linux-kernel - Re: [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active" property

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Message-ID: <4f35bf17-a354-6ffb-fd4a-063027d83ccc@gmail.com>
Date:   Fri, 25 Sep 2020 16:52:50 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Robin Murphy <robin.murphy@....com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     Joerg Roedel <joro@...tes.org>, Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Will Deacon <will@...nel.org>,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] dt-bindings: reserved-memory: Document "active"
 property

25.09.2020 15:39, Robin Murphy пишет:
...
>> Yes, my understanding that this is what Robin suggested here:
>>
>> https://lore.kernel.org/linux-iommu/cb12808b-7316-19db-7413-b7f852a6f8ae@arm.com/
>>
> 
> Just to clarify, what I was talking about there is largely orthogonal to
> the issue here. That was about systems with limited translation
> resources letting translation be specifically opt-in by IOMMU-aware
> drivers. It probably *would* happen to obviate the issue of disrupting
> live DMA at boot time on these particular Tegra platforms, but we still
> need something like Thierry's solution in general, since IOMMU drivers
> may have no other way to determine whether devices are active at boot
> and they have to take care to avoid breaking anything - e.g. SMMUv3 will
> at a bare minimum need to set up *some* form of valid stream table entry
> for the relevant device(s) right at the beginning where we first probe
> and reset the SMMU itself, regardless of what happens with domains and
> addresses later down the line.

Yes, I only meant that yours suggestion also should be useful here.
Anyways, thank you for the clarification :)

I agree that the Thierry's proposal is good! But it needs some more
thought yet because it's not very applicable to the current devices.

>>> The primary goal here is to move towards using the DMA API rather than
>>> the IOMMU API directly, so we don't really have the option of replacing
>>> with an explicitly created domain. Unless we have code in the DMA/IOMMU
>>> code that does this somehow.
>>>
>>> But I'm not sure what would be a good way to mark certain devices as
>>> needing an identity domain by default. Do we still use the reserved-
>>> memory node for that?
>>
>> The reserved-memory indeed shouldn't be needed for resolving the
>> implicit IOMMU problem since we could mark certain devices within the
>> kernel IOMMU driver.
>>
>> I haven't got around to trying to implement the implicit IOMMU support
>> yet, but I suppose we could implement the def_domain_type() hook in the
>> SMMU driver and then return IOMMU_DOMAIN_IDENTITY for the Display/VDE
>> devices. Then the Display/VDE drivers will take over the identity domain
>> and replace it with the explicit domain.
> 
> FWIW I've already cooked up identity domain support for tegra-gart; I
> was planning on tackling it for tegra-smmu as well for the next version
> of my arm default domains series (which will be after the next -rc1 now
> since I'm just about to take some long-overdue holiday).

Very nice! Maybe we will have some more food for the discussion by the
time you'll return. Have a good time!