri, 2 May 2008 13:40:24 -0700
From:	Jesse Barnes <jesse.barnes@...el.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	"Frans Pop" <elendil@...net.nl>, linux-kernel@...r.kernel.org,
	"Ingo Molnar" <mingo@...e.hu>,
	"Packard, Keith" <keith.packard@...el.com>
Subject: Re: [git head] Should X86_PAT really default to yes?

On Friday, May 02, 2008 12:37 pm Pallipadi, Venkatesh wrote:
> >-----Original Message-----
> >From: Frans Pop [mailto:elendil@...net.nl]
> >Sent: Friday, May 02, 2008 12:22 PM
> >To: linux-kernel@...r.kernel.org
> >Cc: Pallipadi, Venkatesh; Ingo Molnar
> >Subject: [git head] Should X86_PAT really default to yes?
> >
> >With X86_PAT enabled, when X is started I get about 40 lines
> >(with varying
> >addresses) like:
> >kernel: Xorg:3358 /dev/mem expected mapping type write-back for
> >807bf000-81000000, got uncached-minus

These messages?  They're coming from the kernel it looks like, from the 
map_devmem routine in pat.c.  I'm not sure they're accurate though; for PCI 
regions /dev/mem is *supposed* to map with UC- and not WB, so maybe this 
function needs to be updated?

> >And when X exits I get a bunch of lines like:
> >kernel: Xorg:3349 freeing invalid memtype 80020000-8002a000
> >
> >I also noticed artifacts (a band of about 2 cm high across the
> >screen) after
> >X goes to black but before the switch to VT1.

This is just a transient issue during VT switch or server exit though, right?  
X functionality isn't affected, and your VTs work fine?  If so, it might not 
be a PAT issue but just a different memory layout or something (and therefore 
it would really just be a cosmetic bug in the X driver).

I really think PAT should be on by default; if you're running into real 
functional or performance problems we'd better get them fixed rather than 
disabling PAT...

Thanks,
Jesse

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