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: Sat, 3 Aug 2019 08:48:12 +0200 [thread overview]
Message-ID: <20190803064812.GA29746@lst.de> (raw)
In-Reply-To: <20190802103803.3qrbhqwxlasojsco@willie-the-truck>
On Fri, Aug 02, 2019 at 11:38:03AM +0100, Will Deacon wrote:
>
> So this boils down to a terminology mismatch. The Arm architecture doesn't have
> anything called "write combine", so in Linux we instead provide what the Arm
> architecture calls "Normal non-cacheable" memory for pgprot_writecombine().
> Amongst other things, this memory type permits speculation, unaligned accesses
> and merging of writes. I found something in the architecture spec about
> non-cachable memory, but it's written in Armglish[1].
>
> pgprot_noncached(), on the other hand, provides what the architecture calls
> Strongly Ordered or Device-nGnRnE memory. This is intended for mapping MMIO
> (i.e. PCI config space) and therefore forbids speculation, preserves access
> size, requires strict alignment and also forces write responses to come from
> the endpoint.
>
> I think the naming mismatch is historical, but on arm64 we wanted to use the
> same names as arm32 so that any drivers using these things directly would get
> the same behaviour.
That all makes sense, but it totally needs a comment. I'll try to draft
one based on this. I've also looked at the arm32 code a bit more, and
it seems arm always (?) supported Normal non-cacheable attribute, but
Linux only optionally uses it for arm v6+ because of fears of drivers
missing barriers. The other really weird things is that in arm32
pgprot_dmacoherent incudes the L_PTE_XN bit, which from my understanding
is the no-execture bit, but pgprot_writecombine does not. This seems to
not very unintentional. So minus that the whole DMA_ATTR_WRITE_COMBІNE
seems to be about flagging old arm specific drivers as having the proper
barriers in places and otherwise is a no-op.
Here is my tentative plan:
- respin this patch with a small fix to handle the
DMA_ATTR_NON_CONSISTENT (as in ignore it unless actually supported),
but keep the name as-is to avoid churn. This should allow 5.3
inclusion and backports
- remove DMA_ATTR_WRITE_COMBINE support from mips, probably also 5.3
material.
- move all architectures but arm over to just define
pgprot_dmacoherent, including a comment with the above explanation
for arm64.
- make DMA_ATTR_WRITE_COMBINE a no-op and schedule it for removal,
thus removing the last instances of arch_dma_mmap_pgprot
next prev parent reply other threads:[~2019-08-03 6:48 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
2019-08-02 10:38 ` Will Deacon
2019-08-03 6:48 ` Christoph Hellwig [this message]
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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