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

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Message-ID: <20150422173728.GJ5622@wotan.suse.de>
Date:	Wed, 22 Apr 2015 19:37:28 +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 12:57:18PM -0400, Doug Ledford wrote:
> 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,

If and only if the system was non-PAT and mtrr_add() failed.

>  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?

Originally the code had it to run pgprot_writecombine() if PAT was going to be
used. After the code changes we check for !cookie which will be true when
cookie is 0 only. In case the cookie was an error, that is if mtrr_add()
failed, then this code would not run because (!negative) is false. The goal was
to trigger a run if the cookie was 0, which can only happen if PAT was enabled.

Please let me know, I'd like to get this right too.

> And what
> about at shutdown when you call qib_disable_wc() and your cookie still
> has an error code in it as well?

Well fortunately arch_phys_wc_del(negative) and arch_phys_wc_del(0) will be
a no-op. Its what helps us remove so much clutter.

 Luis
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