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

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Date: Wed, 27 Mar 2024 20:23:51 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Robin Murphy <robin.murphy@....com>,
 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 19.03.2024 2:53 PM, Robin Murphy wrote:
> 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...

>From my POV as an arch dts maintainer, this often ends up being the only
way to retroactively add some conditional action into the code - the kernel
is supposed to be backwards compatible with older device trees.

And so far it's been almost by luck that all of the smmuv3 implementations
have been a straight copy-and-paste of the reference design (or close enough),
I don't believe this will be for much longer.

Konrad