linux-kernel - Re: [PATCH v2 1/2] dt-bindings: iommu: arm,smmu-v3: Add SC8280XP compatible

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Date: Tue, 19 Mar 2024 13:53:32 +0000
From: Robin Murphy <robin.murphy@....com>
To: Konrad Dybcio <konrad.dybcio@...aro.org>,
 Bjorn Andersson <andersson@...nel.org>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Will Deacon <will@...nel.org>,
 Joerg Roedel <joro@...tes.org>, Rob Herring <robh@...nel.org>
Cc: Marijn Suijten <marijn.suijten@...ainline.org>,
 linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev, Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: iommu: arm,smmu-v3: Add SC8280XP
 compatible

On 2024-03-09 1:31 pm, Konrad Dybcio wrote:
> The smmu-v3 binding currently doesn't differentiate the SoCs it's
> implemented on. This is a poor design choice that may bite in the future,
> should any quirks surface.

That doesn't seem entirely fair to say - the vast majority of bindings 
don't have separate compatibles for every known integration of the same 
implementation in different SoCs. And in this case we don't have 
per-implementation compatibles for quirks and errata because the 
implementation is architecturally discoverable from the SMMU_IIDR register.

We have the whole mess for QCom SMMUv2 because the effective 
*implementation* is a mix of hardware and hypervisor, whose behaviour 
does seem to vary on almost a per-SoC basis. I'm not at all keen to 
start repeating that here without very good reason, and that of 
"documenting" a device which we typically expect to not even be 
accessible isn't really convincing me...

Thanks,
Robin.

> 
> Add a compatible for the instance found on Qualcomm SC8280XP.
> 
> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> ---
>   Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> index 75fcf4cb52d9..f284f7b0c1d8 100644
> --- a/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> +++ b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.yaml
> @@ -20,7 +20,11 @@ properties:
>     $nodename:
>       pattern: "^iommu@[0-9a-f]*"
>     compatible:
> -    const: arm,smmu-v3
> +    oneOf:
> +      - items:
> +          - const: qcom,sc8280xp-smmu-v3
> +          - const: arm,smmu-v3
> +      - const: arm,smmu-v3
>   
>     reg:
>       maxItems: 1
>