linux-kernel - fix default dma_mmap

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Date:   Wed,  7 Aug 2019 09:16:00 +0300
From:   Christoph Hellwig <hch@....de>
To:     iommu@...ts.linux-foundation.org
Cc:     Shawn Anastasio <shawn@...stas.io>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-mips@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: fix default dma_mmap_* pgprot v3

Hi all,

As Shawn pointed out we've had issues with the dma mmap pgprots ever
since the dma_common_mmap helper was added beyong the initial
architectures - we default to uncached mappings, but for devices that
are DMA coherent, or if the DMA_ATTR_NON_CONSISTENT is set (and
supported) this can lead to aliasing of cache attributes.  This patch
fixes that.  My explanation of why this hasn't been much of an issue
is that the dma_mmap_ helpers aren't used widely and mostly just in
architecture specific drivers.

Changes since v2:
 - fix m68knommu compile by inlining dma_prprot helper and providing
   a stub for !CONFIG_MMU
 - fix various typos in the commit messages

Changes since v1:
 - fix handling of DMA_ATTR_NON_CONSISTENT where it is a no-op
   (which is most architectures)
 - remove DMA_ATTR_WRITE_COMBINE on mips, as it seem dangerous as-is