linux-kernel - Re: [PATCH] WIP: HACK: LPAE, BOOTMEM and NO

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Date:	Sat, 29 Jun 2013 20:55:49 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Tejun Heo <tj@...nel.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	linux-arm-kernel@...ts.infradead.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH] WIP: HACK: LPAE, BOOTMEM and NO_BOOTMEM

On Sat, Jun 29, 2013 at 12:29:55PM -0700, Yinghai Lu wrote:
> On Sat, Jun 29, 2013 at 10:57 AM, Tejun Heo <tj@...nel.org> wrote:
> > ( Expanding cc list, original thread is at
> >   http://thread.gmane.org/gmane.linux.kernel/1518046 )
> >
> > Hello,
> >
> > On Sat, Jun 29, 2013 at 06:21:24PM +0100, Russell King - ARM Linux wrote:
> >> Unfortunately, that has not been true on ARM - it's very common for
> >> there to be an offset on physical memory, sometimes of the order of
> >> 3GB or more.  This is because on reset, ARMs start executing the code
> >> at physical address zero, which therefore can't be RAM - and there's
> >> a desire to avoid complex switching games in hardware to temporarily
> >> map ROM there instead of RAM.
> >>
> >> On these SoCs which Santosh is working on, the main physical memory
> >> mapping is above 4GB, with just a small alias below 4GB to allow the
> >> system to boot without the MMU being on, as they may have more than
> >> 4GB of RAM.  As I understand it, the small alias below 4GB is not
> >> suitable for use as a "lowmem" mapping.
> 
> is that 32bit ARM or 64bit ARM?

Only 32-bit has LPAE.  Such things don't make sense on 64-bit CPUs.
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