Re: [PATCH v4 2/2] IB/qib: use arch_phys_wc_add()

["Luis R. Rodriguez" <mcgrof@suse.com>]

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