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

From: Robin Murphy
Date: Fri Mar 09 2018 - 12:36:42 EST


[ +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.