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

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Message-ID: <67ae3a53-3db9-c1bc-2b67-08df14bc15cc@arm.com>
Date:   Tue, 3 Jan 2023 18:12:29 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Vladimir Oltean <vladimir.oltean@....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

On 19/12/2022 12:16 pm, Vladimir Oltean wrote:
> 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)?

This is an ACPI-based machine, where platform device discovery and 
creation is... different :)

SMMUs are among those managed by drivers/acpi/arm64/iort.c

> 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().

Yikes, I'd very much hope that that's not a thing!

Cheers,
Robin.