linux-kernel - Re: [PATCH v9 3/5] iommu/arm-smmu: introduction of ACTLR for custom prefetcher settings

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Date: Wed, 15 May 2024 19:29:29 +0530
From: Bibek Kumar Patro <quic_bibekkum@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@...aro.org>, Rob Clark <robdclark@...il.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
        <dmitry.baryshkov@...aro.org>, <jsnitsel@...hat.com>,
        <quic_bjorande@...cinc.com>, <mani@...nel.org>,
        <quic_eberman@...cinc.com>, <robdclark@...omium.org>,
        <u.kleine-koenig@...gutronix.de>, <robh@...nel.org>,
        <vladimir.oltean@....com>, <quic_pkondeti@...cinc.com>,
        <quic_molvera@...cinc.com>, <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 3/5] iommu/arm-smmu: introduction of ACTLR for custom
 prefetcher settings



On 5/10/2024 6:32 PM, Konrad Dybcio wrote:
> On 10.05.2024 2:52 PM, Bibek Kumar Patro wrote:
>>
>>
>> On 5/1/2024 12:30 AM, Rob Clark wrote:
>>> On Tue, Jan 23, 2024 at 7:00 AM Bibek Kumar Patro
>>> <quic_bibekkum@...cinc.com> wrote:
>>>>
>>>> Currently in Qualcomm  SoCs the default prefetch is set to 1 which allows
>>>> the TLB to fetch just the next page table. MMU-500 features ACTLR
>>>> register which is implementation defined and is used for Qualcomm SoCs
>>>> to have a custom prefetch setting enabling TLB to prefetch the next set
>>>> of page tables accordingly allowing for faster translations.
>>>>
>>>> ACTLR value is unique for each SMR (Stream matching register) and stored
>>>> in a pre-populated table. This value is set to the register during
>>>> context bank initialisation.
>>>>
>>>> Signed-off-by: Bibek Kumar Patro <quic_bibekkum@...cinc.com>
>>>> ---
> 
> [...]
> 
>>>> +
>>>> +               for_each_cfg_sme(cfg, fwspec, j, idx) {
>>>> +                       smr = &smmu->smrs[idx];
>>>> +                       if (smr_is_subset(smr, id, mask)) {
>>>> +                               arm_smmu_cb_write(smmu, cbndx, ARM_SMMU_CB_ACTLR,
>>>> +                                               actlrcfg[i].actlr);
>>>
>>> So, this makes ACTLR look like kind of a FIFO.  But I'm looking at
>>> downstream kgsl's PRR thing (which we'll need to implement vulkan
>>> sparse residency), and it appears to be wanting to set BIT(5) in ACTLR
>>> to enable PRR.
>>>
>>>           val = KGSL_IOMMU_GET_CTX_REG(ctx, KGSL_IOMMU_CTX_ACTLR);
>>>           val |= FIELD_PREP(KGSL_IOMMU_ACTLR_PRR_ENABLE, 1);
>>>           KGSL_IOMMU_SET_CTX_REG(ctx, KGSL_IOMMU_CTX_ACTLR, val);
>>>
>>> Any idea how this works?  And does it need to be done before or after
>>> the ACTLR programming done in this patch?
>>>
>>> BR,
>>> -R
>>>
>>
>> Hi Rob,
>>
>> Can you please help provide some more clarification on the FIFO part? By FIFO are you referring to the storing of ACTLR data in the table?
>>
>> Thanks for pointing to the downstream implementation of kgsl driver for
>> the PRR bit. Since kgsl driver is already handling this PRR bit's
>> setting, this makes setting the PRR BIT(5) by SMMU driver redundant.
> 
> The kgsl driver is not present upstream.
> 

Right kgsl is not present upstream, it would be better to avoid 
configuring the PRR bit and can be handled by kgsl directly in downstream.

>> Thanks for bringing up this point.
>> I will send v10 patch series removing this BIT(5) setting from the ACTLR
>> table.
> 
> I think it's generally saner to configure the SMMU from the SMMU driver..

Yes, agree on this. But since PRR bit is not directly related to SMMU
configuration so I think it would be better to remove this PRR bit
setting from SMMU driver based on my understanding.


Thanks & regards,
Bibek
> 
> Konrad