Re: [PATCH v2 1/3] cpufreq: CPPC: Keep the policy across CPU hotplug

[Sumit Gupta]

On 22/07/26 15:48, Christian Loehle wrote:
> External email: Use caution opening links or attachments
>
>
> On 7/16/26 16:38, Sumit Gupta wrote:
> > Without online()/offline() callbacks, the cpufreq core fully tears
> > down a policy during exit() when its last online CPU is offlined,
> > and rebuilds it during init() when it comes back.
> >
> > Add lightweight online()/offline() callbacks so the core instead
> > keeps the policy live and reuses the driver's cpu_data across
> > CPU hotplug. This avoids re-reading the CPPC capabilities on every
> > offline/online, making CPU hotplug faster.
> >
> > Re-enable CPPC from online(), as it may have been disabled while
> > the CPU was offline.
> >
> > Signed-off-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> > ---
> >   drivers/cpufreq/cppc_cpufreq.c | 27 +++++++++++++++++++++++++++
> >   1 file changed, 27 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> > index f6cea0c54dd9..432c6a6288a7 100644
> > --- a/drivers/cpufreq/cppc_cpufreq.c
> > +++ b/drivers/cpufreq/cppc_cpufreq.c
> > @@ -722,6 +722,31 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
> >        return ret;
> >   }
> >
> > +/*
> > + * With offline() defined, the cpufreq core keeps the policy alive when
> > + * a CPU is hotplugged out.
> > + */
> > +static int cppc_cpufreq_cpu_offline(struct cpufreq_policy *policy)
> > +{
> > +     return 0;
> > +}
> > +
> > +/*
> > + * Re-enable CPPC when the policy's CPU comes back online, since the platform
> > + * may have disabled it while the CPU was offline.
> > + */
> > +static int cppc_cpufreq_cpu_online(struct cpufreq_policy *policy)
> > +{
> > +     unsigned int cpu = policy->cpu;
> > +     int ret;
> > +
> > +     ret = cppc_set_enable(cpu, true);
> > +     if (ret && ret != -EOPNOTSUPP)
> > +             pr_warn("Failed to re-enable CPPC for CPU%d (%d)\n", cpu, ret);
> Keeping the policy alive means cpu_data->perf_ctrls survives hotplug, but the callback does not
> reapply it. A governor restart does not guarantee an immediate target callback, so the controls
> may remain reset indefinitely.
>
> I think the online path needs to restore a complete, current performance-control request before
> returning. MIN/MAX should be recomputed from the policy and the last requested DESIRED value
> clamped into that range.

Agreed.
Will change online() to recompute MIN/MAX from the policy and clamp the
retained DESIRED into that range, as described in my reply on patch 3/3.

> Also, what about suspend/resume?

Good point. On the platform I test on these registers are retained
across both hotplug and suspend, so I left suspend/resume out initially.
But the same mechanism extends to platforms that do reset them.
So I will add it as its own patch in v3 where suspend() saves the
OSPM set values and resume() restores them, reusing the online() path.

Thanks,
Sumit

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