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SubjectRe: [PATCHv2 2/2] iommu/arm-smmu-qcom: Move the adreno smmu specific impl earlier
Hi Bjorn,

On 2021-02-27 00:44, Bjorn Andersson wrote:
> On Fri 26 Feb 12:23 CST 2021, Rob Clark wrote:
> The current logic picks one of:
> 1) is the compatible mentioned in qcom_smmu_impl_of_match[]
> 2) is the compatible an adreno
> 3) no quirks needed
> The change flips the order of these, so the only way I can see this
> change affecting things is if we expected a match on #2, but we got one
> on #1.
> Which implies that the instance that we want to act according to the
> adreno impl was listed in qcom_smmu_impl_of_match[] - which either is
> wrong, or there's a single instance that needs both behaviors.
> (And I believe Jordan's answer confirms the latter - there's a single
> SMMU instance that needs all them quirks at once)

Let me go through the problem statement in case my commit message wasn't
clear. There are two SMMUs (APSS and GPU) on SC7280 and both are SMMU500

APSS SMMU compatible - ("qcom,sc7280-smmu-500", "arm,mmu-500")
GPU SMMU compatible - ("qcom,sc7280-smmu-500", "qcom,adreno-smmu", "arm,mmu-500")

Now if we take SC7180 as an example, GPU SMMU was QSMMU(QCOM SMMU IP)

APSS SMMU compatible - ("qcom,sc7180-smmu-500", "arm,mmu-500")
GPU SMMU compatible - ("qcom,sc7180-smmu-v2", "qcom,adreno-smmu", "qcom,smmu-v2")

Current code sequence without this patch,

if (of_match_node(qcom_smmu_impl_of_match, np))
return qcom_smmu_create(smmu, &qcom_smmu_impl);

if (of_device_is_compatible(np, "qcom,adreno-smmu"))
return qcom_smmu_create(smmu, &qcom_adreno_smmu_impl);

Now if we look at the compatible for SC7180, there is no problem because
the APSS SMMU will match the one in qcom_smmu_impl_of_match[] and GPU SMMU
will match "qcom,adreno-smmu" because the compatible strings are different.
But for SC7280, both the APSS SMMU and GPU SMMU compatible("qcom,sc7280-smmu-500")
are same. So GPU SMMU will match with the one in qcom_smmu_impl_of_match[]
i.e.., "qcom,sc7280-smmu-500" which functionally doesn't cause any problem
but we will miss settings for split pagetables which are part of GPU SMMU
specific implementation.

We can avoid this with yet another new compatible for GPU SMMU something like
"qcom,sc7280-adreno-smmu-500" but since we can handle this easily in the
driver and since the IPs are same, meaning if there was a hardware quirk
required, then we would need to apply to both of them and would this additional
compatible be of any help?

Coming to the part of quirks now, you are right saying both SMMUs will need
to have the same quirks in SC7280 and similar others where both are based on
same IPs but those should probably be *hardware quirks* and if they are
software based like the S2CR quirk depending on the firmware, then it might
not be applicable to both. In case if it is applicable, then as Jordan mentioned,
we can add the same quirks in GPU SMMU implementation.


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