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

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Date:   Wed, 1 Jul 2020 19:56:17 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Krishna Reddy <vdumpa@...dia.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

On 2020-07-01 19:18, Krishna Reddy wrote:
>>> + * 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.

Yeah, I realised later last night that this probably originated from 
forking the whole driver downstream. But even then you could have 
treated the other one as a separate nsmmu with a single instance ;)

Since it does add a bit of confusion to the code and comments, let's 
just keep things simple. I do like Jon's suggestion of actually 
enforcing that the number of "reg" regions exactly matches the number 
expected for the given compatible - I guess for now that means just 
hard-coding 2 and hoping the hardware folks don't cook up any more of 
these...

Robin.