Documentation/cpu-freq/user-guide.txt - pub/scm/linux/kernel/git/kvms390/vfio-ccw - Git at Google

      CPU frequency and voltage scaling code in the Linux(TM) kernel


 		         L i n u x    C P U F r e q

 			     U S E R   G U I D E


 		    Dominik Brodowski  <linux@brodo.de>


    Clock scaling allows you to change the clock speed of the CPUs on the
     fly. This is a nice method to save battery power, because the lower
             the clock speed, the less power the CPU consumes.


 Contents:
 ---------
 1. Supported Architectures and Processors
 1.1 ARM and ARM64
 1.2 x86
 1.3 sparc64
 1.4 ppc
 1.5 SuperH
 1.6 Blackfin

 2. "Policy" / "Governor"?
 2.1 Policy
 2.2 Governor

 3. How to change the CPU cpufreq policy and/or speed
 3.1 Preferred interface: sysfs


 1. Supported Architectures and Processors
 =========================================

 1.1 ARM and ARM64
 -----------------

 Almost all ARM and ARM64 platforms support CPU frequency scaling.

 1.2 x86
 -------

 The following processors for the x86 architecture are supported by cpufreq:

 AMD Elan - SC400, SC410
 AMD mobile K6-2+
 AMD mobile K6-3+
 AMD mobile Duron
 AMD mobile Athlon
 AMD Opteron
 AMD Athlon 64
 Cyrix Media GXm
 Intel mobile PIII and Intel mobile PIII-M on certain chipsets
 Intel Pentium 4, Intel Xeon
 Intel Pentium M (Centrino)
 National Semiconductors Geode GX
 Transmeta Crusoe
 Transmeta Efficeon
 VIA Cyrix 3 / C3
 various processors on some ACPI 2.0-compatible systems [*]
 And many more

 [*] Only if "ACPI Processor Performance States" are available
 to the ACPI<->BIOS interface.


 1.3 sparc64
 -----------

 The following processors for the sparc64 architecture are supported by
 cpufreq:

 UltraSPARC-III


 1.4 ppc
 -------

 Several "PowerBook" and "iBook2" notebooks are supported.
 The following POWER processors are supported in powernv mode:
 POWER8
 POWER9

 1.5 SuperH
 ----------

 All SuperH processors supporting rate rounding through the clock
 framework are supported by cpufreq.

 1.6 Blackfin
 ------------

 The following Blackfin processors are supported by cpufreq:

 BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
 BF531, BF532, BF533, Rev 0.3 or higher
 BF534, BF536, BF537, Rev 0.2 or higher
 BF561, Rev 0.3 or higher
 BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher


 2. "Policy" / "Governor" ?
 ==========================

 Some CPU frequency scaling-capable processor switch between various
 frequencies and operating voltages "on the fly" without any kernel or
 user involvement. This guarantees very fast switching to a frequency
 which is high enough to serve the user's needs, but low enough to save
 power.


 2.1 Policy
 ----------

 On these systems, all you can do is select the lower and upper
 frequency limit as well as whether you want more aggressive
 power-saving or more instantly available processing power.


 2.2 Governor
 ------------

 On all other cpufreq implementations, these boundaries still need to
 be set. Then, a "governor" must be selected. Such a "governor" decides
 what speed the processor shall run within the boundaries. One such
 "governor" is the "userspace" governor. This one allows the user - or
 a yet-to-implement userspace program - to decide what specific speed
 the processor shall run at.


 3. How to change the CPU cpufreq policy and/or speed
 ====================================================

 3.1 Preferred Interface: sysfs
 ------------------------------

 The preferred interface is located in the sysfs filesystem. If you
 mounted it at /sys, the cpufreq interface is located in a subdirectory
 "cpufreq" within the cpu-device directory
 (e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).

 affected_cpus :			List of Online CPUs that require software
 				coordination of frequency.

 cpuinfo_cur_freq :		Current frequency of the CPU as obtained from
 				the hardware, in KHz. This is the frequency
 				the CPU actually runs at.

