Re: [PATCH v2 2/2] [RFC] CPUFreq: Add support for cpu-perf-dependencies

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Oct 8, 2020 at 5:03 PM Ionela Voinescu <ionela.voinescu@arm.com> wrote:
>
> Hi Viresh,
>
> On Thursday 08 Oct 2020 at 16:32:41 (+0530), Viresh Kumar wrote:
> > On 07-10-20, 13:58, Nicola Mazzucato wrote:
> > > Hi Viresh,
> > >
> > > performance controls is what is exposed by the firmware through a protocol that
> > > is not capable of describing hardware (say SCMI). For example, the firmware can
> > > tell that the platform has N controls, but it can't say to which hardware they
> > > are "wired" to. This is done in dt, where, for example, we map these controls
> > > to cpus, gpus, etc.
> > >
> > > Let's focus on cpus.
> > >
> > > Normally we would have N of performance controls (what comes from f/w)
> > > that that correspond to hardware clock/dvfs domains.
> > >
> > > However, some firmware implementations might benefit from having finer
> > > grained information about the performance requirements (e.g.
> > > per-CPU) and therefore choose to present M performance controls to the
> > > OS. DT would be adjusted accordingly to "wire" these controls to cpus
> > > or set of cpus.
> > > In this scenario, the f/w will make aggregation decisions based on the
> > > requests it receives on these M controls.
> > >
> > > Here we would have M cpufreq policies which do not necessarily reflect the
> > > underlying clock domains, thus some s/w components will underperform
> > > (EAS and thermal, for example).
> > >
> > > A real example would be a platform in which the firmware describes the system
> > > having M per-cpu control, and the cpufreq subsystem will have M policies while
> > > in fact these cpus are "performance-dependent" each other (e.g. are in the same
> > > clock domain).
> >
> > If the CPUs are in the same clock domain, they must be part of the
> > same cpufreq policy.
>
> But cpufreq does not currently support HW_ALL (I'm using the ACPI
> coordination type to describe the generic scenario of using hardware
> aggregation and coordination when establishing the clock rate of CPUs).
>
> Adding support for HW_ALL* will involve either bypassing some
> assumptions around cpufreq policies or making core cpufreq changes.
>
> In the way I see it, support for HW_ALL involves either:
>
>  - (a) Creating per-cpu policies in order to allow each of the CPUs to
>    send their own frequency request to the hardware which will do
>    aggregation and clock rate decision at the level of the clock
>    domain.

This has been done for years on many platforms.

>    The PSD domains (ACPI) and the new DT binding will tell
>    which CPUs are actually in the same clock domain for whomever is
>    interested, despite those CPUs not being in the same policy.

And this information hasn't been used so far in those cases.

>    This requires the extra mask that Nicola introduced.

What would you use it for, specifically?

>  - (b) Making deep changes to cpufreq (core/governors/drivers) to allow:

Not an option really.

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