linux-kernel - Re: [PATCH 13/18] iommu/tegra: Add IOMMU_DOMAIN

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Message-ID: <20200827154502.GA1660457@ulmo>
Date:   Thu, 27 Aug 2020 17:45:02 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     hch@....de, joro@...tes.org, linux@...linux.org.uk,
        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,
        jonathanh@...dia.com, vdumpa@...dia.com, digetx@...il.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 13/18] iommu/tegra: Add IOMMU_DOMAIN_DMA support

On Thu, Aug 20, 2020 at 04:08:32PM +0100, Robin Murphy wrote:
> Now that arch/arm is wired up for default domains and iommu-dma,
> implement the corresponding driver-side support for DMA domains.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>  drivers/iommu/tegra-smmu.c | 37 +++++++++++++++++++++----------------
>  1 file changed, 21 insertions(+), 16 deletions(-)

We can't do that yet because it will currently still break for use-cases
such as display where we don't properly set up identity mappings during
boot. The result is that the dma-iommu code will enable translations
before the driver gets a chance to set up any mappings and if the
display controller was left on by the bootloader, scanning out a splash
screen, this causes faults between the point where dma-iommu is being
set up for the display controller and where the display controller
starts mapping its own buffers (rather than the ones mapped by the
bootloader).

That said, I do have a series that I've been carrying around for longer
than I've wanted that does exactly this for Tegra SMMU and I'd prefer if
you could drop this particular change from your series so that I can
keep working on resolving the identity mapping issues first.

Thierry

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