[ARM] mm 3: separate out supersection mappings, avoid for <4GB

Catalin Marinas at ARM Ltd says:
> The CPU architects in ARM intended supersections only as a way to map
> addresses >= 4GB. Supersections are not mandated by the architecture
> and there is no easy way to detect their hardware support at run-time
> (other than checking for a specific core). From the analysis done in
> ARM, there wasn't a clear performance gain by using supersections
> rather than sections (no significant improvement in the TLB misses).

Therefore, we should avoid using supersections unless there's a real
need (iow, we're mapping addresses >= 4GB).

This means that we can simplify create_mapping() a bit since we will
only use supersection mappings for addresses >= 4GB, which means that
the physical, virtual and length must be multiples of the supersection
mapping size.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/ioremap.c

@@ -302,7 +302,7 @@ __ioremap_pfn(unsigned long pfn, unsigned long offset, size_t size,
 #ifndef CONFIG_SMP
 	if (DOMAIN_IO == 0 &&
 	    (((cpu_architecture() >= CPU_ARCH_ARMv6) && (get_cr() & CR_XP)) ||
-	       cpu_is_xsc3()) &&
+	       cpu_is_xsc3()) && pfn >= 0x100000 &&
 	       !((__pfn_to_phys(pfn) | size | addr) & ~SUPERSECTION_MASK)) {
 		area->flags |= VM_ARM_SECTION_MAPPING;
 		err = remap_area_supersections(addr, pfn, size, flags);