DMA_ATTR_WRITE_COMBINE on mips

[Christoph Hellwig]

[I hope the imgtec address still works, but maybe the mips folks know
if it moved to mips]

Hi Alex,

you added DMA_ATTR_WRITE_COMBINE support in dma_mmap_attrs to mips
in commit 8c172467be36f7c9591e59b647e4cd342ce2ef41
("MIPS: Add implementation of dma_map_ops.mmap()"), but that commit
only added the support in mmap, not in dma_alloc_attrs.  This means
the memory is now used in kernel space through KSEG1, and thus uncached,
while for userspace mappings through dma_mmap_* pgprot_writebombine
is used, which creates a write combine mapping, which on some MIPS CPUs
sets the _CACHE_UNCACHED_ACCELERATED pte bit instead of the
_CACHE_UNCACHED one.  I know at least on arm, powerpc and x86 such
mixed page cachability attributes can cause pretty severe problems.
Are they ok on mips?  Or was the DMA_ATTR_WRITE_COMBINE supported
unintended and not correct and we should remove it?