linux-kernel - RE: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual ARM MMU-500 usage

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Message-ID: <BYAPR12MB2822FC263456303302EF30BEB36C0@BYAPR12MB2822.namprd12.prod.outlook.com>
Date:   Wed, 1 Jul 2020 18:18:22 +0000
From:   Krishna Reddy <vdumpa@...dia.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jonathan Hunter <jonathanh@...dia.com>
CC:     Sachin Nikam <Snikam@...dia.com>,
        "nicoleotsuka@...il.com" <nicoleotsuka@...il.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        "Bryan Huntsman" <bhuntsman@...dia.com>,
        "will@...nel.org" <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Pritesh Raithatha" <praithatha@...dia.com>,
        Timo Alho <talho@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Yu-Huan Hsu <YHsu@...dia.com>,
        Thierry Reding <treding@...dia.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Bitan Biswas <bbiswas@...dia.com>
Subject: RE: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual
 ARM MMU-500 usage

>> + * When Linux kernel supports multiple SMMU devices, the SMMU device 
>> +used for
>> + * isochornous HW devices should be added as a separate ARM MMU-500 
>> +device
>> + * in DT and be programmed independently for efficient TLB invalidates.

>I don't understand the "When" there - the driver has always supported multiple independent SMMUs, and it's not something that could be configured out or otherwise disabled. Plus I really don't see why you would ever want to force unrelated SMMUs to be >programmed together - beyond the TLB thing mentioned it would also waste precious context bank resources and might lead to weird device grouping via false stream ID aliasing, with no obvious upside at all.

Sorry, I missed this comment.
During the initial patches, when the iommu_ops were different between, support multiple SMMU drivers at the same is not possible as one of them(that gets probed last) overwrites the platform bus ops. 
On revisiting the original issue, This problem is no longer relevant. At this point, It makes more sense to just get rid of 3rd instance programming in arm-smmu-nvidia.c and just limit it to the SMMU instances that need identical programming.

-KR