linux-kernel - Re: [PATCH v4 2/2] IB/qib: use arch_phys_wc

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Date:	Wed, 22 Apr 2015 17:33:48 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Doug Ledford <dledford@...hat.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	infinipath@...el.com, roland@...nel.org, sean.hefty@...el.com,
	hal.rosenstock@...il.com, linux-rdma@...r.kernel.org,
	luto@...capital.net, mst@...hat.com, linux-kernel@...r.kernel.org,
	cocci@...teme.lip6.fr, Toshi Kani <toshi.kani@...com>,
	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Roland Dreier <roland@...estorage.com>,
	Dennis Dalessandro <dennis.dalessandro@...el.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juergen Gross <jgross@...e.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Antonino Daplas <adaplas@...il.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Stefan Bader <stefan.bader@...onical.com>,
	konrad.wilk@...cle.com, ville.syrjala@...ux.intel.com,
	david.vrabel@...rix.com, jbeulich@...e.com,
	Roger Pau Monné <roger.pau@...rix.com>,
	linux-fbdev@...r.kernel.org, xen-devel@...ts.xensource.com
Subject: Re: [PATCH v4 2/2] IB/qib: use arch_phys_wc_add()

On Wed, Apr 22, 2015 at 09:54:38AM -0400, Doug Ledford wrote:
> On Tue, 2015-04-21 at 14:50 -0700, Luis R. Rodriguez wrote:
> 
> This:
> > +	/* MTRR was used if this is non-zero */
> > +	if (!dd->wc_cookie)
> >  		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> 
> And this:
> > +		dd->wc_cookie = arch_phys_wc_add(pioaddr, piolen);
> > +		if (dd->wc_cookie < 0)
> > +			ret = -EINVAL;
> 
> don't agree on what wc_cookie will be on error.

Can you elaborate? The one below is the one that starts things,
and arch_phys_wc_add() will return 0 on PAT systems. For non-PAT
systems it will return a number > 0 *iff* a valid MTRR was added.
It will return negative onloy on error then.

The change above is meant to replace a check put in place to see
if PAT was enabled. The way we replace this is to ensure that
arch_phys_wc_add() returned 0.

If you disagree it'd be great if you can elaborate why.

  Luis
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