User-visible effect:
 Architectures that choose this method of maintaining cache coherency
(MIPS and xtensa currently) are able to use high memory on cores with
aliasing data cache.  Without this fix such architectures can not use
high memory (in case of xtensa it means that at most 128 MBytes of
physical memory is available).

The problem:
 VIPT cache with way size larger than MMU page size may suffer from
aliasing problem: a single physical address accessed via different
virtual addresses may end up in multiple locations in the cache.
Virtual mappings of a physical address that always get cached in
different cache locations are said to have different colors.  L1 caching
hardware usually doesn't handle this situation leaving it up to
software.  Software must avoid this situation as it leads to data
corruption.

What can be done:
 One way to handle this is to flush and invalidate data cache every time
page mapping changes color.  The other way is to always map physical
page at a virtual address with the same color.  Low memory pages already
have this property.  Giving architecture a way to control color of high
memory page mapping allows reusing of existing low memory cache alias
handling code.

How this is done with this patch:
 Provide hooks that allow architectures with aliasing cache to align
mapping address of high pages according to their color.  Such
architectures may enforce similar coloring of low- and high-memory page
mappings and reuse existing cache management functions to support
highmem.

This code is based on the implementation of similar feature for MIPS by
Leonid Yegoshin.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Marc Gauthier <marc@cadence.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Steven Hill <Steven.Hill@imgtec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>