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

[Shawn Guo <shawnguo2@yeah.net>]

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.

Shawn

> +
>  #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()));
>  /// }
> 
> 
>