linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Fri, 23 Oct 2009 13:31:52 +0900
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...ell.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Tejun Heo <tj@...nel.org>,
	Thomas Schlichter <thomas.schlichter@....de>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Robert Hancock <hancockrwd@...il.com>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
 available

On Thu, 22 Oct 2009 18:53:19 -0700
Eric Anholt <eric@...olt.net> wrote:

> On Thu, 2009-10-22 at 17:11 -0700, Suresh Siddha wrote:
> > On Thu, 2009-10-22 at 16:10 -0700, Jesse Barnes wrote:
> > > On Thu, 22 Oct 2009 14:47:30 -0700
> > > Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> > > 
> > > > On Thu, 2009-10-22 at 08:34 -0700, Eric Anholt wrote:
> > > > > Can we just not create the _wc sysfs entry if we don't have
> > > > > PAT?  I don't think there's userland relying on its presence
> > > > > as opposed to the non-_wc entry.
> > > > 
> > > > Yes indeed. Jesse do you see an issue with this? This is simple
> > > > and clean. Thanks Eric.
> > > 
> > > Yeah, I think that will be fine.  In fact, older versions of
> > > libpciaccess will behave better if we do it that way (iirc it only
> > > allocates an MTRR if the resource_wc file doesn't exist or fails
> > > to get mapped).
> > 
> > Eric, care to send the patch?
> 
> I don't have a patch, I was just suggesting a way to handle the
> submitter's problem that won't involve complicated changes that nobody
> else will be testing since everyone *should* have a graphics driver
> for their graphics hardware.

Here's a quick & dirty version, totally untested.  A cleaner approach
would be to separate the WC mapping routines and hide the return
-EINVAL in arch specific code...

Jesse

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 0f6382f..41010bb 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -23,6 +23,9 @@
 #include <linux/mm.h>
 #include <linux/capability.h>
 #include <linux/pci-aspm.h>
+#ifdef CONFIG_X86
+#include <asm/pat.h>
+#endif
 #include "pci.h"
 
 static int sysfs_initialized;	/* = 0 */
@@ -730,6 +733,10 @@ static int
 pci_mmap_resource_wc(struct kobject *kobj, struct bin_attribute *attr,
 		     struct vm_area_struct *vma)
 {
+#ifdef CONFIG_X86
+	if (!pat_enabled)
+		return -EINVAL;
+#endif
 	return pci_mmap_resource(kobj, attr, vma, 1);
 }
 
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