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Commit 9b48c576 authored by Dominik Brodowski's avatar Dominik Brodowski Committed by Linus Torvalds
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[PATCH] cpufreq (5/5): update documentation 

update CPUfreq documentation, and move it to extra directory.
parent 912fd3b6
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Documentation/00-INDEX
View file @ 9b48c576
......@@ -56,6 +56,8 @@ computone.txt
- info on Computone Intelliport II/Plus Multiport Serial Driver
cpqarray.txt
- info on using Compaq's SMART2 Intelligent Disk Array Controllers.
cpufreq/
- info on CPU frequency and voltage scaling
cris/
- directory with info about Linux on CRIS architecture.
devices.txt
......
Documentation/cpu-freq/core.txt 0 → 100644
View file @ 9b48c576
CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
C P U F r e q C o r e
Dominik Brodowski <linux@brodo.de>
David Kimdon <dwhedon@debian.org>
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. CPUFreq core and interfaces
2. CPUFreq notifiers
1. General Information
=======================
The CPUFreq core code is located in linux/kernel/cpufreq.c. This
cpufreq code offers a standardized interface for the CPUFreq
architecture drivers (those pieces of code that do actual
frequency transitions), as well as to "notifiers". These are device
drivers or other part of the kernel that need to be informed of
policy changes (ex. thermal modules like ACPI) or of all
frequency changes (ex. timing code) or even need to force certain
speed limits (like LCD drivers on ARM architecture). Additionally, the
kernel "constant" loops_per_jiffy is updated on frequency changes
here.
2. CPUFreq notifiers
====================
CPUFreq notifiers conform to the standard kernel notifier interface.
See linux/include/linux/notifier.h for details on notifiers.
There are two different CPUFreq notifiers - policy notifiers and
transition notifiers.
2.1 CPUFreq policy notifiers
----------------------------
These are notified when a new policy is intended to be set. Each
CPUFreq policy notifier is called three times for a policy transition:
1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
they see a need for this - may it be thermal considerations or
hardware limitations.
2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
hardware failure.
3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
- if two hardware drivers failed to agree on a new policy before this
stage, the incompatible hardware shall be shut down, and the user
informed of this.
The phase is specified in the second argument to the notifier.
The third argument, a void *pointer, points to a struct cpufreq_policy
consisting of five values: cpu, min, max, policy and max_cpu_freq. min
and max are the lower and upper frequencies (in kHz) of the new
policy, policy the new policy, cpu the number of the affected CPU or
CPUFREQ_ALL_CPUS for all CPUs; and max_cpu_freq the maximum supported
CPU frequency. This value is given for informational purposes only.
2.2 CPUFreq transition notifiers
--------------------------------
These are notified twice when the CPUfreq driver switches the CPU core
frequency and this change has any external implications.
The second argument specifies the phase - CPUFREQ_PRECHANGE or
CPUFREQ_POSTCHANGE.
The third argument is a struct cpufreq_freqs with the following
values:
cpu - number of the affected CPU or CPUFREQ_ALL_CPUS
old - old frequency
new - new frequency
Documentation/cpu-freq/cpu-drivers.txt 0 → 100644
View file @ 9b48c576
CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
C P U D r i v e r s
- information for developers -
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. What To Do?
1.1 Initialization
1.2 Per-CPU Initialization
1.3 verify
1.4 target or setpolicy?
1.5 target
1.6 setpolicy
2. Frequency Table Helpers
1. What To Do?
==============
So, you just got a brand-new CPU / chipset with datasheets and want to
add cpufreq support for this CPU / chipset? Great. Here are some hints
on what is neccessary:
1.1 Initialization
------------------
First of all, in an __initcall level 7 or later (preferrably
module_init() so that your driver is modularized) function check
whether this kernel runs on the right CPU and the right chipset. If
so, register a struct cpufreq_driver with the CPUfreq core using
cpufreq_register_driver()
What shall this struct cpufreq_driver contain?
cpufreq_driver.name - The name of this driver.
cpufreq_driver.init - A pointer to the per-CPU initialization
function.
cpufreq_driver.verify - A pointer to a "verfication" funciton.
cpufreq_driver.setpolicy _or_
cpufreq_driver.target - See below on the differences.
And optionally
cpufreq_driver.exit - A pointer to a per-CPU cleanup function.
1.2 Per-CPU Initialization
--------------------------
Whenever a new CPU is registered with the device model, or after the
cpufreq driver registers itself, the per-CPU initialization fucntion
cpufreq_driver.init is called. It takes a struct cpufreq_policy
*policy as argument. What to do now?
