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

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Message-ID: <3e655881-bac4-f083-44ed-cfa0a61298d0@arm.com>
Date:   Tue, 30 Jun 2020 13:27:36 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Krishna Reddy <vdumpa@...dia.com>
Cc:     joro@...tes.org, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-tegra@...r.kernel.org, treding@...dia.com, yhsu@...dia.com,
        snikam@...dia.com, praithatha@...dia.com, talho@...dia.com,
        bbiswas@...dia.com, mperttunen@...dia.com, nicolinc@...dia.com,
        bhuntsman@...dia.com, nicoleotsuka@...il.com
Subject: Re: [PATCH v8 2/3] dt-bindings: arm-smmu: Add binding for Tegra194
 SMMU

On 2020-06-30 01:10, Krishna Reddy wrote:
> Add binding for NVIDIA's Tegra194 SoC SMMU topology that is based
> on ARM MMU-500.
> 
> Signed-off-by: Krishna Reddy <vdumpa@...dia.com>
> ---
>   Documentation/devicetree/bindings/iommu/arm,smmu.yaml | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> index d7ceb4c34423b..5b2586ac715ed 100644
> --- a/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> +++ b/Documentation/devicetree/bindings/iommu/arm,smmu.yaml
> @@ -38,6 +38,11 @@ properties:
>                 - qcom,sc7180-smmu-500
>                 - qcom,sdm845-smmu-500
>             - const: arm,mmu-500
> +      - 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.

> +        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?

Robin.

>         - items:
>             - const: arm,mmu-500
>             - const: arm,smmu-v2
>