Re: [PATCH] WIP: HACK: LPAE, BOOTMEM and NO_BOOTMEM

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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@kernel.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.