If necessary, activate the CPUfreq support on your CPU (unlock that
register etc.).
Then, the driver must fill in the following values:
policy->cpuinfo.min_freq _and_
policy->cpuinfo.max_freq - the minimum and maximum frequency
(in kHz) which is supported by
this CPU
policy->cpuinfo.transition_latency the time it takes on this CPU to
switch between two frequencies (if
appropriate, else specify
CPUFREQ_ETERNAL)
policy->cur The current operating frequency of
this CPU (if appropriate)
policy->min,
policy->max,
policy->policy and, if neccessary,
policy->governor must contain the "default policy" for
this CPU. A few moments later,
cpufreq_driver.verify and either
cpufreq_driver.setpolicy or
cpufreq_driver.target is called with
these values.
For setting some of these values, the frequency table helpers might be
helpful. See the section 2 for more information on them.
1.3 verify
------------
When the user decides a new policy (consisting of
"policy,governor,min,max") shall be set, this policy must be validated
so that incompatible values can be corrected. For verifying these
values, a frequency table helper and/or the
cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
int min_freq, unsigned int max_freq) function might be helpful. See
section 2 for details on frequency table helpers.
You need to make sure that at least one valid frequency (or operating
range) is within policy->min and policy->max. If necessary, increase
policy->max fist, and only if this is no solution, decreas policy->min.
1.4 target or setpolicy?
----------------------------
Most cpufreq drivers or even most cpu frequency scaling algorithms
only allow the CPU to be set to one frequency. For these, you use the
->target call.
Some cpufreq-capable processors switch the frequency between certain
limits on their own. These shall use the ->setpolicy call
1.4. target
-------------
The target call has three arguments: struct cpufreq_policy *policy,
unsigned int target_frequency, unsigned int relation.
The CPUfreq driver must set the new frequency when called here. The
actual frequency must be determined using the following rules:
- keep close to "target_freq"
- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
target_freq. ("L for lowest, but no lower than")
- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
target_freq. ("H for highest, but no higher than")
Here again the frequency table helper might assist you - see section 3
for details.
1.5 setpolicy
---------------
The setpolicy call only takes a struct cpufreq_policy *policy as
argument. You need to set the lower limit of the in-processor or
in-chipset dynamic frequency switching to policy->min, the upper limit
to policy->max, and -if supported- select a performance-oriented
setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
the reference implementation in arch/i386/kernel/cpu/cpufreq/longrun.c
2. Frequency Table Helpers
==========================
As most cpufreq processors only allow for being set to a few specific
frequencies, a "frequency table" with some functions might assist in
some work of the processor driver. Such a "frequency table" consists
of an array of struct cpufreq_freq_table entries, with any value in
"index" you want to use, and the corresponding frequency in
"frequency". At the end of the table, you need to add a
cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And
if you want to skip one entry in the table, set the frequency to
CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
order.
By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
struct cpufreq_frequency_table *table);
the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
policy->min and policy->max are set to the same values. This is
helpful for the per-CPU initialization stage.
int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
struct cpufreq_frequency_table *table);
assures that at least one valid frequency is within policy->min and
policy->max, and all other criteria are met. This is helpful for the
->verify call.
int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
struct cpufreq_frequency_table *table,
unsigned int target_freq,
unsigned int relation,
unsigned int *index);
is the corresponding frequency table helper for the ->target
stage. Just pass the values to this function, and the unsigned int
index returns the number of the frequency table entry which contains
the frequency the CPU shall be set to. PLEASE NOTE: This is not the
"index" which is in this cpufreq_table_entry.index, but instead
cpufreq_table[index]. So, the new frequency is
cpufreq_table[index].frequency, and the value you stored into the
frequency table "index" field is
cpufreq_table[index].index.
Documentation/cpu-freq/governors.txt 0 → 100644
View file @ 9b48c576
CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
C P U F r e q G o v e r n o r s
- information for users and developers -
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. What is a CPUFreq Governor?
2. Governors In the Linux Kernel
2.1 Performance
2.2 Powersave
2.3 Userspace
3. The Governor Interface in the CPUfreq Core
1. What Is A CPUFreq Governor?
==============================
Most cpufreq drivers (in fact, all except one, longrun) or even most
cpu frequency scaling algorithms only offer the CPU to be set to one
frequency. In order to offer dynamic frequency scaling, the cpufreq
core must be able to tell these drivers of a "target frequency". So
these specific drivers will be transformed to offer a "->target"
call instead of the existing "->setpolicy" call. For "longrun", all
stays the same, though.
