linux-kernel - DMA_ATTR_WRITE

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Date:   Fri, 2 Aug 2019 08:37:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Alex Smith <alex.smith@...tec.com>
Cc:     Sadegh Abbasi <Sadegh.Abbasi@...tec.com>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>, linux-mips@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: DMA_ATTR_WRITE_COMBINE on mips

[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?