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Date:	Thu, 07 Aug 2014 13:23:14 -0400
From:	Mark Salter <msalter@...hat.com>
To:	Max Filippov <jcmvbkbc@...il.com>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Rob Herring <robh@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Liu hua <sdu.liu@...wei.com>, Rabin Vincent <rabin@....in>,
	Nikolay Borisov <Nikolay.Borisov@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Tomasz Figa <t.figa@...sung.com>,
	Doug Anderson <dianders@...gle.com>,
	Will Deacon <will.deacon@....com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/7] arm: use generic fixmap.h

On Thu, 2014-08-07 at 19:42 +0400, Max Filippov wrote:
> On Thu, Aug 7, 2014 at 7:32 PM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> > On Thu, 7 Aug 2014, Rob Herring wrote:
> >
> >> On Thu, Aug 7, 2014 at 10:15 AM, Max Filippov <jcmvbkbc@...il.com> wrote:
> >> > Hi,
> >> >
> >> > On Wed, Aug 6, 2014 at 11:32 PM, Kees Cook <keescook@...omium.org> wrote:
> >> >> ARM is different from other architectures in that fixmap pages are indexed
> >> >> with a positive offset from FIXADDR_START.  Other architectures index with
> >> >> a negative offset from FIXADDR_TOP.  In order to use the generic fixmap.h
> >> >
> >> > Does anybody know if there's any reason why generic fixmap.h uses negative
> >> > offsets? It complicates things with no obvious benefit if you e.g. try to align
> >> > virtual address in the fixmap region with physical page color (that's why I've
> >> > switched xtensa to positive fixmap addressing in v3.17).
> >>
> >> No, but each arch doing it differently is even more annoying.
> >
> > Why not switching everybody to positive offsets then?
> 
> I can cook a patch if people agree that that'd be good.
> 

I think that would be fine. I think x86 was first and used a negative
negative offset. Others that followed just copied that. When I did the
generic fixmap patch, using a negative offset was the natural thing to
do. Arm was only arch doing it differently.


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