Inconsistent pgprot between dma_alloc_coherent() and dma_mmap_cohernet()

[Takashi Iwai]

Hi,

I noticed using dma_mmap_coherent() on pages allocated via
dma_alloc_coherent() results in a warning on x86:

  map pfn RAM range req uncached-minus for [mem 0x21d500000-0x21d51ffff], got write-back

which comes from arch/x86/mm/pat.c.

Looking at the code, it seems that dma_mmap_coherent() blindly assumes
that the pages are with pgprot_noncached(), but on x86, it's obviously
not.  Maybe some other archs have the same problem.

How should we fix this?  Defining a new pgprot_xxx(), and falling back
to pgprot_noncached if undefined?  Or is it just x86 and do it only
there?


thanks,

Takashi
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