How to decide what frequency within the CPUfreq policy should be used?
That's done using "cpufreq governors". Two are already in this patch
-- they're the already existing "powersave" and "performance" which
set the frequency statically to the lowest or highest frequency,
respectively. At least two more such governors will be ready for
addition in the near future, but likely many more as there are various
different theories and models about dynamic frequency scaling
around. Using such a generic interface as cpufreq offers to scaling
governors, these can be tested extensively, and the best one can be
selected for each specific use.
Basically, it's the following flow graph:
CPU can be set to switch independetly | CPU can only be set
within specific "limits" | to specific frequencies
"CPUfreq policy"
consists of frequency limits (policy->{min,max})
and CPUfreq governor to be used
/ \
/ \
/ the cpufreq governor decides
/ (dynamically or statically)
/ what target_freq to set within
/ the limits of policy->{min,max}
/ \
/ \
Using the ->setpolicy call, Using the ->target call,
the limits and the the frequency closest
"policy" is set. to target_freq is set.
It is assured that it
is within policy->{min,max}
2. Governors In the Linux Kernel
================================
2.1 Performance
---------------
The CPUfreq governor "performance" sets the CPU statically to the
highest frequency within the borders of scaling_min_freq and
scaling_max_freq.
2.1 Powersave
-------------
The CPUfreq governor "powersave" sets the CPU statically to the
lowest frequency within the borders of scaling_min_freq and
scaling_max_freq.
2.2 Userspace
-------------
The CPUfreq governor "userspace" allows the user, or any userspace
program running with UID "root", to set the CPU to a specifc frequency
by making a sysfs file "scaling_setspeed" available in the CPU-device
directory.
3. The Governor Interface in the CPUfreq Core
=============================================
A new governor must register itself with the CPUfreq core using
"cpufreq_register_governor". The struct cpufreq_governor, which has to
be passed to that function, must contain the following values:
governor->name - A unique name for this governor
governor->governor - The governor callback function
governor->owner - .THIS_MODULE for the governor module (if
appropriate)
The governor->governor callback is called with the current (or to-be-set)
cpufreq_policy struct for that CPU, and an unsigned int event. The
following events are currently defined:
CPUFREQ_GOV_START: This governor shall start its duty for the CPU
policy->cpu
CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
policy->cpu
CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
policy->min and policy->max.
If you need other "events" externally of your driver, _only_ use the
cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
CPUfreq core to ensure proper locking.
The CPUfreq governor may call the CPU processor driver using one of
these two functions:
inline int cpufreq_driver_target(struct cpufreq_policy *policy,
unsigned int target_freq,
unsigned int relation);
inline int cpufreq_driver_target_l(struct cpufreq_policy *policy,
unsigned int target_freq,
unsigned int relation);
target_freq must be within policy->min and policy->max, of course.
What's the difference between these two functions? When your governor
still is in a direct code path of a call to governor->governor, the
cpufreq_driver_sem lock is still held in the cpufreq core, and there's
no need to lock it again (in fact, this would cause a deadlock). So
use cpufreq_driver_target only in these cases. In all other cases (for
example, when there's a "daemonized" function that wakes up every
second), use cpufreq_driver_target_l to lock the cpufreq_driver_sem
before the command is passed to the cpufreq processor driver.
Documentation/cpu-freq/index.txt 0 → 100644
View file @ 9b48c576
CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
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.
Documents in this directory:
----------------------------
core.txt - General description of the CPUFreq core and
of CPUFreq notifiers
cpu-drivers.txt - How to implement a new cpufreq processor driver
governors.txt - What are cpufreq governors and how to
implement them?
index.txt - File index, Mailing list and Links (this document)
user-guide.txt - User Guide to CPUFreq
Mailing List
------------
There is a CPU frequency changing CVS commit and general list where
you can report bugs, problems or submit patches. To post a message,
send an email to cpufreq@www.linux.org.uk, to subscribe go to
http://www.linux.org.uk/mailman/listinfo/cpufreq. Previous post to the
mailing list are available to subscribers at
http://www.linux.org.uk/mailman/private/cpufreq/.
