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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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Date: Wed, 1 Jul 2020 20:31:23 +0100
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Cc: Sachin Nikam <Snikam@nvidia.com>,
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On 01/07/2020 20:00, Krishna Reddy wrote:
>>>>> +        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.
> 
> In that case, new binding "nvidia,smmu-v2" can be added with data set to &arm_mmu500 and enumeration would have nvidia,tegra194-smmu and another variant for next generation SoC in future. 

I think you would be better off with nvidia,smmu-500 as smmu-v2 appears
to be something different. I see others have a smmu-v2 but I am not sure
if that is legacy. We have an smmu-500 and so that would seem more
appropriate.

Jon

-- 
nvpublic

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