 cpuinfo_min_freq :		this file shows the minimum operating
 				frequency the processor can run at(in kHz)

 cpuinfo_max_freq :		this file shows the maximum operating
 				frequency the processor can run at(in kHz)

 cpuinfo_transition_latency	The time it takes on this CPU to
 				switch between two frequencies in nano
 				seconds. If unknown or known to be
 				that high that the driver does not
 				work with the ondemand governor, -1
 				(CPUFREQ_ETERNAL) will be returned.
 				Using this information can be useful
 				to choose an appropriate polling
 				frequency for a kernel governor or
 				userspace daemon. Make sure to not
 				switch the frequency too often
 				resulting in performance loss.

 related_cpus :			List of Online + Offline CPUs that need software
 				coordination of frequency.

 scaling_available_frequencies : List of available frequencies, in KHz.

 scaling_available_governors :	this file shows the CPUfreq governors
 				available in this kernel. You can see the
 				currently activated governor in

 scaling_cur_freq :		Current frequency of the CPU as determined by
 				the governor and cpufreq core, in KHz. This is
 				the frequency the kernel thinks the CPU runs
 				at.

 scaling_driver :		this file shows what cpufreq driver is
 				used to set the frequency on this CPU

 scaling_governor,		and by "echoing" the name of another
 				governor you can change it. Please note
 				that some governors won't load - they only
 				work on some specific architectures or
 				processors.

 scaling_min_freq and
 scaling_max_freq		show the current "policy limits" (in
 				kHz). By echoing new values into these
 				files, you can change these limits.
 				NOTE: when setting a policy you need to
 				first set scaling_max_freq, then
 				scaling_min_freq.

 scaling_setspeed		This can be read to get the currently programmed
 				value by the governor. This can be written to
 				change the current frequency for a group of
 				CPUs, represented by a policy. This is supported
 				currently only by the userspace governor.

 bios_limit :			If the BIOS tells the OS to limit a CPU to
 				lower frequencies, the user can read out the
 				maximum available frequency from this file.
 				This typically can happen through (often not
 				intended) BIOS settings, restrictions
 				triggered through a service processor or other
 				BIOS/HW based implementations.
 				This does not cover thermal ACPI limitations
 				which can be detected through the generic
 				thermal driver.

 If you have selected the "userspace" governor which allows you to
 set the CPU operating frequency to a specific value, you can read out
 the current frequency in

 scaling_setspeed.		By "echoing" a new frequency into this
 				you can change the speed of the CPU,
 				but only within the limits of
 				scaling_min_freq and scaling_max_freq.
	CPU frequency and voltage scaling code in the Linux(TM) kernel


	L i n u x C P U F r e q

	U S E R G U I D E


	Dominik Brodowski <linux@brodo.de>



	Clock scaling allows you to change the clock speed of the CPUs on the
	fly. This is a nice method to save battery power, because the lower
	the clock speed, the less power the CPU consumes.


	Contents:
	---------
	1. Supported Architectures and Processors
	1.1 ARM and ARM64
	1.2 x86
	1.3 sparc64
	1.4 ppc
	1.5 SuperH
	1.6 Blackfin

	2. "Policy" / "Governor"?
	2.1 Policy
	2.2 Governor

	3. How to change the CPU cpufreq policy and/or speed
	3.1 Preferred interface: sysfs



	1. Supported Architectures and Processors
	=========================================

	1.1 ARM and ARM64
	-----------------

	Almost all ARM and ARM64 platforms support CPU frequency scaling.

	1.2 x86
	-------

	The following processors for the x86 architecture are supported by cpufreq:

	AMD Elan - SC400, SC410
	AMD mobile K6-2+
	AMD mobile K6-3+
	AMD mobile Duron
	AMD mobile Athlon
	AMD Opteron
	AMD Athlon 64
	Cyrix Media GXm
	Intel mobile PIII and Intel mobile PIII-M on certain chipsets
	Intel Pentium 4, Intel Xeon
	Intel Pentium M (Centrino)
	National Semiconductors Geode GX
	Transmeta Crusoe
	Transmeta Efficeon
	VIA Cyrix 3 / C3
	various processors on some ACPI 2.0-compatible systems [*]
	And many more

	[*] Only if "ACPI Processor Performance States" are available
	to the ACPI<->BIOS interface.


	1.3 sparc64
	-----------

	The following processors for the sparc64 architecture are supported by
	cpufreq:

	UltraSPARC-III


	1.4 ppc
	-------

	Several "PowerBook" and "iBook2" notebooks are supported.
	The following POWER processors are supported in powernv mode:
	POWER8
	POWER9

	1.5 SuperH
	----------

	All SuperH processors supporting rate rounding through the clock
	framework are supported by cpufreq.