Links
-----
the FTP archives:
* ftp://ftp.linux.org.uk/pub/linux/cpufreq/
how to access the CVS repository:
* http://cvs.arm.linux.org.uk/
the CPUFreq Mailing list:
* http://www.linux.org.uk/mailman/listinfo/cpufreq
Clock and voltage scaling for the SA-1100:
* http://www.lart.tudelft.nl/projects/scaling
Documentation/cpu-freq/user-guide.txt 0 → 100644
View file @ 9b48c576
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
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
3.2 Deprecated interfaces
1. Supported Architectures and Processors
=========================================
1.1 ARM
-------
The following ARM processors are supported by cpufreq:
ARM Integrator
ARM-SA1100
ARM-SA1110
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+
Cyrix Media GXm
Intel mobile PIII [*] and Intel mobile PIII-M on certain chipsets
Intel Pentium 4, Intel Xeon
National Semiconductors Geode GX
Transmeta Crusoe
varios processors on some ACPI 2.0-compatible systems [**]
[*] only certain Intel mobile PIII processors are supported. If you
know that you own a speedstep-capable processor, pass the option
"speedstep_coppermine=1" to the module speedstep.o
[**] 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
2. "Policy" / "Governor" ?
==========================
Some CPU frequency scaling-capable processor switch between varios
frequencies and operating voltages "on the fly" without any kernel or
user involvement. This guarantuees 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 avaialble 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 the
cpu-device directory (e.g. /sys/devices/sys/cpu0/ 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)
scaling_driver : this file shows what cpufreq driver is
used to set the frequency on this CPU
available_scaling_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.
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.
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.
3.2 Deprecated Interfaces
-------------------------
Depending on your kernel configuration, you might find the following
cpufreq-related files:
/proc/cpufreq
/proc/sys/cpu/*/speed
/proc/sys/cpu/*/speed-min
/proc/sys/cpu/*/speed-max
These are files for deprecated interfaces to cpufreq, which offer far
less functionality. Because of this, these interfaces aren't described
here.
Documentation/cpufreq deleted 100644 → 0
View file @ 912fd3b6
CPU frequency and voltage scaling code in the Linux(TM) kernel
L i n u x C P U F r e q
Dominik Brodowski <linux@brodo.de>
David Kimdon <dwhedon@debian.org>
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
2. User interface
2.1 /proc/cpufreq interface [2.6]
2.2. /proc/sys/cpu/ interface [2.4]
3. CPUFreq core and interfaces
3.1 General information
3.2 CPUFreq notifiers
3.3 CPUFreq architecture drivers
4. Mailing list and Links
1. Supported architectures
==========================
ARM:
ARM Integrator, SA 1100, SA1110
--------------------------------
This driver will be ported to new CPUFreq core soon, so
far it will not work.
AMD Elan:
SC400, SC410
--------------------------------
You need to specify the highest allowed CPU frequency as
a module parameter ("max_freq") or as boot parameter
("elanfreq="). Else the available speed range will be
limited to the speed at which the CPU runs while this
module is loaded.
VIA Cyrix Longhaul:
VIA Samuel/CyrixIII, VIA Cyrix Samuel/C3,
VIA Cyrix Ezra, VIA Cyrix Ezra-T
--------------------------------
If you do not want to scale the Front Side Bus or voltage,
pass the module parameter "dont_scale_fsb 1" or
"dont_scale_voltage 1". Additionally, it is advised that
you pass the current Front Side Bus speed (in MHz) to
this module as module parameter "current_fsb", e.g.
"current_fsb 133" for a Front Side Bus speed of 133 MHz.
Intel SpeedStep:
certain mobile Intel Pentium III (Coppermine), and all mobile
Intel Pentium III-M (Tualatin) and mobile Intel Pentium 4 P4-Ms.
--------------------------------
Unfortunately only modern Intel ICH2-M and ICH3-M chipsets are
supported.
P4 CPU Clock Modulation:
Intel Pentium 4 Xeon processors
---------------------------------
Note that you can only switch the speed of two logical CPUs at
once - but each phyiscal CPU may have different throttling levels.
PowerNow! K6:
mobile AMD K6-2+ / mobile K6-3+:
--------------------------------
No known issues.
Transmeta Crusoe Longrun:
Transmeta Crusoe processors:
--------------------------------
It is recommended to use the 2.6. /proc/cpufreq interface when
using this driver
2. User Interface
=================
2.1 /proc/cpufreq interface [2.6]
***********************************
Starting in the patches for kernel 2.5.33, CPUFreq uses a "policy"
interface /proc/cpufreq.
When you "cat" this file, you'll find something like:
--
minimum CPU frequency - maximum CPU frequency - policy
CPU 0 1200000 ( 75%) - 1600000 (100%) - performance
--
This means the current policy allows this CPU to be run anywhere
between 1.2 GHz (the value is in kHz) and 1.6 GHz with an eye towards
performance.
