linux-kernel - Re: ioremap_uc() followed by set_memory_wc()

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Date:	Wed, 22 Apr 2015 14:53:11 -0400
From:	Doug Ledford <dledford@...hat.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Andy Lutomirski <luto@...capital.net>,
	mike.marciniszyn@...el.com, infinipath@...el.com,
	linux-rdma@...r.kernel.org, awalls@...metrocast.net,
	Toshi Kani <toshi.kani@...com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>,
	Roland Dreier <roland@...estorage.com>,
	Juergen Gross <jgross@...e.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Borislav Petkov <bp@...e.de>, Mel Gorman <mgorman@...e.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Davidlohr Bueso <dbueso@...e.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ville Syrj?l? <syrjala@....fi>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	linux-media@...r.kernel.org, X86 ML <x86@...nel.org>,
	mcgrof@...not-panic.com
Subject: Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

On Wed, 2015-04-22 at 10:17 -0600, Jason Gunthorpe wrote:
> On Wed, Apr 22, 2015 at 05:23:28PM +0200, Luis R. Rodriguez wrote:
> > On Tue, Apr 21, 2015 at 11:39:39PM -0600, Jason Gunthorpe wrote:
> > > On Wed, Apr 22, 2015 at 01:39:07AM +0200, Luis R. Rodriguez wrote:
> > > > > Mike, do you think the time is right to just remove the iPath driver?
> > > > 
> > > > With PAT now being default the driver effectively won't work
> > > > with write-combining on modern kernels. Even if systems are old
> > > > they likely had PAT support, when upgrading kernels PAT will work
> > > > but write-combing won't on ipath.
> > > 
> > > Sorry, do you mean the driver already doesn't get WC? Or do you mean
> > > after some more pending patches are applied?
> > 
> > No, you have to consider the system used and the effects of calls used
> > on the driver in light of this table:
> 
> So, just to be clear:
> 
> At some point Linux started setting the PAT bits during
> ioremap_nocache, which overrides MTRR, and at that point the driver
> became broken on all PAT capable systems?
> 
> Not only that, but we've only just noticed it now, and no user ever
> complained?
> 
> So that means either no users exist, or all users are on non-PAT
> systems?
> 
> This driver only works on x86-64 systems. Are there any x86-64 systems
> that are not PAT capable? IIRC even the first Opteron had PAT, but my
> memory is fuzzy from back then :|
> 
> > Another option in order to enable this type of checks at run time
> > and still be able to build the driver on standard distributions and
> > just prevent if from loading on PAT systems is to have some code in
> > place which would prevent the driver from loading if PAT was
> > enabled, this would enable folks to disable PAT via a kernel command
> > line option, and if that was used then the driver probe would
> > complete.
> 
> This seems like a reasonble option to me. At the very least we might
> learn if anyone is still using these cards.
> 
> I'd also love to remove the driver if it turns out there are actually
> no users. qib substantially replaces it except for a few very old
> cards.

To be precise, the split is that ipath powers the old HTX bus cards that
only work in AMD systems, qib is all PCI-e cards.  I still have a few
HTX cards, but I no longer have any systems with HTX slots, so we
haven't even used this driver in testing for 3 or 4 years now.  And
these are all old SDR cards, where the performance numbers were 800MB/s
with WC enabled, 50MB/s without it.

> Mike?
> 
> Jason
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-- 
Doug Ledford <dledford@...hat.com>
              GPG KeyID: 0E572FDD



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