linux-kernel - Re: use of fixmap on non-x86/sh?

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Date:	Sat, 8 Dec 2007 20:44:26 +0000
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Kumar Gala <galak@...nel.crashing.org>
Cc:	davem@...emloft.net, wli@...omorphy.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: use of fixmap on non-x86/sh?

On Fri, Nov 30, 2007 at 04:14:55PM -0600, Kumar Gala wrote:

> Ben and I are talking about using fixmap on ppc for similar applications to 
> it use on x86.  However in poking around other arch's (sparc, mips) they 
> appear to have some support but not as complete as x86.
>
> For example both SPARC & MIPS reference __set_fixmap() in asm/fixmap.h but 
> I can't find an implementation on either.
>
> So I was wondering if there was some reason fixmap isn't as well supported 
> or if its just used for a specific function on those SPARC, MIPS, etc. and 
> they dont need as much functionality out of it as x86 does.

MIPS uses fixmap for two purposes:

 o In some cases it is possibly to avoid or optimize a cacheflush by creating
   a temporary mapping.  The mechanism works very similar to what x86 and
   MIPS use for atomic kmaps.
 o Highmem.

Both manipulate pagetables directly or even insert entries directly into
the TLB without pagetables as intermediate stage so __set_fixmap is indeed
unused.

  Ralf
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