[PATCH v2 5/9] PM / ACPI: Provide option to disable direct_complete for ACPI devices

[Rafael J. Wysocki]

On Thursday, August 24, 2017 10:19:43 AM CEST Ulf Hansson wrote:
> On 24 August 2017 at 01:39, Rafael J. Wysocki <rafael at kernel.org> wrote:
> > On Wed, Aug 23, 2017 at 4:42 PM, Ulf Hansson <ulf.hansson at linaro.org> wrote:
> >> In some cases a driver for an ACPI device needs to be able to prevent the
> >> ACPI PM domain from using the direct_complete path during system sleep.
> >>
> >> One typical case is when the driver for the device needs its device to stay
> >> runtime enabled, during the __device_suspend phase. This isn't the case
> >> when the direct_complete path is being executed by the PM core, as it then
> >> disables runtime PM for the device in __device_suspend(). Any following
> >> attempts to runtime resume the device after that point, just fails.
> >
> > OK, that is a problem.
> >
> >> A workaround to this problem is to let the driver runtime resume its device
> >> from its ->prepare() callback, as that would prevent the direct_complete
> >> path from being executed. However, that may often be a waste, especially if
> >> it turned out that no one really needed the device.
> >>
> >> For this reason, invent acpi_dev_disable|enable_direct_complete(), to allow
> >> drivers to inform the ACPI PM domain to change its default behaviour during
> >> system sleep, and thus control whether it may use the direct_complete path
> >> or not.
> >
> > But I'm not sure this is the right place to address it as it very well
> > may affect a PCI device too.
> >
> > Also, this is about the device and not about its ACPI companion
> > object, so putting the flag in there is somewhat unclean, so to speak.
> >
> > It looks like we need a flag in dev_pm_info saying something along the
> > lines of "my system suspend callback can deal with runtime PM" that
> > will cause the direct_complete update to occur in
> > __device_suspend_late() instead of __device_suspend().
> 
> I realize that in the end this turns out to be a comparison between
> the runtime PM centric path and the direct_complete path while
> implementing system sleep. In patch 9, there is some more explanation
> around this, however if you like I can elaborate even more about
> this!?
> 
> Regarding making changes to the PM core and adding more flags to the
> dev_pm_info etc, I am not sure we really want that. Isn't it already
> complex enough?

Maybe it is.

> My point is, that I am trying to improve the behavior of the ACPI PM
> domain by enabling the runtime PM centric path for it, and even if
> there is something similar that could be done for PCI, I don't think
> we should need involvement by the PM core.

Well, this generally simply doesn't work.

The whole "runtime PM centric approach" idea is generally fine by me,
but the fact today is that there are drivers not ready for it.  Which
is why there is the direct_complete thing (it may be regarded as a
sort-of workaround for the unreadiness of drivers if you will).

Now, buy adding the no_direct_complete flag just to the ACPI PM domain
you basically overlook the fact that this potentially affects the parents
of the devices in question by preventing direct_complete from being set
for them.  And those parents may not be in the ACPI PM domain in principle,
so the problem needs to be addressed in the core.

Thanks,
Rafael