[PATCH RFC 0/3] PM / Domains: Generic PM domains for cpus

Lina Iyer [off-list ref] writes:

This is an attempt to provide a generic PM domain for cpus, so as to allow the
cpu domain to be powered off, when all the cpus are in power down state during
cpuidle. 

You don't use the word "cluster" here, but it might be helpful to
summarize this as using a genpd to model a cluster.  A cluster contains
CPUs, but also additional devices/logic like L2$, GIC, PMUs, floating
point units, CoreSight, etc. etc.

The rationale behind the change is that newer SoCs can power down the
cpus for very short sleep times to save on leakage power. The domain which
usually has the cpus, a second level cache and some peripheral hardware is
powered by a rail that can also be turned off when the cpus are not in
use.

This last sentence doesn't read well.  I think you meant something like:

In the domain which has the CPUs, a second level cache and some
peripheral hardware might share the power rail with the CPUs so could
also be turned off when the cpus are not in use.

Devices for each cpu, L2 and other related blocks could be attached to the
domain and when there are no uses of these devices (device idle) the domain
could also be powered off. Generic PM domains provides all the backend needed
for such an organization.

In the first 2 patches, I make genpd usable in atomic context as well. CPUIdle

s/as well//

runs with irqs disabled and therefore use of mutexes in the current genpd
implementation is a limitation. But, not all PM domains need to operate in irq
safe context, those that need to can specify explicitly the irq safe
requirement of the genpd at init. Devices and sub-domains that attach to an irq
safe genpd also have to be irq safe.

The third patch, adds a generic PM domain for the cpus. GenPD provider can be

drop the ',' 

A GenPD provider...

specified in the DT and individual cpus that are part of the domain would be
the domain consumers. A new API pm_cpu_domain_init() has been introduced that
would initialize the genpd and attach cpus specified as consumers in the DT to
the domain. 

Hmm, doesn't the of_genpd_* stuff already handle this?

What about non-CPU consumers in the same domain?  

When the cpus enter their idle state cpu_pm notifications are sent
out for that cpu. The last cpu to send cpu_pm notification would trigger the
domain power_off callback and the first cpu to come up would trigger the genpd
power_on callback.  Generally, the power_off callback is where the caches are
flushed in preparation for a power down and the domain hardware configured to
power down when the cpu finishes execution. In the power_on callback, the
domain hardware is reset to a state to allow cpus to be active or entire idle
states individually.

A future addition to this feature, could be a new genpd governor for the
specific case of the cpu domain.  The governor may look up the time available
to sleep between the last cpu down and the first cpu up, in determining if it
would be more efficient to just keep the domain powered on.

This patch is based on Ulf's patch for simplifying domain power down states
[1], which removes a bunch of complexity in genpd, simplifying atomic genpd.

Kevin