From: Christoph Hellwig <hch@lst.de>
To: Will Deacon <will@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
iommu@lists.linux-foundation.org,
Shawn Anastasio <shawn@anastas.io>,
Michael Ellerman <mpe@ellerman.id.au>,
Russell King <linux@armlinux.org.uk>,
Catalin Marinas <catalin.marinas@arm.com>,
Robin Murphy <robin.murphy@arm.com>,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_*
Date: Fri, 2 Aug 2019 10:14:41 +0200 [thread overview]
Message-ID: <20190802081441.GA9725@lst.de> (raw)
In-Reply-To: <20190801164411.kmsl4japtfkgvzxe@willie-the-truck>
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.
next prev parent reply other threads:[~2019-08-02 8:14 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 14:21 fix default dma_mmap_* pgprot Christoph Hellwig
2019-08-01 14:21 ` [PATCH] dma-mapping: fix page attributes for dma_mmap_* Christoph Hellwig
2019-08-01 16:23 ` Will Deacon
2019-08-01 16:34 ` Christoph Hellwig
2019-08-01 16:44 ` Will Deacon
2019-08-02 8:14 ` Christoph Hellwig [this message]
2019-08-02 10:38 ` Will Deacon
2019-08-03 6:48 ` Christoph Hellwig
2019-08-06 16:08 ` Will Deacon
2019-08-06 16:45 ` Russell King - ARM Linux admin
2019-08-06 16:48 ` Russell King - ARM Linux admin
2019-08-07 6:14 ` Christoph Hellwig
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