Subject: Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_*

From: Christoph Hellwig <[email-masked]>
Date: Fri, 2 Aug 2019 10:14:41 +0200

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.