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

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Date:	Wed, 22 Apr 2015 09:53:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Mike Marciniszyn <infinipath@...el.com>,
	linux-rdma@...r.kernel.org, Andy Walls <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>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
Subject: Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

On Wed, Apr 22, 2015 at 8:23 AM, Luis R. Rodriguez <mcgrof@...e.com> 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:
>
> ----------------------------------------------------------------------
> MTRR Non-PAT   PAT    Linux ioremap value        Effective memory type
> ----------------------------------------------------------------------
>                                                   Non-PAT |  PAT
>      PAT
>      |PCD
>      ||PWT
>      |||
> WC   000      WB      _PAGE_CACHE_MODE_WB            WC   |   WC
> WC   001      WC      _PAGE_CACHE_MODE_WC            WC*  |   WC
> WC   010      UC-     _PAGE_CACHE_MODE_UC_MINUS      WC*  |   UC
> WC   011      UC      _PAGE_CACHE_MODE_UC            UC   |   UC
> ----------------------------------------------------------------------
>
> (*) denotes implementation defined and is discouraged
>
> ioremap_nocache() will use _PAGE_CACHE_MODE_UC_MINUS by default today,
> in the future we want to flip the switch and make _PAGE_CACHE_MODE_UC
> the default. When that flip occurs it will mean ipath cannot get
> write-combining on both non-PAT and PAT systems. Now that is for
> the future, lets review the current situation for ipath.
>
> For PAT capable systems if mtrr_add() is used today on a Linux system on a
> region mapped with ioremap_nocache() that will mean you effectively nullify the
> mtrr_add() effect as the combinatorial effect above yields an effective memory
> type of UC.

Are you sure?  I thought that ioremap_nocache currently is UC-, so
mtrr_add + ioremap_nocache gets WC even on PAT systems.

Going forward, when mtrr_add is gone, this will change, of course.

--Andy
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