Ulf Hansson [off-list ref] writes:

In cases when the PM domain isn't powered off when genpd's ->prepare()
callback is invoked, genpd runtime resumes and disables runtime PM for the
device. This behaviour was needed when genpd managed intermediate states
during the power off sequence, as to maintain proper low power states of
devices during system PM suspend/resume.

The commit ba2bbfbf6307 ("PM / Domains: Remove intermediate states from
the power off sequence"), enables genpd to improve its behaviour in this
regards.

The PM core disables runtime PM at __device_suspend_late() before it calls
a system PM "late" callback for a device. When resuming a device, after a
corresponding "early" callback has been invoked, the PM core re-enables
runtime PM.

By changing genpd to allow runtime PM according to the same system PM
phases as the PM core, devices can be runtime resumed by its corresponding

grammar nit: s/its/their/

subsystem/driver when really needed.

In this way, genpd no longer need to runtime resume the device from its
->prepare() callback. In most cases that avoids unnecessary and energy-
wasting operations of runtime resuming devices that have nothing to do,
only to runtime suspend them shortly after.

Although, because of changing this behaviour in genpd and due to that
genpd powers on the PM domain unconditionally in the system PM resume
"noirq" phase, it could potentially cause a PM domain to stay powered on
even if it's unused after the system has resumed. To avoid this, let's
schedule a power off work when genpd's system PM ->complete() callback has
been invoked for the last device in the PM domain.

IIUC, this should be the end of the changelog for this patch.  The rest
was moved to a new patch, no?

Another issue that arises due to this change in genpd, concerns those
platforms/PM domains that makes use of genpd's device ->stop|start()
callbacks. In these scenarios, the corresponding subsystem/driver needs to
invoke pm_runtime_force_suspend() from a system PM suspend callback to
allow genpd's ->runtime_suspend() to be invoked for an active device, else
genpd won't "stop" a device that has been "started".

The subsystem/driver also needs to invoke pm_runtime_force_resume()
in a system PM resume callback, to restore the runtime PM state for the
device and to re-enable runtime PM.

Signed-off-by: Ulf Hansson <redacted>

Other than the changelog nits above, LGTM.

Reviewed-by: Kevin Hilman <khilman@baylibre.com>

Kevin