Re: [PATCH v3 1/4] cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition latency

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

On Sun, Sep 28, 2025 at 5:24 AM Shawn Guo <shawnguo2@yeah.net> wrote:
>
> On Fri, Sep 26, 2025 at 12:12:37PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > Commit a755d0e2d41b ("cpufreq: Honour transition_latency over
> > transition_delay_us") caused platforms where cpuinfo.transition_latency
> > is CPUFREQ_ETERNAL to get a very large transition latency whereas
> > previously it had been capped at 10 ms (and later at 2 ms).
> >
> > This led to a user-observable regression between 6.6 and 6.12 as
> > described by Shawn:
> >
> > "The dbs sampling_rate was 10000 us on 6.6 and suddently becomes
> >  6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these platforms
> >  because the default transition delay was dropped [...].
> >
> >  It slows down dbs governor's reacting to CPU loading change
> >  dramatically.  Also, as transition_delay_us is used by schedutil
> >  governor as rate_limit_us, it shows a negative impact on device
> >  idle power consumption, because the device gets slightly less time
> >  in the lowest OPP."
> >
> > Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as
> > cpuinfo.transition_latency was that it would be capped by the core,
> > but they may as well return a default transition latency value instead
> > of CPUFREQ_ETERNAL and the core need not do anything with it.
> >
> > Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make
> > all of the drivers in question use it instead of CPUFREQ_ETERNAL.  Also
> > update the related Rust binding.
> >
> > Fixes: a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us")
> > Closes: https://lore.kernel.org/linux-pm/20250922125929.453444-1-shawnguo2@yeah.net/
> > Reported-by: Shawn Guo <shawnguo@kernel.org>
> > Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
> > Reviewed-by: Jie Zhan <zhanjie9@hisilicon.com>
> > Cc: 6.6+ <stable@vger.kernel.org> # 6.6+
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >
> > v1 -> v3:
> >    * Add updates of the Rust version of cpufreq-dt and Rust binding
> >    * Update the changelog
> >    * Add tags from Mario Limonciello and Jie Zhan
> >
> > ---
> >  drivers/cpufreq/cpufreq-dt.c          |    2 +-
> >  drivers/cpufreq/imx6q-cpufreq.c       |    2 +-
> >  drivers/cpufreq/mediatek-cpufreq-hw.c |    2 +-
> >  drivers/cpufreq/rcpufreq_dt.rs        |    2 +-
> >  drivers/cpufreq/scmi-cpufreq.c        |    2 +-
> >  drivers/cpufreq/scpi-cpufreq.c        |    2 +-
> >  drivers/cpufreq/spear-cpufreq.c       |    2 +-
> >  include/linux/cpufreq.h               |    3 +++
> >  rust/kernel/cpufreq.rs                |    7 ++++---
> >  9 files changed, 14 insertions(+), 10 deletions(-)
> >
> > --- a/drivers/cpufreq/cpufreq-dt.c
> > +++ b/drivers/cpufreq/cpufreq-dt.c
> > @@ -104,7 +104,7 @@ static int cpufreq_init(struct cpufreq_p
> >
> >       transition_latency = dev_pm_opp_get_max_transition_latency(cpu_dev);
> >       if (!transition_latency)
> > -             transition_latency = CPUFREQ_ETERNAL;
> > +             transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       cpumask_copy(policy->cpus, priv->cpus);
> >       policy->driver_data = priv;
> > --- a/drivers/cpufreq/imx6q-cpufreq.c
> > +++ b/drivers/cpufreq/imx6q-cpufreq.c
> > @@ -442,7 +442,7 @@ soc_opp_out:
> >       }
> >
> >       if (of_property_read_u32(np, "clock-latency", &transition_latency))
> > -             transition_latency = CPUFREQ_ETERNAL;
> > +             transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       /*
> >        * Calculate the ramp time for max voltage change in the
> > --- a/drivers/cpufreq/mediatek-cpufreq-hw.c
> > +++ b/drivers/cpufreq/mediatek-cpufreq-hw.c
> > @@ -309,7 +309,7 @@ static int mtk_cpufreq_hw_cpu_init(struc
> >
> >       latency = readl_relaxed(data->reg_bases[REG_FREQ_LATENCY]) * 1000;
> >       if (!latency)
> > -             latency = CPUFREQ_ETERNAL;
> > +             latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       policy->cpuinfo.transition_latency = latency;
> >       policy->fast_switch_possible = true;
> > --- a/drivers/cpufreq/rcpufreq_dt.rs
> > +++ b/drivers/cpufreq/rcpufreq_dt.rs
> > @@ -123,7 +123,7 @@ impl cpufreq::Driver for CPUFreqDTDriver
> >
> >          let mut transition_latency = opp_table.max_transition_latency_ns() as u32;
> >          if transition_latency == 0 {
> > -            transition_latency = cpufreq::ETERNAL_LATENCY_NS;
> > +            transition_latency = cpufreq::DEFAULT_TRANSITION_LATENCY_NS;
> >          }
> >
> >          policy
> > --- a/drivers/cpufreq/scmi-cpufreq.c
> > +++ b/drivers/cpufreq/scmi-cpufreq.c
> > @@ -294,7 +294,7 @@ static int scmi_cpufreq_init(struct cpuf
> >
> >       latency = perf_ops->transition_latency_get(ph, domain);
> >       if (!latency)
> > -             latency = CPUFREQ_ETERNAL;
> > +             latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       policy->cpuinfo.transition_latency = latency;
> >
> > --- a/drivers/cpufreq/scpi-cpufreq.c
> > +++ b/drivers/cpufreq/scpi-cpufreq.c
> > @@ -157,7 +157,7 @@ static int scpi_cpufreq_init(struct cpuf
> >
> >       latency = scpi_ops->get_transition_latency(cpu_dev);
> >       if (!latency)
> > -             latency = CPUFREQ_ETERNAL;
> > +             latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       policy->cpuinfo.transition_latency = latency;
> >
> > --- a/drivers/cpufreq/spear-cpufreq.c
> > +++ b/drivers/cpufreq/spear-cpufreq.c
> > @@ -182,7 +182,7 @@ static int spear_cpufreq_probe(struct pl
> >
> >       if (of_property_read_u32(np, "clock-latency",
> >                               &spear_cpufreq.transition_latency))
> > -             spear_cpufreq.transition_latency = CPUFREQ_ETERNAL;
> > +             spear_cpufreq.transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
> >
> >       cnt = of_property_count_u32_elems(np, "cpufreq_tbl");
> >       if (cnt <= 0) {
> > --- a/include/linux/cpufreq.h
> > +++ b/include/linux/cpufreq.h
> > @@ -32,6 +32,9 @@
> >   */
> >
> >  #define CPUFREQ_ETERNAL                      (-1)
> > +
> > +#define CPUFREQ_DEFAULT_TANSITION_LATENCY_NS NSEC_PER_MSEC
>
> Typo of TANSITION, should be CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS.

