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

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Message-ID: <9b406a7c-57b8-4b5f-8fbc-714560cce8cf@quicinc.com>
Date:   Fri, 17 Nov 2023 10:57:19 +0530
From:   Bibek Kumar Patro <quic_bibekkum@...cinc.com>
To:     Robin Murphy <robin.murphy@....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 11/15/2023 8:23 PM, Robin Murphy wrote:
> 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.
> 

I see, this looks like probably we need a separate patch altogether for
this cleanup, as "cfg" is used in other fault handling places as well as 
i can see and is introduced as a part of different patch. Should we 
scope this work for a separate patch if it's okay?

Thanks,
Bibek
>>>> +
>>>>       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 {
>>>>   };
>>>>