linux-kernel - Re: [PATCH v2 1/3] iommu/arm-smmu: introduction of ACTLR for custom prefetcher settings

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Date:   Wed, 15 Nov 2023 14:53:04 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Bibek Kumar Patro <quic_bibekkum@...cinc.com>, will@...nel.org,
        joro@...tes.org, dmitry.baryshkov@...aro.org, a39.skl@...il.com,
        konrad.dybcio@...aro.org, quic_pkondeti@...cinc.com,
        quic_molvera@...cinc.com
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, qipl.kernel.upstream@...cinc.com
Subject: Re: [PATCH v2 1/3] iommu/arm-smmu: introduction of ACTLR for custom
 prefetcher settings

On 2023-11-15 1:54 pm, Bibek Kumar Patro wrote:
> 
>>> @@ -467,6 +505,9 @@ static struct arm_smmu_device 
>>> *qcom_smmu_create(struct arm_smmu_device *smmu,
>>>       qsmmu->smmu.impl = impl;
>>>       qsmmu->cfg = data->cfg;
>>>
>>> +    if (data->actlrcfg && (data->actlrcfg->size))
>>> +        qsmmu->actlrcfg = data->actlrcfg;
>>
>> Do we really need to replicate multiple parts of the data, or would it 
>> be sensible to just replace qsmmu->cfg with qsmmu->data and handle the 
>> further dereferences in the places that want them?
>>
> 
> Mm, could not understand this properly. :( Could you help explain more 
> please?
> As per my understanding aren't data and qsmmu different structures.
> qcom_smmu is a superset of arm_smmu housing additonal properties
> and qcom_smmu_match_data is kind of a superset of arm_smmu_impl with
> additional specific implmentations, so both needs to be in place?
> Apologies if I understood your statement incorrectly.

My point is that the data is static and constant, so there's really no 
point storing multiple pointers into different bits of it. So rather than:

	qsmmu->cfg = data->cfg;
	qssmu->actlrcfg = data->actlrcfg;
	...
	do_something(qsmmu->cfg);
	...
	do_other_thing(qsmmu->actlrcfg);

we can just store the one pointer and have:

	qsmmu->data = data;
	...
	do_something(qsmmu->data->cfg);
	...
	do_other_thing(qsmmu->data->actlrcfg);

Thanks,
Robin.

>>> +
>>>       return &qsmmu->smmu;
>>>   }
>>>
>>> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.h 
>>> b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.h
>>> index 593910567b88..4b6862715070 100644
>>> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.h
>>> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.h
>>> @@ -9,6 +9,7 @@
>>>   struct qcom_smmu {
>>>       struct arm_smmu_device smmu;
>>>       const struct qcom_smmu_config *cfg;
>>> +    const struct actlr_config *actlrcfg;
>>>       bool bypass_quirk;
>>>       u8 bypass_cbndx;
>>>       u32 stall_enabled;
>>> @@ -25,6 +26,7 @@ struct qcom_smmu_config {
>>>   };
>>>