Re: [PATCH v2] cpufreq: Handle CPUFREQ_ETERNAL with a default transition latency

[Shawn Guo <shawnguo2@yeah.net>]

On Mon, Sep 22, 2025 at 08:31:56PM +0200, Rafael J. Wysocki wrote:
> What about the appended (untested) change instead?

I'm trying to address a regression with a fix to be ported for stable
kernel.  Not really sure it's a good idea to mix up with cleanup
changes.

Shawn

> 
> With a follow-up one to replace CPUFREQ_ETERNAL with something internal
> to CPPC.
> 
> ---
>  Documentation/admin-guide/pm/cpufreq.rst                  |    4 ----
>  Documentation/cpu-freq/cpu-drivers.rst                    |    3 +--
>  Documentation/translations/zh_CN/cpu-freq/cpu-drivers.rst |    3 +--
>  Documentation/translations/zh_TW/cpu-freq/cpu-drivers.rst |    3 +--
>  drivers/cpufreq/cppc_cpufreq.c                            |   14 ++++++++++++--
>  drivers/cpufreq/cpufreq-dt.c                              |    2 +-
>  drivers/cpufreq/imx6q-cpufreq.c                           |    2 +-
>  drivers/cpufreq/mediatek-cpufreq-hw.c                     |    2 +-
>  drivers/cpufreq/scmi-cpufreq.c                            |    2 +-
>  drivers/cpufreq/scpi-cpufreq.c                            |    2 +-
>  drivers/cpufreq/spear-cpufreq.c                           |    2 +-
>  include/linux/cpufreq.h                                   |    7 ++++---
>  12 files changed, 25 insertions(+), 21 deletions(-)
> 
> --- a/Documentation/admin-guide/pm/cpufreq.rst
> +++ b/Documentation/admin-guide/pm/cpufreq.rst
> @@ -274,10 +274,6 @@ are the following:
>  	The time it takes to switch the CPUs belonging to this policy from one
>  	P-state to another, in nanoseconds.
>  
> -	If unknown or if known to be so high that the scaling driver does not
> -	work with the `ondemand`_ governor, -1 (:c:macro:`CPUFREQ_ETERNAL`)
> -	will be returned by reads from this attribute.
> -
>  ``related_cpus``
>  	List of all (online and offline) CPUs belonging to this policy.
>  
> --- a/Documentation/cpu-freq/cpu-drivers.rst
> +++ b/Documentation/cpu-freq/cpu-drivers.rst
> @@ -109,8 +109,7 @@ Then, the driver must fill in the follow
>  +-----------------------------------+--------------------------------------+
>  |policy->cpuinfo.transition_latency | the time it takes on this CPU to	   |
>  |				    | switch between two frequencies in	   |
> -|				    | nanoseconds (if appropriate, else	   |
> -|				    | specify CPUFREQ_ETERNAL)		   |
> +|				    | nanoseconds                          |
>  +-----------------------------------+--------------------------------------+
>  |policy->cur			    | The current operating frequency of   |
>  |				    | this CPU (if appropriate)		   |
> --- a/Documentation/translations/zh_CN/cpu-freq/cpu-drivers.rst
> +++ b/Documentation/translations/zh_CN/cpu-freq/cpu-drivers.rst
> @@ -112,8 +112,7 @@ CPUfreq核心层注册一个cpufreq_driv
>  |                                   |                                      |
>  +-----------------------------------+--------------------------------------+
>  |policy->cpuinfo.transition_latency | CPU在两个频率之间切换所需的时间，以  |
> -|                                   | 纳秒为单位（如不适用，设定为         |
> -|                                   | CPUFREQ_ETERNAL）                    |
> +|                                   | 纳秒为单位                    |
>  |                                   |                                      |
>  +-----------------------------------+--------------------------------------+
>  |policy->cur                        | 该CPU当前的工作频率(如适用)          |
> --- a/Documentation/translations/zh_TW/cpu-freq/cpu-drivers.rst
> +++ b/Documentation/translations/zh_TW/cpu-freq/cpu-drivers.rst
> @@ -112,8 +112,7 @@ CPUfreq核心層註冊一個cpufreq_driv
>  |                                   |                                      |
>  +-----------------------------------+--------------------------------------+
>  |policy->cpuinfo.transition_latency | CPU在兩個頻率之間切換所需的時間，以  |
> -|                                   | 納秒爲單位（如不適用，設定爲         |
> -|                                   | CPUFREQ_ETERNAL）                    |
> +|                                   | 納秒爲單位                    |
>  |                                   |                                      |
>  +-----------------------------------+--------------------------------------+
>  |policy->cur                        | 該CPU當前的工作頻率(如適用)          |
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -308,6 +308,16 @@ static int cppc_verify_policy(struct cpu
>  	return 0;
>  }
>  
> +static unsigned int get_transition_latency(unsigned int cpu)
> +{
> +	unsigned int transition_latency_ns = cppc_get_transition_latency(cpu);
> +
> +	if (transition_latency_ns == CPUFREQ_ETERNAL)
> +		return CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS / NSEC_PER_USEC;
> +
> +	return transition_latency_ns / NSEC_PER_USEC;
> +}
> +
>  /*
>   * The PCC subspace describes the rate at which platform can accept commands
>   * on the shared PCC channel (including READs which do not count towards freq
> @@ -330,12 +340,12 @@ static unsigned int cppc_cpufreq_get_tra
>  			return 10000;
>  		}
>  	}
> -	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
> +	return get_transition_latency(cpu);
>  }
>  #else
>  static unsigned int cppc_cpufreq_get_transition_delay_us(unsigned int cpu)
>  {
> -	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
> +	return get_transition_latency(cpu);
>  }
>  #endif
>  
> --- 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/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
> @@ -26,12 +26,13 @@
>   *********************************************************************/
>  /*
>   * Frequency values here are CPU kHz
> - *
> - * Maximum transition latency is in nanoseconds - if it's unknown,
> - * CPUFREQ_ETERNAL shall be used.
>   */
>  
> +/* Represents unknown transition latency */
>  #define CPUFREQ_ETERNAL			(-1)
> +
> +#define CPUFREQ_DEFAULT_TANSITION_LATENCY_NS	NSEC_PER_MSEC
> +
>  #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)
> 
> 
> 
>