linux-kernel - Re: [patch] bootmem: use MAX_DMA

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Date:	Fri, 28 Jul 2006 16:58:45 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch] bootmem: use MAX_DMA_ADDRESS instead of LOW32LIMIT

On Fri, 2006-07-28 at 15:13 +0200, Ingo Molnar wrote:
> > -#define LOW32LIMIT 0xffffffff
> 
> >  		if ((ptr = __alloc_bootmem_core(bdata, size,
> > -						 align, goal, LOW32LIMIT)))
> > +						 align, goal, MAX_DMA_ADDRESS)))
> 
> but this limits things to 16MB on i686. Are you sure this wont break 
> anything?

That is something we should not do. MAX_DMA_ADDRESS is not the correct
value, it says something about the DMA limitations. LOW32LIMIT says
something about the cpu addressing limitations which is a completly
different thing. I think it would be best to introduce an architecture
overridable define like LOW_ADDRESS_LIMIT. The default is 4GB-1, for
s390 it is 2GB-1. The current name is misleading LOW32LIMIT indicates
that the address for alloc_bootmem_low objects has 32 bits, which isn't
true for s390.

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.

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