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Subject: Re: [PATCH v8 2/3] dt-bindings: arm-smmu: Add binding for Tegra194
 SMMU
To: Krishna Reddy <vdumpa@nvidia.com>, Robin Murphy <robin.murphy@arm.com>
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From: Jon Hunter <jonathanh@nvidia.com>
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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
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