linux-kernel - Re: [PATCH v2 2/4] pmdomain: Respect the CPU system-wakeup QoS limit during s2idle

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Message-ID: <CAPDyKFrrhw5vMYLEWJ5LRphVzwPwjiU-n=tdbgOtOmFSXGd0GQ@mail.gmail.com>
Date: Thu, 30 Oct 2025 13:00:18 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-pm@...r.kernel.org, Vincent Guittot <vincent.guittot@...aro.org>, 
	Peter Zijlstra <peterz@...radead.org>, Kevin Hilman <khilman@...libre.com>, 
	Pavel Machek <pavel@...nel.org>, Len Brown <len.brown@...el.com>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Saravana Kannan <saravanak@...gle.com>, 
	Maulik Shah <quic_mkshah@...cinc.com>, Prasad Sodagudi <psodagud@...cinc.com>, 
	Dhruva Gole <d-gole@...com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] pmdomain: Respect the CPU system-wakeup QoS limit
 during s2idle

On Thu, 30 Oct 2025 at 11:45, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Oct 16, 2025 at 5:19 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > A CPU system-wakeup QoS limit may have been requested by user-space. To
> > avoid breaking this constraint when entering a low-power state during
> > s2idle through genpd, let's extend the corresponding genpd governor for
> > CPUs. More precisely, during s2idle let the genpd governor select a
> > suitable low-power state, by taking into account the QoS limit.
> >
> > Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> > ---
> >
> > Changes in v2:
> >         - Limite the change to the genpd governor for CPUs.
> >
> > ---
> >  drivers/pmdomain/core.c     | 10 ++++++++--
> >  drivers/pmdomain/governor.c | 27 +++++++++++++++++++++++++++
> >  include/linux/pm_domain.h   |  1 +
> >  3 files changed, 36 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pmdomain/core.c b/drivers/pmdomain/core.c
> > index 61c2277c9ce3..4fd546ef0448 100644
> > --- a/drivers/pmdomain/core.c
> > +++ b/drivers/pmdomain/core.c
> > @@ -1425,8 +1425,14 @@ static void genpd_sync_power_off(struct generic_pm_domain *genpd, bool use_lock,
> >                         return;
> >         }
> >
> > -       /* Choose the deepest state when suspending */
> > -       genpd->state_idx = genpd->state_count - 1;
> > +       if (genpd->gov && genpd->gov->system_power_down_ok) {
> > +               if (!genpd->gov->system_power_down_ok(&genpd->domain))
> > +                       return;
> > +       } else {
> > +               /* Default to the deepest state. */
> > +               genpd->state_idx = genpd->state_count - 1;
> > +       }
> > +
> >         if (_genpd_power_off(genpd, false)) {
> >                 genpd->states[genpd->state_idx].rejected++;
> >                 return;
> > diff --git a/drivers/pmdomain/governor.c b/drivers/pmdomain/governor.c
> > index 39359811a930..bd1b9d66d4a5 100644
> > --- a/drivers/pmdomain/governor.c
> > +++ b/drivers/pmdomain/governor.c
> > @@ -415,9 +415,36 @@ static bool cpu_power_down_ok(struct dev_pm_domain *pd)
> >         return false;
> >  }
> >
> > +static bool cpu_system_power_down_ok(struct dev_pm_domain *pd)
> > +{
> > +       s64 constraint_ns = cpu_wakeup_latency_qos_limit() * NSEC_PER_USEC;
>
> I'm not sure why genpd needs to take cpu_wakeup_latency_qos_limit()
> into account directly.
>
> It should be told by cpuidle which state has been selected on the CPU
> side and it should not go any deeper than that anyway.

For PSCI OS-initiated mode, cpuidle doesn't know about the states that
may be shared among a group of CPUs.

Instead, those states are controlled through the PM domain topology by
genpd and its governor, hence this is needed too.

>
> > +       struct generic_pm_domain *genpd = pd_to_genpd(pd);
> > +       int state_idx = genpd->state_count - 1;
> > +
> > +       if (!(genpd->flags & GENPD_FLAG_CPU_DOMAIN)) {
> > +               genpd->state_idx = state_idx;
> > +               return true;
> > +       }
> > +
> > +       /* Find the deepest state for the latency constraint. */
> > +       while (state_idx >= 0) {
> > +               s64 latency_ns = genpd->states[state_idx].power_off_latency_ns +
> > +                                genpd->states[state_idx].power_on_latency_ns;
> > +
> > +               if (latency_ns <= constraint_ns) {
> > +                       genpd->state_idx = state_idx;
> > +                       return true;
> > +               }
> > +               state_idx--;
> > +       }
> > +
> > +       return false;
> > +}
> > +
> >  struct dev_power_governor pm_domain_cpu_gov = {
> >         .suspend_ok = default_suspend_ok,
> >         .power_down_ok = cpu_power_down_ok,
> > +       .system_power_down_ok = cpu_system_power_down_ok,
> >  };
> >  #endif
> >
> > diff --git a/include/linux/pm_domain.h b/include/linux/pm_domain.h
> > index f67a2cb7d781..93ba0143ca47 100644
> > --- a/include/linux/pm_domain.h
> > +++ b/include/linux/pm_domain.h
> > @@ -153,6 +153,7 @@ enum genpd_sync_state {
> >  };
> >
> >  struct dev_power_governor {
> > +       bool (*system_power_down_ok)(struct dev_pm_domain *domain);
> >         bool (*power_down_ok)(struct dev_pm_domain *domain);
> >         bool (*suspend_ok)(struct device *dev);
> >  };
> > --
> > 2.43.0
> >

Kind regards
Uffe