[net-next-2.6.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
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
-------

The following ARM processors are supported by cpufreq:

ARM Integrator
ARM-SA1100
ARM-SA1110
Intel PXA


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 [*]

[*] 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).

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

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

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.

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

scaling_available_frequencies : List of available frequencies, in KHz.

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.

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

related_cpus :			List of CPUs that need some sort of frequency
				coordination, whether software or hardware.

scaling_driver :		Hardware driver for cpufreq.

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.

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
	21	1.1 ARM
	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
	40	1.1 ARM
	41	-------
	42
	43	The following ARM processors are supported by cpufreq:
	44
	45	ARM Integrator
	46	ARM-SA1100
	47	ARM-SA1110
9e2697ff	48	Intel PXA
1da177e4 LT	49
	50
	51	1.2 x86
	52	-------
	53
	54	The following processors for the x86 architecture are supported by cpufreq:
	55
	56	AMD Elan - SC400, SC410
	57	AMD mobile K6-2+
	58	AMD mobile K6-3+
	59	AMD mobile Duron
	60	AMD mobile Athlon
	61	AMD Opteron
	62	AMD Athlon 64
	63	Cyrix Media GXm
	64	Intel mobile PIII and Intel mobile PIII-M on certain chipsets
	65	Intel Pentium 4, Intel Xeon
	66	Intel Pentium M (Centrino)
	67	National Semiconductors Geode GX
	68	Transmeta Crusoe
	69	Transmeta Efficeon
	70	VIA Cyrix 3 / C3
	71	various processors on some ACPI 2.0-compatible systems [*]
	72
	73	[*] Only if "ACPI Processor Performance States" are available
	74	to the ACPI<->BIOS interface.
	75
	76
	77	1.3 sparc64
	78	-----------
	79
	80	The following processors for the sparc64 architecture are supported by
	81	cpufreq:
	82
	83	UltraSPARC-III
	84
	85
	86	1.4 ppc
	87	-------
	88
	89	Several "PowerBook" and "iBook2" notebooks are supported.
	90
	91
	92	1.5 SuperH
	93	----------
	94
8a655053 PM	95	All SuperH processors supporting rate rounding through the clock
8a655053 PM	96	framework are supported by cpufreq.
1da177e4	97
121fe86b RG	98	1.6 Blackfin
	99	------------
	100
	101	The following Blackfin processors are supported by cpufreq:
	102
	103	BF522, BF523, BF524, BF525, BF526, BF527, Rev 0.1 or higher
	104	BF531, BF532, BF533, Rev 0.3 or higher
	105	BF534, BF536, BF537, Rev 0.2 or higher
	106	BF561, Rev 0.3 or higher
	107	BF542, BF544, BF547, BF548, BF549, Rev 0.1 or higher
	108
1da177e4 LT	109
	110	2. "Policy" / "Governor" ?
	111	==========================
	112
	113	Some CPU frequency scaling-capable processor switch between various
	114	frequencies and operating voltages "on the fly" without any kernel or
	115	user involvement. This guarantees very fast switching to a frequency
	116	which is high enough to serve the user's needs, but low enough to save
	117	power.
	118
	119
	120	2.1 Policy
	121	----------
	122
	123	On these systems, all you can do is select the lower and upper
	124	frequency limit as well as whether you want more aggressive
	125	power-saving or more instantly available processing power.
	126
	127
	128	2.2 Governor
	129	------------
	130
	131	On all other cpufreq implementations, these boundaries still need to
	132	be set. Then, a "governor" must be selected. Such a "governor" decides
	133	what speed the processor shall run within the boundaries. One such
	134	"governor" is the "userspace" governor. This one allows the user - or
	135	a yet-to-implement userspace program - to decide what specific speed
	136	the processor shall run at.
	137
	138
	139	3. How to change the CPU cpufreq policy and/or speed
	140	====================================================
	141
	142	3.1 Preferred Interface: sysfs
	143	------------------------------
	144
	145	The preferred interface is located in the sysfs filesystem. If you
	146	mounted it at /sys, the cpufreq interface is located in a subdirectory
	147	"cpufreq" within the cpu-device directory
	148	(e.g. /sys/devices/system/cpu/cpu0/cpufreq/ for the first CPU).
	149
	150	cpuinfo_min_freq : this file shows the minimum operating
	151	frequency the processor can run at(in kHz)
	152	cpuinfo_max_freq : this file shows the maximum operating
	153	frequency the processor can run at(in kHz)
ed129784 TR	154	cpuinfo_transition_latency The time it takes on this CPU to
	155	switch between two frequencies in nano
	156	seconds. If unknown or known to be
	157	that high that the driver does not
	158	work with the ondemand governor, -1
	159	(CPUFREQ_ETERNAL) will be returned.
	160	Using this information can be useful
	161	to choose an appropriate polling
	162	frequency for a kernel governor or
	163	userspace daemon. Make sure to not
	164	switch the frequency too often
	165	resulting in performance loss.
1da177e4 LT	166	scaling_driver : this file shows what cpufreq driver is
	167	used to set the frequency on this CPU
	168
	169	scaling_available_governors : this file shows the CPUfreq governors
	170	available in this kernel. You can see the
	171	currently activated governor in
	172
	173	scaling_governor, and by "echoing" the name of another
	174	governor you can change it. Please note
	175	that some governors won't load - they only
	176	work on some specific architectures or
	177	processors.
605400a8	178
da470db1 NC	179	cpuinfo_cur_freq : Current frequency of the CPU as obtained from
	180	the hardware, in KHz. This is the frequency
	181	the CPU actually runs at.
605400a8 DW	182
	183	scaling_available_frequencies : List of available frequencies, in KHz.
	184
9c9a43ed	185	scaling_min_freq and
1da177e4 LT	186	scaling_max_freq show the current "policy limits" (in
	187	kHz). By echoing new values into these
	188	files, you can change these limits.
9c9a43ed MD	189	NOTE: when setting a policy you need to
	190	first set scaling_max_freq, then
	191	scaling_min_freq.
1da177e4	192
605400a8 DW	193	affected_cpus : List of CPUs that require software coordination
	194	of frequency.
	195
	196	related_cpus : List of CPUs that need some sort of frequency
	197	coordination, whether software or hardware.
	198
	199	scaling_driver : Hardware driver for cpufreq.
	200
da470db1 NC	201	scaling_cur_freq : Current frequency of the CPU as determined by
	202	the governor and cpufreq core, in KHz. This is
	203	the frequency the kernel thinks the CPU runs
	204	at.
1da177e4	205
e2f74f35 TR	206	bios_limit : If the BIOS tells the OS to limit a CPU to
	207	lower frequencies, the user can read out the
	208	maximum available frequency from this file.
	209	This typically can happen through (often not
	210	intended) BIOS settings, restrictions
	211	triggered through a service processor or other
	212	BIOS/HW based implementations.
	213	This does not cover thermal ACPI limitations
	214	which can be detected through the generic
	215	thermal driver.
	216
1da177e4 LT	217	If you have selected the "userspace" governor which allows you to
	218	set the CPU operating frequency to a specific value, you can read out
	219	the current frequency in
	220
	221	scaling_setspeed. By "echoing" a new frequency into this
	222	you can change the speed of the CPU,
	223	but only within the limits of
	224	scaling_min_freq and scaling_max_freq.