Re: [PATCH 7/9] PM / ACPI: Enable the runtime PM centric approach for system sleep

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Thursday, June 22, 2017 11:42:11 AM Ulf Hansson wrote:
> On 21 June 2017 at 23:47, Rafael J. Wysocki <rafael@kernel.org> wrote:
> > On Wed, Jun 21, 2017 at 9:21 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> >> This change extends the interpretation of the ACPI's no_direct_complete
> >> flag to be used to enable the so called runtime PM centric approach, for
> >> devices being attached to the ACPI PM domain.
> >>
> >> The principle behind the runtime PM centric approach is to re-use the
> >> runtime PM callbacks to implement system sleep for drivers/subsystems.
> >> Moreover, using the runtime PM centric approach gives an optimized
> >> behaviour around avoiding to wake up a device from its low power state
> >> during system sleep, unless really needed.
> >>
> >> To deploy the runtime PM centric approach for a subsystem/driver, the
> >> following adaptations needs to be made.
> >>
> >> First, the runtime PM callbacks may be called when runtime PM has been
> >> disabled for the device. This serves as an indication for the callbacks to
> >> understand they are running in the system sleep sequence, instead of in the
> >> regular runtime PM path. In some cases, a callback needs to take different
> >> actions depending in what path it is being executed in, as is the case for
> >> the ACPI PM domain.
> >>
> >> In particular for the ACPI PM domain's ->runtime_suspend|resume()
> >> callbacks, when those finds runtime PM being disabled for the device, it
> >> instead executes the same operations as normally being run when
> >> ->suspend_late() and ->resume_early() callbacks are invoked during system
> >> sleep.
> >>
> >> Second, at the PM domain level, it is expected that the driver for the
> >> device makes use of pm_runtime_force_suspend|resume(), to re-use the
> >> runtime PM callbacks to put the device into low power state and to wake it
> >> up when needed during system sleep.
> >
> > What if it doesn't do that?
> >
> > Do all drivers of devices that may fall into the ACPI PM domain use
> > pm_runtime_force_suspend|resume()?
> 
> No, no - the runtime PM centric path is optional by all ACPI
> devices/drivers. The default is still for the ACPI PM domain to try
> the direct_complete path.
> 
> However if an ACPI device/driver (i2c designware in this case) likes
> to do that, they need to inform the ACPI PM domain about it. Then they
> call acpi_dev_disable_direct_complete() and makes use of
> pm_runtime_force_suspend|resume() to deal with system sleep.
> 
> Does that makes sense?

Overall, yes, it does, but then it should be made clear that when you use
"no_direct_complete" (all what you are going to call that eventually), you
also must use pm_runtime_force_suspend|resume() as your sleep callbacks.

Otherwise things may not work correctly if my understanding is correct.

Thanks,
Rafael