Re: [Discussion] Performance levels of power domains

[Kevin Hilman <khilman@baylibre.com>]

"Rafael J. Wysocki" <rafael@kernel.org> writes:

> On Wed, Oct 26, 2016 at 9:00 PM, Kevin Hilman <khilman@baylibre.com> wrote:
>> Viresh Kumar <viresh.kumar@linaro.org> writes:
>>
>>> Hi Guys,
>>>
>>> I wanted to involve you guys to get a discussion going
>>> for a problem we want to solve, and so this mail.
>>>
>>>
>>> Platform details:
>>>
>>> Some of the Qualcom SoCs have the option to configure
>>> the performance level of their Power Domains. The performance
>>> levels are identified by integer values (lets say 0-9, 0 being the lowest).
>>>
>>> Another M3 core handles the *real* voltage scaling based on the input
>>> received (from software) in terms of these performance levels. The M3
>>> core translates the levels into a range of voltages (corners) and selects
>>> the right one by itself.
>>>
>>> Software needs to provide the performance level for the entire domain
>>> to the M3 core and so software also needs to handle performance requests
>>> from all the devices that lie in the domain X and find a Performance Level P,
>>> which can satisfy all the devices (normally the highest requrested level).
>>>
>>>
>>> Problem statement:
>>>
>>> As we aren't dealing with Voltages here, we can't really get the benefits
>>> of the Regulators framework. The regulators are managed internally
>>> by the M3 core. All we need is a way for software to comeout with inputs
>>> for the M3 core.
>>>
>>> The OPP framework can be used to include performance levels for
>>> each OPP (frequency) entry.
>>>
>>> But what framework can be used to select performance level of power
>>> domains ?
>>>
>>> By name, power-domain or genpd looks to be the right choice, but until
>>> now it is only managing power-on and power-off of devices and domains.
>>
>> genpd has also recently been extended to support multiple states, though
>> those are still idle states, not active (performance) states.
>>
>>> Should we extend that (along with runtime PM), or do something else?
>>
>> Yes.  As I've suggested to qcom/linaro folks (off-list discussions), I
>> think extending genpd to handle performance states is a logical
>> extension.  Otherwise, you will be (re)inventing something that looks an
>> awful lot like genpd anyways.
>
> On the Intel side we also have a mechanism to tell the processor about
> the power/performance preference and it would be good to have a common
> way to do it on all platforms and genpd doesn't look like a
> particularly good place for that.
>
>> The other related framework is per-device PM QoS which could be used to
>> set constraints on specific devices, and the genpd governors would then
>> be responsible for looking at the constraints and changing states as
>> needed.
>
> Right.
>
> Let's talk about this at the LPC.

Sounds good.

Kevin