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

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Message-Id: <1256273896.2569.10.camel@sbs-t61>
Date:	Thu, 22 Oct 2009 21:58:15 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Eric Anholt <eric@...olt.net>,
	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, 2009-10-22 at 21:31 -0700, Jesse Barnes wrote:
> 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 How about this patch? Doing this in x86 is cleaner.

I would like Acks/sign-offs-by Thomas, Eric and Jesse, if it is ok with
this patch and works.

thanks,
suresh
---

From: Suresh Siddha <suresh.b.siddha@...el.com>
Subject: x86, pat: return EINVAL for pci mmap WC request for !pat_enabled

Thomas Schlichter reported:
> X.org uses libpciaccess which tries to mmap with write combining enabled via
> /sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, the
> kernel does fall back to uncached mmap. Then libpciaccess thinks it succeeded
> mapping with write combining enabled and does not set up suited MTRR entries.
> ;-(

Instead of silently mapping pci mmap region as UC minus in the case
of !pat_enabled and wc request, we can return error. Eric Anholt mentioned
that caller (like X) typically follows up with UC minus pci mmap request and
if there is a free mtrr slot, caller will manage adding WC mtrr.

Jesse Barnes says:
> 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).

Reported-by: Thomas Schlichter <thomas.schlichter@....de>
Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
---

diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index b22d13b..a672f12 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -282,6 +282,15 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
 		return -EINVAL;
 
 	prot = pgprot_val(vma->vm_page_prot);
+
+	/*
+ 	 * Return error if pat is not enabled and write_combine is requested.
+ 	 * Caller can followup with UC MINUS request and add a WC mtrr if there
+ 	 * is a free mtrr slot.
+ 	 */
+	if (!pat_enabled && write_combine)
+		return -EINVAL;
+
 	if (pat_enabled && write_combine)
 		prot |= _PAGE_CACHE_WC;
 	else if (pat_enabled || boot_cpu_data.x86 > 3)


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