linux-kernel - Inconsistent pgprot between dma_alloc_coherent() and dma_mmap

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Message-ID: <s5htx27p1lv.wl-tiwai@suse.de>
Date:	Mon, 10 Nov 2014 15:33:00 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org
Subject: Inconsistent pgprot between dma_alloc_coherent() and dma_mmap_cohernet()

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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