Re: [PATCH v2 1/2] ARM: common: Introduce PM domains for CPUs/clusters

[Lina Iyer <lina.iyer@linaro.org>]

On Thu, Aug 13 2015 at 11:29 -0600, Rob Herring wrote:
>On Wed, Aug 12, 2015 at 2:00 PM, Lina Iyer <lina.iyer@linaro.org> wrote:
>> Define and add Generic PM domains (genpd) for 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.
>>
>> Cc: Stephen Boyd <sboyd@codeaurora.org>
>> Cc: Kevin Hilman <khilman@linaro.org>
>> Cc: Ulf Hansson <ulf.hansson@linaro.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
>> Signed-off-by: Kevin Hilman <khilman@linaro.org>
>> Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
>> ---
>> Changes since v1:
>>
>> - Function name changes and split out common code
>> - Use cpu,pd for now. Removed references to ARM. Open to recommendations.
>> - Still located in arch/arm/common/. May move to a more appropriate location.
>> - Platform drivers can directly call of_init_cpu_domain() without using
>>   compatibles.
>> - Now maintains a list of CPU PM domains.
>
>[...]
>
>> +static int __init of_pm_domain_attach_cpus(void)
>> +{
>> +       int cpuid, ret;
>> +
>> +       /* Find any CPU nodes with a phandle to this power domain */
>> +       for_each_possible_cpu(cpuid) {
>> +               struct device *cpu_dev;
>> +               struct of_phandle_args pd_args;
>> +
>> +               cpu_dev = get_cpu_device(cpuid);
>> +               if (!cpu_dev) {
>> +                       pr_warn("%s: Unable to get device for CPU%d\n",
>> +                                       __func__, cpuid);
>> +                       return -ENODEV;
>> +               }
>> +
>> +               /*
>> +                * We are only interested in CPUs that can be attached to
>> +                * PM domains that are cpu,pd compatible.
>> +                */
>
>Under what conditions would the power domain for a cpu not be cpu,pd compatible?
>
Mostly never. But I dont want to assume and attach a CPU to its domain
that I am not concerned with.

>Why can't the driver handling the power domain register with gen_pd
>and the cpu_pd as the driver is going to be aware of which domains are
>for cpus.
They could and like Renesas they would. They could have an intricate
hierarchy of domains that they may want to deal with in their platform
drivers. Platforms could define the CPU devices as IRQ-safe and attach
it to their domains. Ensure the reference count of the online and
running CPUs are correct and they are good to go. They also would attach
the CPU devices to the domain and everything would work as they would
here. Its just repeated code across platforms that we are trying to
avoid.

>While there could be h/w such that all power domains within
>a chip have a nice uniform programming model, I'd guess that is the
>exception, not the rule. First, chips I have worked on were not that
>way. CPU related and peripheral related domains are handled quite
>differently. 
>
Agreed. These are very SoC specific components and would require
platform specific programming. But most SoCs, would need to do many
other things like suspending debuggers, reducing clocks, GIC save and
restore, cluster state determination etc that are only getting more
generalized. We have generalized ARM CPU idle, I see this as the
platform for the next set of power saving in an SoC.

>Second, often the actions on the CPU power domains don't
>take effect until a WFI, so you end up with a different programming
>sequence.
>
How so? The last core going down (in this case, when the domain is
suspended the last core is the last device that does a _put() on the
domain) would determine and perform the power domain's programming
sequence, in the context of the last CPU. A sequence that would only be
effected in hardware when the the CPU executes WFI. I dont see why it
would be any different than how it is today.

Thanks,
Lina