On 21 June 2017 at 23:47, Rafael J. Wysocki [off-list ref] wrote:

On Wed, Jun 21, 2017 at 9:21 PM, Ulf Hansson [off-list ref] wrote:

quoted

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