	1.6 Blackfin
	------------

	The following Blackfin processors are supported by cpufreq:

	BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
	BF531, BF532, BF533, Rev 0.3 or higher
	BF534, BF536, BF537, Rev 0.2 or higher
	BF561, Rev 0.3 or higher
	BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher


	2. "Policy" / "Governor" ?
	==========================

	Some CPU frequency scaling-capable processor switch between various
	frequencies and operating voltages "on the fly" without any kernel or
	user involvement. This guarantees very fast switching to a frequency
	which is high enough to serve the user's needs, but low enough to save
	power.


	2.1 Policy
	----------

	On these systems, all you can do is select the lower and upper
	frequency limit as well as whether you want more aggressive
	power-saving or more instantly available processing power.


	2.2 Governor
	------------

	On all other cpufreq implementations, these boundaries still need to
	be set. Then, a "governor" must be selected. Such a "governor" decides
	what speed the processor shall run within the boundaries. One such
	"governor" is the "userspace" governor. This one allows the user - or
	a yet-to-implement userspace program - to decide what specific speed
	the processor shall run at.


	3. How to change the CPU cpufreq policy and/or speed
	====================================================

	3.1 Preferred Interface: sysfs
	------------------------------

	The preferred interface is located in the sysfs filesystem. If you
	mounted it at /sys, the cpufreq interface is located in a subdirectory
	"cpufreq" within the cpu-device directory
	(e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).

	affected_cpus : List of Online CPUs that require software
	coordination of frequency.

	cpuinfo_cur_freq : Current frequency of the CPU as obtained from
	the hardware, in KHz. This is the frequency
	the CPU actually runs at.

	cpuinfo_min_freq : this file shows the minimum operating
	frequency the processor can run at(in kHz)

	cpuinfo_max_freq : this file shows the maximum operating
	frequency the processor can run at(in kHz)

	cpuinfo_transition_latency The time it takes on this CPU to
	switch between two frequencies in nano
	seconds. If unknown or known to be
	that high that the driver does not
	work with the ondemand governor, -1
	(CPUFREQ_ETERNAL) will be returned.
	Using this information can be useful
	to choose an appropriate polling
	frequency for a kernel governor or
	userspace daemon. Make sure to not
	switch the frequency too often
	resulting in performance loss.

	related_cpus : List of Online + Offline CPUs that need software
	coordination of frequency.

	scaling_available_frequencies : List of available frequencies, in KHz.

	scaling_available_governors : this file shows the CPUfreq governors
	available in this kernel. You can see the
	currently activated governor in

	scaling_cur_freq : Current frequency of the CPU as determined by
	the governor and cpufreq core, in KHz. This is
	the frequency the kernel thinks the CPU runs
	at.

	scaling_driver : this file shows what cpufreq driver is
	used to set the frequency on this CPU

	scaling_governor, and by "echoing" the name of another
	governor you can change it. Please note
	that some governors won't load - they only
	work on some specific architectures or
	processors.

	scaling_min_freq and
	scaling_max_freq show the current "policy limits" (in
	kHz). By echoing new values into these
	files, you can change these limits.
	NOTE: when setting a policy you need to
	first set scaling_max_freq, then
	scaling_min_freq.

	scaling_setspeed This can be read to get the currently programmed
	value by the governor. This can be written to
	change the current frequency for a group of
	CPUs, represented by a policy. This is supported
	currently only by the userspace governor.

	bios_limit : If the BIOS tells the OS to limit a CPU to
	lower frequencies, the user can read out the
	maximum available frequency from this file.
	This typically can happen through (often not
	intended) BIOS settings, restrictions
	triggered through a service processor or other
	BIOS/HW based implementations.
	This does not cover thermal ACPI limitations
	which can be detected through the generic
	thermal driver.

	If you have selected the "userspace" governor which allows you to
	set the CPU operating frequency to a specific value, you can read out
	the current frequency in

	scaling_setspeed. By "echoing" a new frequency into this
	you can change the speed of the CPU,
	but only within the limits of
	scaling_min_freq and scaling_max_freq.