Re: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs

From	David Woodhouse <dwmw2@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs
Date	2017-05-26 15:00 +0200
Message-ID	<tLn0u-60Y-13@gated-at.bofh.it> (permalink)
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On Fri, 2017-05-26 at 11:57 +0000, Deucher, Alexander wrote:
> 
> FWIW, the GPU driver does not actually use ATS at the moment so I
> don't think we should see any ATS transactions.

That's a confusing sentence. The "GPU driver", if you mean software
running in the OS, wouldn't be expected to have anything to do with
ATS.

ATS is something that the CPU itself (or its DMA engine) would do.
Instead of just performing a DMA transaction to a given bus address,
and letting the IOMMU do the translation, the hardware might choose to
first perform an IOTLB lookup, and then later do the actual DMA
transaction to the pre-translated, raw physical address. Which kind of
makes a mockery of any kind of protection the IOMMU is supposed to give
you, but does shave a cycle or two of latency off the DMA when it
finally happens, since the translation can be done in advance.

Thread

RE: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs "Deucher, Alexander" <Alexander.Deucher@amd.com> - 2017-05-26 14:00 +0200
  Re: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs David Woodhouse <dwmw2@infradead.org> - 2017-05-26 15:00 +0200
    RE: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs "Deucher, Alexander" <Alexander.Deucher@amd.com> - 2017-05-26 18:10 +0200

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