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Subject: Re: [PATCH V2] powerpc/infiniband: Use cache inhibitted and guarded
 mapping on powerpc
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>, paulus@samba.org,
 mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, Mike Marciniszyn
 <infinipath@intel.com>, Doug Ledford <dledford@redhat.com>, Sean Hefty
 <sean.hefty@intel.com>,
 Hal Rosenstock <hal.rosenstock@gmail.com>, linux-rdma@vger.kernel.org
Date: Sat, 23 Apr 2016 18:25:49 +1000
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On Fri, 2016-04-22 at 10:09 -0600, Jason Gunthorpe wrote:
> On Fri, Apr 22, 2016 at 08:47:56AM +1000, Benjamin Herrenschmidt
> wrote:
> > 
> > On Wed, 2016-04-20 at 03:58 -0400, Aneesh Kumar K.V wrote:
> > > 
> > > The driver was requesting for a writethrough mapping. But with
> > > thoses
> > > flags we will end up with a SAO mapping because we now have
> > > memory
> > > conherence always enabled. ie, the existing mapping will end up
> > > with
> > > a WIMG value 0b1110 which is Strong Access Order.
> > > 
> > > Update this to use cache inhibitted guarded mapping
> > Why guarded ? If it's performance sensitive (and the driver has
> > appropriate barriers where needed), you will get write combining
> > without guarded, you won't with it.
> This driver uses uncached write combining on x86

Right so if you want something similar on power, use the _wc variant,
and make sure you have the appropriate barriers when you need to
break combining.

Cheers,
Ben.