[mirror_ubuntu-artful-kernel.git] / Documentation / cpu-freq / user-guide.txt

     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.


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.
Commit	Line	Data
1da177e4 LT	1	CPU frequency and voltage scaling code in the Linux(TM) kernel
	2
	3
	4	L i n u x C P U F r e q
	5
	6	U S E R G U I D E
	7
	8
	9	Dominik Brodowski <linux@brodo.de>
	10
	11
	12
	13	Clock scaling allows you to change the clock speed of the CPUs on the
	14	fly. This is a nice method to save battery power, because the lower
	15	the clock speed, the less power the CPU consumes.
	16
	17
	18	Contents:
	19	---------
	20	1. Supported Architectures and Processors
7de962c0	21	1.1 ARM and ARM64
1da177e4 LT	22	1.2 x86
	23	1.3 sparc64
	24	1.4 ppc
	25	1.5 SuperH
121fe86b	26	1.6 Blackfin
1da177e4 LT	27
	28	2. "Policy" / "Governor"?
	29	2.1 Policy
	30	2.2 Governor
	31
	32	3. How to change the CPU cpufreq policy and/or speed
	33	3.1 Preferred interface: sysfs
1da177e4 LT	34
	35
	36
	37	1. Supported Architectures and Processors
	38	=========================================
	39
7de962c0 VK	40	1.1 ARM and ARM64
7de962c0 VK	41	-----------------
1da177e4	42
7de962c0	43	Almost all ARM and ARM64 platforms support CPU frequency scaling.
1da177e4 LT	44
	45	1.2 x86
	46	-------
	47
	48	The following processors for the x86 architecture are supported by cpufreq:
	49
	50	AMD Elan - SC400, SC410
	51	AMD mobile K6-2+
	52	AMD mobile K6-3+
	53	AMD mobile Duron
	54	AMD mobile Athlon
	55	AMD Opteron
	56	AMD Athlon 64
	57	Cyrix Media GXm
	58	Intel mobile PIII and Intel mobile PIII-M on certain chipsets
	59	Intel Pentium 4, Intel Xeon
	60	Intel Pentium M (Centrino)
	61	National Semiconductors Geode GX
	62	Transmeta Crusoe
	63	Transmeta Efficeon
	64	VIA Cyrix 3 / C3
	65	various processors on some ACPI 2.0-compatible systems [*]
7de962c0	66	And many more
1da177e4 LT	67
	68	[*] Only if "ACPI Processor Performance States" are available
	69	to the ACPI<->BIOS interface.
	70
	71
	72	1.3 sparc64
	73	-----------
	74
	75	The following processors for the sparc64 architecture are supported by
	76	cpufreq:
	77
	78	UltraSPARC-III
	79
	80
	81	1.4 ppc
	82	-------
	83
	84	Several "PowerBook" and "iBook2" notebooks are supported.
	85
	86
	87	1.5 SuperH
	88	----------
	89
8a655053 PM	90	All SuperH processors supporting rate rounding through the clock
8a655053 PM	91	framework are supported by cpufreq.
1da177e4	92
121fe86b RG	93	1.6 Blackfin
	94	------------
	95
	96	The following Blackfin processors are supported by cpufreq:
	97
	98	BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
	99	BF531, BF532, BF533, Rev 0.3 or higher
	100	BF534, BF536, BF537, Rev 0.2 or higher
	101	BF561, Rev 0.3 or higher
	102	BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
	103
1da177e4 LT	104
	105	2. "Policy" / "Governor" ?
	106	==========================
	107
	108	Some CPU frequency scaling-capable processor switch between various
	109	frequencies and operating voltages "on the fly" without any kernel or
	110	user involvement. This guarantees very fast switching to a frequency
	111	which is high enough to serve the user's needs, but low enough to save
	112	power.
	113
	114
	115	2.1 Policy
	116	----------
	117
	118	On these systems, all you can do is select the lower and upper
	119	frequency limit as well as whether you want more aggressive
	120	power-saving or more instantly available processing power.
