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

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Message-ID: <f5779af2-d690-45da-b223-352f2cc91b8a@quicinc.com>
Date: Thu, 30 May 2024 14:51:29 +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 5/28/2024 6:29 PM, Konrad Dybcio wrote:
> 
> 
> On 5/15/24 15:59, Bibek Kumar Patro wrote:
>>
>>
>> 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.
> 
> No! Upstream is not a dumping ground to reduce your technical debt.
> 
> There is no kgsl driver upstream, so this ought to be handled here, in
> the iommu driver (as poking at hardware A from driver B is usually not good
> practice).
> 

Okay, so I see this point now. Driver B need to use hardware A's driver
exposed interface only to interact with the hardware functionality
instead of directly poking it. Agree on this, it looks to be an
appropriate approach.

>>
>>>> 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.
> 
> Why is it not related? We still don't know what it does.
> 

By not related, I meant to say this bit is used for GFX implementation 
instead of direct SMMU related configuration.

Thanks & regards,
Bibek

> Konrad