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

[Ulf Hansson]

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?

Kind regards
Uffe