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

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Message-ID: <1495803280.26190.37.camel@infradead.org>
Date:   Fri, 26 May 2017 13:54:40 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     "Deucher, Alexander" <Alexander.Deucher@....com>,
        'Joerg Roedel' <jroedel@...e.de>
Cc:     'Joerg Roedel' <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Daniel Drake <drake@...lessm.com>,
        Samuel Sieb <samuel@...b.net>
Subject: Re: [PATCH v2] PCI: Add ATS-disable quirk for AMD Stoney GPUs

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.

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