	121
	122
	123	2.2 Governor
	124	------------
	125
	126	On all other cpufreq implementations, these boundaries still need to
	127	be set. Then, a "governor" must be selected. Such a "governor" decides
	128	what speed the processor shall run within the boundaries. One such
	129	"governor" is the "userspace" governor. This one allows the user - or
	130	a yet-to-implement userspace program - to decide what specific speed
	131	the processor shall run at.
	132
	133
	134	3. How to change the CPU cpufreq policy and/or speed
	135	====================================================
	136
	137	3.1 Preferred Interface: sysfs
	138	------------------------------
	139
	140	The preferred interface is located in the sysfs filesystem. If you
	141	mounted it at /sys, the cpufreq interface is located in a subdirectory
	142	"cpufreq" within the cpu-device directory
	143	(e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
	144
7de962c0 VK	145	affected_cpus : List of Online CPUs that require software
	146	coordination of frequency.
	147
	148	cpuinfo_cur_freq : Current frequency of the CPU as obtained from
	149	the hardware, in KHz. This is the frequency
	150	the CPU actually runs at.
	151
1da177e4 LT	152	cpuinfo_min_freq : this file shows the minimum operating
1da177e4 LT	153	frequency the processor can run at(in kHz)
7de962c0	154
1da177e4 LT	155	cpuinfo_max_freq : this file shows the maximum operating
1da177e4 LT	156	frequency the processor can run at(in kHz)
7de962c0	157
ed129784 TR	158	cpuinfo_transition_latency The time it takes on this CPU to
	159	switch between two frequencies in nano
	160	seconds. If unknown or known to be
	161	that high that the driver does not
	162	work with the ondemand governor, -1
	163	(CPUFREQ_ETERNAL) will be returned.
	164	Using this information can be useful
	165	to choose an appropriate polling
	166	frequency for a kernel governor or
	167	userspace daemon. Make sure to not
	168	switch the frequency too often
	169	resulting in performance loss.
7de962c0 VK	170
	171	related_cpus : List of Online + Offline CPUs that need software
	172	coordination of frequency.
	173
	174	scaling_available_frequencies : List of available frequencies, in KHz.
1da177e4 LT	175
	176	scaling_available_governors : this file shows the CPUfreq governors
	177	available in this kernel. You can see the
	178	currently activated governor in
	179
7de962c0 VK	180	scaling_cur_freq : Current frequency of the CPU as determined by
	181	the governor and cpufreq core, in KHz. This is
	182	the frequency the kernel thinks the CPU runs
	183	at.
	184
	185	scaling_driver : this file shows what cpufreq driver is
	186	used to set the frequency on this CPU
	187
1da177e4 LT	188	scaling_governor, and by "echoing" the name of another
	189	governor you can change it. Please note
	190	that some governors won't load - they only
	191	work on some specific architectures or
	192	processors.
605400a8	193
9c9a43ed	194	scaling_min_freq and
1da177e4 LT	195	scaling_max_freq show the current "policy limits" (in
	196	kHz). By echoing new values into these
	197	files, you can change these limits.
9c9a43ed MD	198	NOTE: when setting a policy you need to
	199	first set scaling_max_freq, then
	200	scaling_min_freq.
1da177e4	201
7de962c0 VK	202	scaling_setspeed This can be read to get the currently programmed
	203	value by the governor. This can be written to
	204	change the current frequency for a group of
	205	CPUs, represented by a policy. This is supported
	206	currently only by the userspace governor.
1da177e4	207
e2f74f35 TR	208	bios_limit : If the BIOS tells the OS to limit a CPU to
	209	lower frequencies, the user can read out the
	210	maximum available frequency from this file.
	211	This typically can happen through (often not
	212	intended) BIOS settings, restrictions
	213	triggered through a service processor or other
	214	BIOS/HW based implementations.
	215	This does not cover thermal ACPI limitations
	216	which can be detected through the generic
	217	thermal driver.
	218
1da177e4 LT	219	If you have selected the "userspace" governor which allows you to
	220	set the CPU operating frequency to a specific value, you can read out
	221	the current frequency in
	222
	223	scaling_setspeed. By "echoing" a new frequency into this
	224	you can change the speed of the CPU,
	225	but only within the limits of
	226	scaling_min_freq and scaling_max_freq.