mm, x86: document return values of mapping funcs

mm, x86: document return values of mapping funcs

This patchset enhances MTRR checks for the kernel huge I/O mapping, which
was enabled by the patchset below:

  https://lkml.org/lkml/2015/3/3/589

The following functional changes are made in patch 7/7.
 - Allow pud_set_huge() and pmd_set_huge() to create a huge page
   mapping to a range covered by a single MTRR entry of any memory
   type.
 - Log a pr_warn() message when a specified PMD map range spans more
   than a single MTRR entry.  Drivers should make a mapping request
   aligned to a single MTRR entry when the range is covered by MTRRs.

This patch (of 7):

Document the return values of KVA mapping functions, pud_set_huge(),
pmd_set_huge, pud_clear_huge() and pmd_clear_huge().

Simplify the conditions to select HAVE_ARCH_HUGE_VMAP in the Kconfig,
since X86_PAE depends on X86_32.

There is no functional change in this patch.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Robert Elliott <Elliott@hp.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>