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

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Date:	Sat, 1 Dec 2007 10:03:50 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Kumar Gala <galak@...nel.crashing.org>
Cc:	davem@...emloft.net, wli@...omorphy.com, mingo@...hat.com,
	ralf@...ux-mips.org, 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.
> 
That's probably because people got lazy with copying around the
definitions -- perhaps surprisingly this happens quite frequently in arch
headers ;-)

MIPS has a fixrange_init() which does things in more or less one shot.
__set_fixmap() is a good abstraction if you're interested in poking at
individual fixmaps, but at least the kmap fixmaps have special handling all
over the place (look for kmap_pte in the various highmem implementations),
and there are few fixmaps otherwise.

> 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.
> 
There are of course things that make this more attractive on x86,
especially with regards to the global bit and preservation across a TLB
flush, there's a note in arch/sh/mm/init.c above __set_fixmap() about
that. fixmap doesn't really have any special behaviour that makes an
architecture implementation problematic at least.
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