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Subject: Re: [RFC][PATCH 3/8] 4xx MMU
From: Josh Boyer <jwboyer@linux.vnet.ibm.com>
To: Arnd Bergmann <arnd@arndb.de>
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Date: Wed, 11 Jul 2007 16:15:57 -0500
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On Wed, 2007-07-11 at 22:56 +0200, Arnd Bergmann wrote:
> On Wednesday 11 July 2007, Josh Boyer wrote:
> > +#elif defined(CONFIG_40x)
> > +/* 40x-style software loaded TLB */
> > +#  include <asm/mmu-4xx.h>
> >  #elif defined(CONFIG_44x)
> >  /* 44x-style software loaded TLB */
> >  #  include <asm/mmu-44x.h>
> 
> If you call it mmu-4xx, shouldn't it be used
> for 44x as well? I would think this either
> should be

No.  I was following the established convention that's been there for
years.  Which sucks I suppose for people not familiar with 40x/44x.
That would be a lot of stuff to change though...  e.g.:

arch/powerpc/platforms/4xx -> arch/powerpc/platforms/40x
arch/powerpc/kernel/head_4xx.S -> arch/powerpc/platforms/head_40x.S

etc.  I'd need a git tree to do that cleanly.  I've been toying with
creating one for powerpc4xx anyway so I might do that.

> 
> > +#elif defined(CONFIG_4xx)
> > +/* 40x-style software loaded TLB */
> > +#  include <asm/mmu-4xx.h>
> > -#elif defined(CONFIG_44x)
> > -/* 44x-style software loaded TLB */
> > -#  include <asm/mmu-44x.h>
> 
> or 
> 
> > +#elif defined(CONFIG_40x)
> > +/* 40x-style software loaded TLB */
> > +#  include <asm/mmu-40x.h>
> >  #elif defined(CONFIG_44x)
> >  /* 44x-style software loaded TLB */
> >  #  include <asm/mmu-44x.h>

I'll probably change it to this.  Will look a bit odd, given that C file
is arch/powerpc/mm/4xx_mmu.c.

> Is it actually feasible to get to a point where
> you can build a kernel that boots on both
> 40x and 44x, or is it just too different?

The MMUs are entirely different.  40x has real-mode and is 32-bit.  44x
is always virtual and is 36-bit.

josh