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

From: Doug Ledford <dledford@redhat.com>
To: "Luis R. Rodriguez" <mcgrof@suse.com>
Cc: "Luis R. Rodriguez" <mcgrof@do-not-panic.com>,
	infinipath@intel.com, roland@kernel.org, sean.hefty@intel.com,
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	"Toshi Kani" <toshi.kani@hp.com>,
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	"Dennis Dalessandro" <dennis.dalessandro@intel.com>,
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	konrad.wilk@oracle.com, ville.syrjala@linux.intel.com,
	david.vrabel@citrix.com, jbeulich@suse.com,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	linux-fbdev@vger.kernel.org, xen-devel@lists.xensource.com
Subject: Re: [PATCH v4 2/2] IB/qib: use arch_phys_wc_add()
Date: Wed, 22 Apr 2015 12:57:18 -0400	[thread overview]
Message-ID: <1429721838.45956.142.camel@redhat.com> (raw)
In-Reply-To: <20150422153348.GD5622@wotan.suse.de>

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On Wed, 2015-04-22 at 17:33 +0200, Luis R. Rodriguez wrote:
> 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.

Maybe I'm missing something, but in qib_enable_wc() you store the return
from arch_phys_wc_add into wc_cookie.  That return is negative, so you
return from qib_enable_wc() to qib_init_one(), they see the ret value,
they print out a warning about bad performance, then they clear the
return value and continue with device initialization.

In all of this though, wc_cookie is never cleared and so it still has
the error condition in it.  Then, much later at run time, you call
mmap_piobufs() and you check the contents of wc_cookie, and if it's
non-0 (which is still will be), you do the wrong thing, right?  And what
about at shutdown when you call qib_disable_wc() and your cookie still
has an error code in it as well?

-- 
Doug Ledford <dledford@redhat.com>
              GPG KeyID: 0E572FDD

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