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

From: Konrad Dybcio <konrad.dybcio@linaro.org>
To: Bibek Kumar Patro <quic_bibekkum@quicinc.com>,
	Rob Clark <robdclark@gmail.com>
Cc: will@kernel.org, robin.murphy@arm.com, joro@8bytes.org,
	dmitry.baryshkov@linaro.org, jsnitsel@redhat.com,
	quic_bjorande@quicinc.com, mani@kernel.org,
	quic_eberman@quicinc.com, robdclark@chromium.org,
	u.kleine-koenig@pengutronix.de, robh@kernel.org,
	vladimir.oltean@nxp.com, quic_pkondeti@quicinc.com,
	quic_molvera@quicinc.com, linux-arm-msm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v9 3/5] iommu/arm-smmu: introduction of ACTLR for custom prefetcher settings
Date: Tue, 28 May 2024 14:59:51 +0200	[thread overview]
Message-ID: <7140cdb8-eda4-4dcd-b5e3-c4acdd01befb@linaro.org> (raw)
In-Reply-To: <0a867cd1-8d99-495e-ae7e-a097fc9c00e9@quicinc.com>

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@quicinc.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@quicinc.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).

> 
>>> 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

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