To change the policy, "echo" the desired new policy into
/proc/cpufreq. Use one of the following formats:
cpu_nr:min_freq:max_freq:policy
cpu_nr%min_freq%max_freq%policy
min_freq:max_freq:policy
min_freq%max_freq%policy
with cpu_nr being the CPU which shall be affected, min_freq and
max_freq the lower and upper limit of the CPU core frequency in kHz,
and policy either "performance" or "powersave".
A few examples:
root@notebook:#echo -n "0:0:0:powersave" > /proc/cpufreq
sets the CPU #0 to the lowest supported frequency.
root@notebook:#echo -n "1%100%100%performance" > /proc/cpufreq
sets the CPU #1 to the highest supported frequency.
root@notebook:#echo -n "1000000:2000000:performance" > /proc/cpufreq
to set the frequency of all CPUs between 1 GHz and 2 GHz and to
the policy "performance".
Please note that the values you "echo" into /proc/cpufreq are
validated first, and may be limited by hardware or thermal
considerations. Because of this, a read from /proc/cpufreq might
differ from what was written into it.
When you read /proc/cpufreq for the first time after a CPUFreq driver
has been initialized, you'll see the "default policy" for this
driver. If this does not suit your needs, you can pass a boot
parameter to the cpufreq core. Use the following syntax for this:
"cpufreq=min_freq:max_freq:policy", i.e. you may not chose a
specific CPU and you need to specify the limits in kHz and not in
per cent.
2.2 /proc/cpufreq interface [2.4]
***********************************
Previsiously (and still available as a config option), CPUFreq used
a "sysctl" interface which is located in
/proc/sys/cpu/0/
/proc/sys/cpu/1/ ... (SMP only)
In these directories, you will find three files of importance for
CPUFreq: speed-max, speed-min and speed:
speed shows the current CPU frequency in kHz,
speed-min the minimum supported CPU frequency, and
speed-max the maximum supported CPU frequency.
To change the CPU frequency, "echo" the desired CPU frequency (in kHz)
to speed. For example, to set the CPU speed to the lowest/highest
allowed frequency do:
root@notebook:# cat /proc/sys/cpu/0/speed-min > /proc/sys/cpu/0/speed
root@notebook:# cat /proc/sys/cpu/0/speed-max > /proc/sys/cpu/0/speed
3. CPUFreq core and interfaces
===============================
3.1 General information
*************************
The CPUFreq core code is located in linux/kernel/cpufreq.c. This
cpufreq code offers a standardized interface for the CPUFreq
architecture drivers (those pieces of code that do actual
frequency transitions), as well as to "notifiers". These are device
drivers or other part of the kernel that need to be informed of
policy changes (like thermal modules like ACPI) or of all
frequency changes (like timing code) or even need to force certain
speed limits (like LCD drivers on ARM architecture). Additionally, the
kernel "constant" loops_per_jiffy is updated on frequency changes
here.
3.2 CPUFreq notifiers
***********************
CPUFreq notifiers conform to the standard kernel notifier interface.
See linux/include/linux/notifier.h for details on notifiers.
There are two different CPUFreq notifiers - policy notifiers and
transition notifiers.
3.2.1 CPUFreq policy notifiers
******************************
These are notified when a new policy is intended to be set. Each
CPUFreq policy notifier is called three times for a policy transition:
1.) During CPUFREQ_ADJUST all CPUFreq notifiers may change the limit if
they see a need for this - may it be thermal considerations or
hardware limitations.
2.) During CPUFREQ_INCOMPATIBLE only changes may be done in order to avoid
hardware failure.
3.) And during CPUFREQ_NOTIFY all notifiers are informed of the new policy
- if two hardware drivers failed to agree on a new policy before this
stage, the incompatible hardware shall be shut down, and the user
informed of this.
The phase is specified in the second argument to the notifier.
The third argument, a void *pointer, points to a struct cpufreq_policy
consisting of five values: cpu, min, max, policy and max_cpu_freq. Min
and max are the lower and upper frequencies (in kHz) of the new
policy, policy the new policy, cpu the number of the affected CPU or
CPUFREQ_ALL_CPUS for all CPUs; and max_cpu_freq the maximum supported
CPU frequency. This value is given for informational purposes only.
3.2.2 CPUFreq transition notifiers
**********************************
These are notified twice when the CPUfreq driver switches the CPU core
frequency and this change has any external implications.