Yes, thanks!

Fixed already because CIs caught it yesterday.

I'm not sure how this happened though.  I must have mangled the patch
right before sending it because I had tested the whole patchset before
posting it.

> > +
> >  #define CPUFREQ_NAME_LEN             16
> >  /* Print length for names. Extra 1 space for accommodating '\n' in prints */
> >  #define CPUFREQ_NAME_PLEN            (CPUFREQ_NAME_LEN + 1)
> > --- a/rust/kernel/cpufreq.rs
> > +++ b/rust/kernel/cpufreq.rs
> > @@ -39,7 +39,8 @@ use macros::vtable;
> >  const CPUFREQ_NAME_LEN: usize = bindings::CPUFREQ_NAME_LEN as usize;
> >
> >  /// Default transition latency value in nanoseconds.
> > -pub const ETERNAL_LATENCY_NS: u32 = bindings::CPUFREQ_ETERNAL as u32;
> > +pub const DEFAULT_TRANSITION_LATENCY_NS: u32 =
> > +        bindings::CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS as u32;
> >
> >  /// CPU frequency driver flags.
> >  pub mod flags {
> > @@ -400,13 +401,13 @@ impl TableBuilder {
> >  /// The following example demonstrates how to create a CPU frequency table.
> >  ///
> >  /// ```
> > -/// use kernel::cpufreq::{ETERNAL_LATENCY_NS, Policy};
> > +/// use kernel::cpufreq::{DEFAULT_TRANSITION_LATENCY_NS, Policy};
> >  ///
> >  /// fn update_policy(policy: &mut Policy) {
> >  ///     policy
> >  ///         .set_dvfs_possible_from_any_cpu(true)
> >  ///         .set_fast_switch_possible(true)
> > -///         .set_transition_latency_ns(ETERNAL_LATENCY_NS);
> > +///         .set_transition_latency_ns(DEFAULT_TRANSITION_LATENCY_NS);
> >  ///
> >  ///     pr_info!("The policy details are: {:?}\n", (policy.cpu(), policy.cur()));
> >  /// }
> >
> >
> >
>