linux-kernel - Re: [RFC PATCH v1 0/6] Resolve unwanted DMA backing with IOMMU

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Message-ID: <20180727090328.GH28088@arm.com>
Date:   Fri, 27 Jul 2018 10:03:28 +0100
From:   Will Deacon <will.deacon@....com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Mikko Perttunen <cyndis@...si.fi>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Nicolas Chauvet <kwizart@...il.com>,
        devicetree@...r.kernel.org, nouveau@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org, dri-devel@...ts.freedesktop.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/6] Resolve unwanted DMA backing with IOMMU

On Fri, Jul 27, 2018 at 10:25:13AM +0200, Joerg Roedel wrote:
> On Fri, Jul 27, 2018 at 02:16:18AM +0300, Dmitry Osipenko wrote:
> > The proposed solution adds a new option to the base device driver
> > structure that allows device drivers to explicitly convey to the drivers
> > core that the implicit IOMMU backing for devices must not happen.
> 
> Why is IOMMU mapping a problem for the Tegra GPU driver?
> 
> If we add something like this then it should not be the choice of the
> device driver, but of the user and/or the firmware.

Agreed, and it would still need somebody to configure an identity domain so
that transactions aren't aborted immediately. We currently allow the
identity domain to be used by default via a command-line option, so I guess
we'd need a way for firmware to request that on a per-device basis.

Will