linux-kernel - Re: [PATCH v8 2/3] dt-bindings: arm-smmu: Add binding for Tegra194 SMMU

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Message-ID: <0d4f46d6-6a4e-bca0-bcf3-0e22a950e57b@nvidia.com>
Date:   Wed, 1 Jul 2020 19:47:36 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Krishna Reddy <vdumpa@...dia.com>,
        Robin Murphy <robin.murphy@....com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Thierry Reding <treding@...dia.com>,
        "Yu-Huan Hsu" <YHsu@...dia.com>, Sachin Nikam <Snikam@...dia.com>,
        Pritesh Raithatha <praithatha@...dia.com>,
        Timo Alho <talho@...dia.com>,
        Bitan Biswas <bbiswas@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        Bryan Huntsman <bhuntsman@...dia.com>,
        "nicoleotsuka@...il.com" <nicoleotsuka@...il.com>
Subject: Re: [PATCH v8 2/3] dt-bindings: arm-smmu: Add binding for Tegra194
 SMMU


On 01/07/2020 19:28, Krishna Reddy wrote:
>>> +      - description: NVIDIA SoCs that use more than one "arm,mmu-500"
>> Hmm, there must be a better way to word that to express that it only applies to the sets of SMMUs that must be programmed identically, and not any other independent MMU-500s that might also happen to be in the same SoC.
> 
> Let me reword it to "NVIDIA SoCs that must program multiple MMU-500s identically".
> 
>>> +        items:
>>> +          - enum:
>>> +              - nvdia,tegra194-smmu
>>> +          - const: arm,mmu-500
> 
>> Is the fallback compatible appropriate here? If software treats this as a standard MMU-500 it will only program the first instance (because the second isn't presented as a separate MMU-500) - is there any way that isn't going to blow up?
> 
> When compatible is set to both nvidia,tegra194-smmu and arm,mmu-500, implementation override ensure that both instances are programmed. Isn't it? I am not sure I follow your comment fully.

The problem is, if for some reason someone had a Tegra194, but only set
the compatible string to 'arm,mmu-500' it would assume that it was a
normal arm,mmu-500 and only one instance would be programmed. We always
want at least 2 of the 3 instances programmed and so we should only
match 'nvidia,tegra194-smmu'. In fact, I think that we also need to
update the arm_smmu_of_match table to add 'nvidia,tegra194-smmu' with
the data set to &arm_mmu500.

Jon

-- 
nvpublic