linux-kernel - Re: [PATCH v8 3/5] iommu/arm-smmu: Invoke pm_runtime during probe, add/remove device

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Message-ID: <ae34ca46-8699-bdc7-7d5c-c402e75be97d@arm.com>
Date:   Fri, 9 Mar 2018 17:36:31 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Vivek Gautam <vivek.gautam@...eaurora.org>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Will Deacon <will.deacon@....com>,
        Rob Clark <robdclark@...il.com>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jcrouse@...eaurora.org, Stephen Boyd <sboyd@...eaurora.org>,
        Sricharan R <sricharan@...eaurora.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Archit Taneja <architt@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v8 3/5] iommu/arm-smmu: Invoke pm_runtime during probe,
 add/remove device

[ +Lorenzo ]

On 09/03/18 04:50, Tomasz Figa wrote:
[...]
>>> Now we need a way to do the check. Perhaps for the time being it would
>>> be enough to just check for the power-domains property in DT?
>>
>>
>> AFAICS, it might be as simple as arm_smmu_probe() doing this:
>>
>>          /*
>>           * We want to avoid touching dev->power.lock in fastpaths unless
>>           * it's really going to do something useful - pm_runtime_enabled()
>>           * can serve as an ideal proxy for that decision.
>>           */
>>          if (dev->pm_domain)
>>                  pm_runtime_enable(dev);
>>
>> or maybe even just gate all the calls with "if (smmu->dev.pm_domain)"
>> directly (like pcie-mediatek does), but I'm not sure which would be
>> conceptually cleaner.
> 
> Okay, that was easier than I expected. Thanks. :)
> 
> Actually, there is one more thing that might need rechecking. Are you
> sure that dev->pm_domain is NULL for the devices, for which we don't
> want runtime PM to be enabled? I think ACPI was mentioned and ACPI
> includes the concept of PM domains.

Thanks for pointing that out - thankfully, I've confirmed that the SMMUs 
on my Juno don't have dev->pm_domain set when booting with ACPI, and 
double-checking the ACPI code I think we're OK here. Since the SMMUs are 
only described in the static IORT table and not in the ACPI namespace, 
they won't have the ACPI companion device that acpi_dev_pm_attach() 
looks for, and thus should always be ignored. Lorenzo, do I have that right?

Robin.