NAME

cpufreq — CPU frequency control framework

SYNOPSIS

device cpufreq

#include <sys/cpu.h>
int
cpufreq_levels(device_t dev, struct cf_level *levels, int *count);
int
cpufreq_set(device_t dev, const struct cf_level *level, int priority);
int
cpufreq_get(device_t dev, struct cf_level *level);
int
cpufreq_drv_settings(device_t dev, struct cf_setting *sets, int *count);
int
cpufreq_drv_type(device_t dev, int *type);
int
cpufreq_drv_set(device_t dev, const struct cf_setting *set);
int
cpufreq_drv_get(device_t dev, struct cf_setting *set);

DESCRIPTION

The cpufreq 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.conf(5).

SYSCTL VARIABLES

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 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.
dev.cpu.%d.freq
Current active CPU frequency in MHz.
dev.cpu.%d.freq_levels
Currently available levels for the CPU (frequency/power usage). Values are in units of MHz and milliwatts.
dev.DEVICE.%d.freq_settings
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.
debug.cpufreq.lowest
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.
debug.cpufreq.verbose
Print verbose messages. This setting is also accessible via a tunable with the same name.

SUPPORTED DRIVERS

The following device drivers offer absolute frequency control via the cpufreq interface. Usually, only one of these can be active at a time.
acpi_perf
ACPI CPU performance states
est
Intel Enhanced SpeedStep
ichss
Intel SpeedStep for ICH
powernow
AMD PowerNow! and Cool'n'Quiet for K7 and K8
smist
Intel SMI-based SpeedStep for PIIX4
The following device drivers offer relative frequency control and have an additive effect:
acpi_throttle
ACPI CPU throttling
p4tcc
Pentium 4 Thermal Control Circuitry

KERNEL INTERFACE

Kernel components can query and set CPU frequencies through the cpufreq kernel interface. This involves obtaining a cpufreq device, calling cpufreq_levels() to get the currently available frequency levels, checking the current level with cpufreq_get(), and setting a new one from the list with cpufreq_set(). Each level may actually reference more than one cpufreq driver but kernel components do not need to be aware of this. The total_set element of 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 cpufreq_levels() method takes a cpufreq device and an empty array of levels. The 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 count will allow, it should return E2BIG.
The cpufreq_get() method takes a pointer to space to store a level. After successful completion, the output will be the current active level and is equal to one of the levels returned by cpufreq_levels().
The cpufreq_set() method takes a pointer a level and attempts to activate it. The priority (i.e., CPUFREQ_PRIO_KERN) tells cpufreq whether to override previous settings while activating this level. If 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 cpufreq_set() is called with a NULL level, the saved level will be restored. If there is no saved level, cpufreq_set() will return ENXIO. If priority is lower than the current active level's priority, this method returns EPERM.

DRIVER INTERFACE

Kernel drivers offering hardware-specific CPU frequency control export their individual settings through the cpufreq driver interface. This involves implementing these methods: cpufreq_drv_settings(), cpufreq_drv_type(), cpufreq_drv_set(), and 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 cpufreq framework.
The cpufreq_drv_settings() method returns an array of currently available settings, each of type 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, 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 count will allow, it should return E2BIG.
The 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 cpufreq_drv_set() to activate them.
The cpufreq_drv_set() method takes a driver setting and makes it active. If the setting is invalid or not currently available, it should return EINVAL.
The cpufreq_drv_get() method returns the currently-active driver setting. The struct cf_setting returned must be valid for passing to cpufreq_drv_set(), including all elements being filled out correctly. If the driver cannot infer the current setting (even by estimating it with cpu_est_clockrate()) then it should set all elements to CPUFREQ_VAL_UNKNOWN.

SEE ALSO

acpi(4), est(4), timecounters(4), powerd(8), sysctl(8)

AUTHORS

Nate Lawson
Bruno Ducrot contributed the powernow driver.

BUGS

The following drivers have not yet been converted to the cpufreq 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.

Recommended readings

Pages related to cpufreq you should read also:
acpi(4) est(4) longrun(4) powerd(8) rc.conf(5) sysctl(8) timecounters(4)

Questions & Answers

Helpful answers and articles about cpufreq you may found on these sites:
Stack Overflow Server Fault Super User Unix & Linux Ask Ubuntu Network Engineering DevOps Raspberry Pi Webmasters Google Search