Modify MIPS page table entry (PTE) initialisation so that cachability
bits are set only once, using is_cacheable_mem() to determine what
caching properties are required, rather than also unconditionally
setting PTE_C_CACHE in init_pte_prot(). As PTE_C_CACHE |
PTE_C_UNCACHED == PTE_C_CACHE, this meant that all userspace memory
mappings of device memory (incorrectly) used caching TLB entries.
This is arguably not quite what we want, even though it is (more)
consistent with the MIPS pmap design: PTE caching properties should
be derived from machine-independent page table attributes, but this
is a substantially more complex change as the MIPS pmap doesn't yet
know about page attributes, causing it to ignore requests by device
drivers that want uncached userspace memory mappings as they
describe memory-mapped FIFOs or shared memory with a device not
participating in the cache coherence scheme.
This fixes cacheability issues (specifically, undesired and
unrequested caching) seen in userspace memory mappings of Avalon SoC
bus device memory on BERI MIPS.
Discussed with: jmallett, alc
Sponsored by: DARPA, AFRL
MFC after: 3 days
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