The second argument specifies the phase - CPUFREQ_PRECHANGE or
CPUFREQ_POSTCHANGE.
The third argument is a struct cpufreq_freqs with the following
values:
cpu - number of the affected CPU or CPUFREQ_ALL_CPUS
old - old frequency
new - new frequency
3.3 CPUFreq architecture drivers
**********************************
CPUFreq architecture drivers are the pieces of kernel code that
actually perform CPU frequency transitions. These need to be
initialized separately (separate initcalls), and may be
modularized. They interact with the CPUFreq core in the following way:
cpufreq_register()
------------------
cpufreq_register registers an arch driver to the CPUFreq core. Please
note that only one arch driver may be registered at any time. -EBUSY
is returned when an arch driver is already registered. The argument to
cpufreq_register, struct cpufreq_driver *driver, is described later.
cpufreq_unregister()
--------------------
cpufreq_unregister unregisters an arch driver, e.g. on module
unloading. Please note that there is no check done that this is called
from the driver which actually registered itself to the core, so
please only call this function when you are sure the arch driver got
registered correctly before.
cpufreq_notify_transition()
---------------------------
On "dumb" hardware where only fixed frequency can be set, the driver
must call cpufreq_notify_transition() once before, and once after the
actual transition.
struct cpufreq_driver
---------------------
On initialization, the arch driver is supposed to pass a pointer
to a struct cpufreq_driver *cpufreq_driver consisting of the following
entries:
cpufreq_verify_t verify: This is a pointer to a function with the
following definition:
int verify_function (struct cpufreq_policy *policy).
This function must verify the new policy is within the limits
supported by the CPU, and at least one supported CPU is within
this range. It may be useful to use cpufreq.h /
cpufreq_verify_within_limits for this. If this is called with
CPUFREQ_ALL_CPUS, and there is no common subset of frequencies
for all CPUs, exit with an error.
cpufreq_setpolicy_t setpolicy: This is a pointer to a function with
the following definition:
int setpolicy_function (struct cpufreq_policy *policy).
This function must set the CPU to the new policy. If it is a
"dumb" CPU which only allows fixed frequencies to be set, it
shall set it to the lowest within the limit for
CPUFREQ_POLICY_POWERSAVE, and to the highest for
CPUFREQ_POLICY_PERFORMANCE. Once CONFIG_CPU_FREQ_DYNAMIC is
implemented, it can use a dynamic method to adjust the speed
between the lower and upper limit.
struct cpufreq_policy *policy: This is an array of NR_CPUS struct
cpufreq_policies, containing the current policies set for these
CPUs. Note that policy[cpu].max_cpu_freq must contain the
absolute maximum CPU frequency supported by the specified cpu.
In case the driver is expected to run with the 2.4.-style API
(/proc/sys/cpu/.../), two more values must be passed
#ifdef CONFIG_CPU_FREQ_24_API
unsigned int cpu_min_freq[NR_CPUS];
unsigned int cpu_cur_freq[NR_CPUS];
#endif
with cpu_min_freq[cpu] being the minimum CPU frequency
supported by the CPU; and the entries in cpu_cur_freq
reflecting the current speed of the appropriate CPU.
Some Requirements to CPUFreq architecture drivers
-------------------------------------------------
* Only call cpufreq_register() when the ability to switch CPU
frequencies is _verified_ or can't be missing. Also, all
other initialization must be done beofre this call, as
cpfureq_register calls the driver's verify and setpolicy code for
each CPU.
* cpufreq_unregister() may only be called if cpufreq_register() has
been successfully(!) called before.
* kfree() the struct cpufreq_driver only after the call to
cpufreq_unregister(), unless cpufreq_register() failed.
4. Mailing list and Links
*************************
Mailing List
------------
There is a CPU frequency changing CVS commit and general list where
you can report bugs, problems or submit patches. To post a message,
send an email to cpufreq@www.linux.org.uk, to subscribe go to
http://www.linux.org.uk/mailman/listinfo/cpufreq. Previous post to the
mailing list are available to subscribers at
http://www.linux.org.uk/mailman/private/cpufreq/.
Links
-----
the FTP archives:
* ftp://ftp.linux.org.uk/pub/linux/cpufreq/
how to access the CVS repository:
* http://cvs.arm.linux.org.uk/
the CPUFreq Mailing list:
* http://www.linux.org.uk/mailman/listinfo/cpufreq
Clock and voltage scaling for the SA-1100:
* http://www.lart.tudelft.nl/projects/scaling
CPUFreq project homepage
* http://www.brodo.de/cpufreq/
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