cpufreq(4) CPU frequency control framework


device cpufreq

In sys/cpu.h Ft int Fn cpufreq_levels device_t dev struct cf_level *levels int *count Ft int Fn cpufreq_set device_t dev const struct cf_level *level int priority Ft int Fn cpufreq_get device_t dev struct cf_level *level Ft int Fo cpufreq_drv_settings Fa device_t dev Fa struct cf_setting *sets Fa int *count Fc Ft int Fn cpufreq_drv_type device_t dev int *type Ft int Fn cpufreq_drv_set device_t dev const struct cf_setting *set Ft int Fn cpufreq_drv_get device_t dev struct cf_setting *set


The driver provides a unified kernel and user interface to CPU frequency control drivers. It combines multiple drivers offering different settings into a single interface of all possible levels. Users can access this interface directly via sysctl(8) or by indicating to /etc/rc.d/power_profile that it should switch settings when the AC line state changes via rc.conf5.


These settings may be overridden by kernel drivers requesting alternate settings. If this occurs, the original values will be restored once the condition has passed (e.g., the system has cooled sufficiently). If a sysctl cannot be set due to an override condition, it will return Er EPERM .

The frequency cannot be changed if TSC is in use as the timecounter. This is because the timecounter system needs to use a source that has a constant rate. The timecounter source can be changed with the kern.timecounter.hardware sysctl. Available modes are in kern.timecounter.choice sysctl entry.

Current active CPU frequency in MHz.
Currently available levels for the CPU (frequency/power usage). Values are in units of MHz and milliwatts.
Currently available settings for the driver (frequency/power usage). Values are in units of MHz and milliwatts. This is helpful for understanding which settings are offered by which driver for debugging purposes.
Lowest CPU frequency in MHz to offer to users. This setting is also accessible via a tunable with the same name. This can be used to disable very low levels that may be unusable on some systems.
Print verbose messages. This setting is also accessible via a tunable with the same name.


The following device drivers offer absolute frequency control via the interface. Usually, only one of these can be active at a time.

ACPI CPU performance states
Intel Enhanced SpeedStep
Intel SpeedStep for ICH
AMD PowerNow! and Cool'n'Quiet for K7 and K8
Intel SMI-based SpeedStep for PIIX4

The following device drivers offer relative frequency control and have an additive effect:

ACPI CPU throttling
Pentium 4 Thermal Control Circuitry


Kernel components can query and set CPU frequencies through the kernel interface. This involves obtaining a device, calling Fn cpufreq_levels to get the currently available frequency levels, checking the current level with Fn cpufreq_get , and setting a new one from the list with Fn cpufreq_set . Each level may actually reference more than one driver but kernel components do not need to be aware of this. The total_set element of Vt struct cf_level provides a summary of the frequency and power for this level. Unknown or irrelevant values are set to CPUFREQ_VAL_UNKNOWN

The Fn cpufreq_levels method takes a device and an empty array of Fa levels . The Fa count value should be set to the number of levels available and after the function completes, will be set to the actual number of levels returned. If there are more levels than Fa count will allow, it should return Er E2BIG .

The Fn cpufreq_get method takes a pointer to space to store a Fa level . After successful completion, the output will be the current active level and is equal to one of the levels returned by Fn cpufreq_levels .

The Fn cpufreq_set method takes a pointer a Fa level and attempts to activate it. The Fa priority (i.e., CPUFREQ_PRIO_KERN tells whether to override previous settings while activating this level. If Fa priority is higher than the current active level, that level will be saved and overridden with the new level. If a level is already saved, the new level is set without overwriting the older saved level. If Fn cpufreq_set is called with a NULL Fa level , the saved level will be restored. If there is no saved level, Fn cpufreq_set will return Er ENXIO . If Fa priority is lower than the current active level's priority, this method returns Er EPERM .


Kernel drivers offering hardware-specific CPU frequency control export their individual settings through the driver interface. This involves implementing these methods: Fn cpufreq_drv_settings , Fn cpufreq_drv_type , Fn cpufreq_drv_set , and Fn cpufreq_drv_get . Additionally, the driver must attach a device as a child of a CPU device so that these methods can be called by the framework.

The Fn cpufreq_drv_settings method returns an array of currently available settings, each of type Vt struct cf_setting . The driver should set unknown or irrelevant values to CPUFREQ_VAL_UNKNOWN All the following elements for each setting should be returned:

struct cf_setting {
        int     freq;   /* CPU clock in MHz or 100ths of a percent. */
        int     volts;  /* Voltage in mV. */
        int     power;  /* Power consumed in mW. */
        int     lat;    /* Transition latency in us. */
        device_t dev;   /* Driver providing this setting. */

On entry to this method, Fa count contains the number of settings that can be returned. On successful completion, the driver sets it to the actual number of settings returned. If the driver offers more settings than Fa count will allow, it should return Er E2BIG .

The Fn cpufreq_drv_type method indicates the type of settings it offers, either CPUFREQ_TYPE_ABSOLUTE or CPUFREQ_TYPE_RELATIVE Additionally, the driver may set the CPUFREQ_FLAG_INFO_ONLY flag if the settings it provides are information for other drivers only and cannot be passed to Fn cpufreq_drv_set to activate them.

The Fn cpufreq_drv_set method takes a driver setting and makes it active. If the setting is invalid or not currently available, it should return Er EINVAL .

The Fn cpufreq_drv_get method returns the currently-active driver setting. The Vt struct cf_setting returned must be valid for passing to Fn cpufreq_drv_set , including all elements being filled out correctly. If the driver cannot infer the current setting (even by estimating it with Fn cpu_est_clockrate ) then it should set all elements to CPUFREQ_VAL_UNKNOWN


An Nate Lawson An Bruno Ducrot contributed the powernow driver.


The following drivers have not yet been converted to the interface: longrun(4).

Notification of CPU and bus frequency changes is not implemented yet.

When multiple CPUs offer frequency control, they cannot be set to different levels and must all offer the same frequency settings.