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

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Date: Thu, 30 May 2024 14:51:40 +0530
From: Bibek Kumar Patro <quic_bibekkum@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Konrad Dybcio
	<konrad.dybcio@...aro.org>
CC: Rob Clark <robdclark@...il.com>, <will@...nel.org>, <robin.murphy@....com>,
        <joro@...tes.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 6:36 PM, Dmitry Baryshkov wrote:
> On Tue, May 28, 2024 at 02:59:51PM +0200, 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).
> 
> I'd second the request here. If another driver has to control the
> behaviour of another driver, please add corresponding API for that.
> 

Ack, I understood this point now. Will add an interface for gfx to carry 
out the PRR bit implementation through smmu driver.

Thanks & regards,
Bibek

>>
>>>
>>>>> 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.
>>
>> Konrad
>