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SubjectRe: [PATCH] dma-mapping: fix page attributes for dma_mmap_*
On Thu, Aug 01, 2019 at 05:44:12PM +0100, Will Deacon wrote:
> > > Although arch_dma_mmap_pgprot() is a bit of a misnomer now that it only
> > > gets involved in the non-coherent case.
> >
> > A better name is welcome.
>
> How about arch_dma_noncoherent_mmap_pgprot() ? Too long?

Sounds a little long yes. And doesn't fix the additional problem that
we don't just it for mmap but also for the in-kernel remapping these
days.

> > But my worry is how this interacts with architectures that have an
> > uncached segment (mips, nios2, microblaze, extensa) where we'd have
> > the kernel access DMA_ATTR_WRITE_COMBINE mappigns using the uncached
> > segment, and userspace mmaps using pgprot_writecombine, which could
> > lead to aliasing issues. But then again mips already supports
> > DMA_ATTR_WRITE_COMBINE, so this must be ok somehow. I guess I'll
> > need to field that question to the relevant parties.
>
> Or it's always been busted and happens to work out in practice...

I've sent a ping to the mips folks. While we'are at it: arm64
and arm32 (optionally) map dma coherent allocations as write combine.
I suspect this hasn't always just been busted but intentional (of course!),
but is there any chance to get a quote from the arm architecture spec
on why this is fine as it looks rather confusion?

Also if we assume mips is buggy DMA_ATTR_WRITE_COMBINE really just seems
to be there for old arm platforms, which makes the scope pretty limited.

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