linux-kernel - Re: [RFC PATCH] arm64: dts: ls1028a: mark ARM SMMU as DMA coherent

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Date:   Mon, 19 Dec 2022 14:16:18 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     devicetree@...r.kernel.org, iommu@...ts.linux.dev,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-kernel@...r.kernel.org, Michael Walle <michael@...le.cc>
Subject: Re: [RFC PATCH] arm64: dts: ls1028a: mark ARM SMMU as DMA coherent

Hi Robin,

On Wed, Dec 14, 2022 at 08:33:10PM +0000, Robin Murphy wrote:
> > Does looking at the CTTW bit make any sense for MMU-500?
> 
> In general, yes. The result above does imply that NXP have inadvertently set
> cfg_cttw wrong. For the avoidance of doubt, here's another MMU-500 showing
> SMMU_IDR0.CTTW set:
> 
> [    3.014972] arm-smmu arm-smmu.0.auto: probing hardware configuration...
> [    3.014974] arm-smmu arm-smmu.0.auto: SMMUv2 with:
> [    3.014976] arm-smmu arm-smmu.0.auto:        stage 2 translation
> [    3.014977] arm-smmu arm-smmu.0.auto:        coherent table walk
> [    3.014979] arm-smmu arm-smmu.0.auto:        stream matching with 128 register groups
> [    3.014981] arm-smmu arm-smmu.0.auto:        128 context banks (128 stage-2 only)
> [    3.014984] arm-smmu arm-smmu.0.auto:        Supported page sizes: 0x60211000
> [    3.014986] arm-smmu arm-smmu.0.auto:        Stage-2: 48-bit IPA -> 48-bit PA

Thanks for the explanations and the patch you've sent separately.

I have a side question, why is the dev_name() of your SMMU set to
"arm-smmu.0.auto" (determined by PLATFORM_DEVID_AUTO if I'm not mistaken)?
I'm asking because I would like to study the mechanism through which
your SMMU platform device get probed, to make sure that it's not
possible, during shutdown, for both platform_driver :: shutdown()
and platform_driver :: remove() methods to get called by the driver core.
This is generally not disallowed, and even possible if the entity who
registers these platform devices has its ->shutdown() method pointing
at ->remove().