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

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Message-ID: <51b2bd40-888d-4ee4-956f-c5239c5be9e9@linaro.org>
Date: Fri, 10 May 2024 15:02:46 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Bibek Kumar Patro <quic_bibekkum@...cinc.com>,
 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 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.

> 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..

Konrad