linux-kernel - Re: [PATCH 12/18] iommu/tegra-gart: Add IOMMU_DOMAIN

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Date:   Mon, 24 Aug 2020 00:42:02 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Robin Murphy <robin.murphy@....com>, hch@....de, joro@...tes.org,
        linux@...linux.org.uk
Cc:     will@...nel.org, inki.dae@...sung.com, sw0312.kim@...sung.com,
        kyungmin.park@...sung.com, m.szyprowski@...sung.com,
        agross@...nel.org, bjorn.andersson@...aro.org,
        thierry.reding@...il.com, jonathanh@...dia.com, vdumpa@...dia.com,
        matthias.bgg@...il.com, yong.wu@...iatek.com,
        geert+renesas@...der.be, magnus.damm@...il.com, t-kristo@...com,
        s-anna@...com, laurent.pinchart@...asonboard.com,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        linux-samsung-soc@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/18] iommu/tegra-gart: Add IOMMU_DOMAIN_DMA support

21.08.2020 03:28, Robin Murphy пишет:
...
>> Will a returned NULL tell to IOMMU core that implicit domain shouldn't
>> be used? Is it possible to leave this driver as-is?
> 
> The aim of this patch was just to make the conversion without functional
> changes wherever possible, i.e. maintain an equivalent to the existing
> ARM behaviour of allocating its own implicit domains for everything. It
> doesn't represent any judgement of whether that was ever appropriate for
> this driver in the first place ;)
> 
> Hopefully my other reply already covered the degree of control drivers
> can have with proper default domains, but do shout if anything wasn't
> clear.

Thank you for the detailed comments! I wasn't watching closely all the
recent iommu/ changes and yours clarification is very helpful!

My current understanding is that the GART driver will need to support
the IOMMU_DOMAIN_IDENTITY and set def_domain_type to
IOMMU_DOMAIN_IDENTITY for all devices.

Meanwhile, today's upstream drivers don't use GART, hence this patch
should be okay. Although, it's a bit unlikely that the IOMMU_DOMAIN_DMA
type will ever be useful for the GART, and thus, I'm still thinking that
will be a bit nicer to keep GART driver as-is for now.