Re: [Discussion] Performance levels of power domains

[Kevin Hilman <khilman@baylibre.com>]

Viresh Kumar <viresh.kumar@linaro.org> writes:

> On 27 October 2016 at 15:41, Rafael J. Wysocki <rafael@kernel.org> wrote:
>> 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.
>
> Any updates from LPC on this ?

The short version: we agreed that extending genpd to support performance
states/levels is the right way to go.

Kevin