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Subject: Re: [PATCH v8 3/5] iommu/arm-smmu: Invoke pm_runtime during probe,
 add/remove device
To: Tomasz Figa <tfiga@chromium.org>
Cc: Vivek Gautam <vivek.gautam@codeaurora.org>,
        Joerg Roedel <joro@8bytes.org>, Rob Herring <robh+dt@kernel.org>,
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        Archit Taneja <architt@codeaurora.org>,
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        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
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From: Robin Murphy <robin.murphy@arm.com>
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Date: Fri, 9 Mar 2018 17:36:31 +0000
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[ +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.