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

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Thu, Aug 13, 2015 at 04:45:03PM +0100, Lina Iyer wrote:
> On Thu, Aug 13 2015 at 09:01 -0600, Lorenzo Pieralisi wrote:
> >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.
> >
> CPUs have phandles to their domains. That is how the relationship
> between the domain provider (power-controller) and the consumer (CPU) is
> established.
> 
> >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" ?
> >
> Nope, not for calling a cluster, a cluster :)
> 
> This compatible is used to define a generic behavior of the CPU domain
> controller (in addition to the platform specific behavior of the domain
> power controller). The kernel activities for such power controller are
> generally the same which otherwise would be repeated across platforms.

What activities ? CPU PM notifiers ?

Thanks,
Lorenzo