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Date:   Mon, 15 Mar 2021 00:31:57 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     akhilpo@codeaurora.org, iommu@lists.linux-foundation.org,
        jcrouse@codeaurora.org, linux-arm-kernel@lists.infradead.org,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        robdclark@gmail.com, robin.murphy@arm.com, will@kernel.org
Subject: Re: [PATCHv2 2/2] iommu/arm-smmu-qcom: Move the adreno smmu specific
 impl earlier
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On 2021-03-12 04:59, Bjorn Andersson wrote:
> On Sat 27 Feb 07:53 CST 2021, Sai Prakash Ranjan wrote:
> 
>> 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
>> (ARM SMMU IP).
>> 
>> 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)
>> and APSS SMMU was SMMU500(ARM 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?
>> 
> 
> No, I think you're doing the right thing of having them both. I just
> didn't remember us doing that.
> 
>> 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.
>> 
> 
> I do suspect that at some point (probably sooner than later) we'd have
> to support both inheriting of stream from the bootloader and the Adreno
> "quirks" in the same instance.
> 
> But for now this is okay to me.
> 

Sure, let me know if you or anyone face any issues without it and I will
add it. I will resend this series with the dt-bindings patch for sc7280 
smmu
which wasn't cc'd to smmu folks by mistake.

Thanks,
Sai

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