linux-kernel - Re: [PATCH] cpufreq: CPPC: Mark driver with NEED_UPDATE

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Message-ID: <CAJZ5v0iUjOitwHYFqRgQY=h+Z-5of+3myDGDJKC8GcSOOSwYAA@mail.gmail.com>
Date: Tue, 22 Jul 2025 12:15:27 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>, Prashant Malani <pmalani@...gle.com>
Cc: open list <linux-kernel@...r.kernel.org>, 
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Beata Michalska <beata.michalska@....com>, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] cpufreq: CPPC: Mark driver with NEED_UPDATE_LIMITS flag

On Tue, Jul 22, 2025 at 8:03 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 22-07-25, 05:55, Prashant Malani wrote:
> > AMU counters on certain CPPC-based platforms tend to yield inaccurate
> > delivered performance measurements on systems that are idle/mostly idle.
> > This results in an inaccurate frequency being stored by cpufreq in its
> > policy structure when the CPU is brought online. [1]
> >
> > Consequently, if the userspace governor tries to set the frequency to a
> > new value, there is a possibility that it would be the erroneous value
> > stored earlier. In such a scenario, cpufreq would assume that the
> > requested frequency has already been set and return early, resulting in
> > the correct/new frequency request never making it to the hardware.
> >
> > Since the operating frequency is liable to this sort of inconsistency,
> > mark the CPPC driver with CPUFREQ_NEED_UPDATE_LIMITS so that it is always
> > invoked when a target frequency update is requested.
> >
> > [1] https://lore.kernel.org/linux-pm/20250619000925.415528-3-pmalani@google.com/
> >
> > Cc: Beata Michalska <beata.michalska@....com>
> > Cc: Viresh Kumar <viresh.kumar@...aro.org>
> > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Signed-off-by: Prashant Malani <pmalani@...gle.com>
> > ---
> >  drivers/cpufreq/cppc_cpufreq.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> > index a1fd0ff22bc5..4a17162a392d 100644
> > --- a/drivers/cpufreq/cppc_cpufreq.c
> > +++ b/drivers/cpufreq/cppc_cpufreq.c
> > @@ -910,7 +910,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
> >  };
> >
> >  static struct cpufreq_driver cppc_cpufreq_driver = {
> > -     .flags = CPUFREQ_CONST_LOOPS,
> > +     .flags = CPUFREQ_CONST_LOOPS | CPUFREQ_NEED_UPDATE_LIMITS,
> >       .verify = cppc_verify_policy,
> >       .target = cppc_cpufreq_set_target,
> >       .get = cppc_cpufreq_get_rate,
>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

Applied as 6.17 material, thanks!