[PATCH] powerpc/dma: Fix invalid DMA mmap behavior

Arnd Bergmann arnd at arndb.de
Fri Jul 19 21:18:13 AEST 2019

On Thu, Jul 18, 2019 at 11:52 AM Christoph Hellwig <hch at lst.de> wrote:
> On Thu, Jul 18, 2019 at 10:49:34AM +0200, Christoph Hellwig wrote:
> > On Thu, Jul 18, 2019 at 01:45:16PM +1000, Oliver O'Halloran wrote:
> > > > Other than m68k, mips, and arm64, everybody else that doesn't have
> > > > ARCH_NO_COHERENT_DMA_MMAP set uses this default implementation, so
> > > > I assume this behavior is acceptable on those architectures.
> > >
> > > It might be acceptable, but there's no reason to use pgport_noncached
> > > if the platform supports cache-coherent DMA.
> > >
> > > Christoph (+cc) made the change so maybe he saw something we're missing.
> >
> > I always found the forcing of noncached access even for coherent
> > devices a little odd, but this was inherited from the previous
> > implementation, which surprised me a bit as the different attributes
> > are usually problematic even on x86.  Let me dig into the history a
> > bit more, but I suspect the righ fix is to default to cached mappings
> > for coherent devices.
> Ok, some history:
> The generic dma mmap implementation, which we are effectively still
> using today was added by:
> commit 64ccc9c033c6089b2d426dad3c56477ab066c999
> Author: Marek Szyprowski <m.szyprowski at samsung.com>
> Date:   Thu Jun 14 13:03:04 2012 +0200
>     common: dma-mapping: add support for generic dma_mmap_* calls
> and unconditionally uses pgprot_noncached in dma_common_mmap, which is
> then used as the fallback by dma_mmap_attrs if no ->mmap method is
> present.  At that point we already had the powerpc implementation
> that only uses pgprot_noncached for non-coherent mappings, and
> the arm one, which uses pgprot_writecombine if DMA_ATTR_WRITE_COMBINE
> is set and otherwise pgprot_dmacoherent, which seems to be uncached.
> Arm did support coherent platforms at that time, but they might have
> been an afterthought and not handled properly.

Cache-coherent devices are still very rare on 32-bit ARM.

Among the callers of dma_mmap_coherent(), almost all are in platform
specific device drivers that only ever run on noncoherent ARM SoCs,
which explains why nobody would have noticed problems.

There is also a difference in behavior between ARM and PowerPC
when dealing with mismatched cacheability attributes: If the same
page is mapped as both cached and uncached to, this may
cause silent undefined behavior on ARM, while PowerPC should
enter a checkstop as soon as it notices.


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