[PATCH 5/9] ARM: common: Introduce PM domains for CPUs/clusters

[lorenzo.pieralisi@arm.com (Lorenzo Pieralisi)]

On Thu, Aug 06, 2015 at 04:14:51AM +0100, Rob Herring wrote:
> On Tue, Aug 4, 2015 at 6:35 PM, Lina Iyer <lina.iyer@linaro.org> wrote:
> > Define and add Generic PM domains (genpd) for ARM CPU clusters. Many new
> > SoCs group CPUs as clusters. Clusters share common resources like GIC,
> > power rail, caches, VFP, Coresight etc. When all CPUs in the cluster are
> > idle, these shared resources may also be put in their idle state.
> >
> > The idle time between the last CPU entering idle and a CPU resuming
> > execution is an opportunity for these shared resources to be powered
> > down. Generic PM domain provides a framework for defining such power
> > domains and attach devices to the domain. When the devices in the domain
> > are idle at runtime, the domain would also be suspended and resumed
> > before the first of the devices resume execution.
> >
> > We define a generic PM domain for each cluster and attach CPU devices in
> > the cluster to that PM domain. The DT definitions for the SoC describe
> > this relationship. Genpd callbacks for power_on and power_off can then
> > be used to power up/down the shared resources for the domain.
> 
> [...]
> 
> > +ARM CPU Power domains
> > +
> > +The device tree allows describing of CPU power domains in a SoC. In ARM SoC,
> > +CPUs may be grouped as clusters. A cluster may have CPUs, GIC, Coresight,
> > +caches, VFP and power controller and other peripheral hardware. Generally,
> > +when the CPUs in the cluster are idle/suspended, the shared resources may also
> > +be suspended and resumed before any of the CPUs resume execution.
> > +
> > +CPUs are the defined as the PM domain consumers and there is a PM domain
> > +provider for the CPUs. Bindings for generic PM domains (genpd) is described in
> > +[1].
> > +
> > +The ARM CPU PM domain follows the same binding convention as any generic PM
> > +domain. Additional binding properties are -
> > +
> > +- compatible:
> > +       Usage: required
> > +       Value type: <string>
> > +       Definition: Must also have
> > +                       "arm,pd"
> > +               inorder to initialize the genpd provider as ARM CPU PM domain.
> 
> A compatible string should represent a particular h/w block. If it is
> generic, it should represent some sort of standard programming
> interface (e.g, AHCI, EHCI, etc.). This doesn't seem to be either and
> is rather just a mapping of what "driver" you want to use.
> 
> I would expect that identifying a cpu's or cluster's power domain
> would be done by a phandle between the cpu/cluster node and power
> domain node. But I've not really looked at the power domain bindings
> so who knows.

I would expect the same, meaning that a cpu node, like any other device
node would have a phandle pointing at the respective HW power domain.

I do not really understand why we want a "generic" CPU power domain, what
purpose does it serve ? Creating a collection of cpu devices that we
can call "cluster" ?

Thanks,
Lorenzo