linux-kernel - Re: [PATCH v18 0/5] iommu/arm-smmu: introduction of ACTLR implementation for Qualcomm SoCs

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Message-ID: <1db3d7f5-0cdf-449c-bbc3-dc40c89ab27f@oss.qualcomm.com>
Date: Thu, 13 Nov 2025 19:25:22 +0530
From: Charan Teja Kalla <charan.kalla@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Bibek Kumar Patro <bibek.patro@....qualcomm.com>,
        Enric Balletbo i Serra <eballetb@...hat.com>
Cc: robdclark@...il.com, will@...nel.org, robin.murphy@....com,
        joro@...tes.org, jgg@...pe.ca, jsnitsel@...hat.com, robh@...nel.org,
        krzysztof.kozlowski@...aro.org, quic_c_gdjako@...cinc.com,
        dmitry.baryshkov@...aro.org, iommu@...ts.linux.dev,
        linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Maxime Ripard <mripard@...nel.org>,
        Eric Chanudet <echanude@...hat.com>
Subject: Re: [PATCH v18 0/5] iommu/arm-smmu: introduction of ACTLR
 implementation for Qualcomm SoCs



On 11/12/2025 7:04 PM, Konrad Dybcio wrote:
>> Hi Eric,
>>
>> If a device has multiple SIDs, all serving the same functionality and grouped under the same "iommus" field, for example:
>>
>> iommus = <&apps_smmu, 0x2141, 0x0>,
>>           <&apps_smmu, 0x25c1, 0x0>,
>>           <&apps_smmu, 0x2161, 0x0>;
>>
>> In this case, all the SIDs will be associated with the same context bank. Even if the three SIDs have different ACTLR settings, since SMMU_CB_ACTLR is per CB setting, all SIDs attached to that bank will share the same ACTLR configuration. This is why we designed it to be "per device / per compatible".
> Does that suggest the settings may be slightly suboptimal?
> 
Or it is limitation to use the ACTLR?

> There's some work being done to allow more granular association of
> the passed SIDs:
> 
> https://lore.kernel.org/linux-arm-msm/20250928171718.436440-1-
> charan.kalla@....qualcomm.com/
Sorry, I am unable to link this limitation for actlr setting with the
work. Can you elaborate please?

IIUC, unless the SIDs are totally separated per actlr settings and
attached to CB(which are limited), this can't be achieved...but may be a
question here to check is it really a __valid__ to associate a different
actlr settings SID to use the same CB?