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

[Ulf Hansson]

On 22 June 2017 at 16:32, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
> On Thursday, June 22, 2017 11:42:11 AM Ulf Hansson wrote:
>> On 21 June 2017 at 23:47, Rafael J. Wysocki <rafael at kernel.org> wrote:
>> > On Wed, Jun 21, 2017 at 9:21 PM, Ulf Hansson <ulf.hansson at 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.

Okay, so I will clarify that in the header of
acpi_dev_disable_direct_complete() and I guess I should also update
some documentation around ACPI.

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

Correct.

Kind regards
Uffe