mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson
synced 2025-10-31 16:38:31 +00:00
992caacf11
This patch fixes typos in various Documentation txts. The patch addresses some words starting with the letters 'N'-'P'. Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Adrian Bunk <bunk@stusta.de>
222 lines
8.5 KiB
Plaintext
222 lines
8.5 KiB
Plaintext
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>
|
|
some additions and corrections by Nico Golde <nico@ngolde.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
|
|
2.4 Ondemand
|
|
2.5 Conservative
|
|
|
|
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 independently | 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.
|
|
|
|
|
|
2.2 Powersave
|
|
-------------
|
|
|
|
The CPUfreq governor "powersave" sets the CPU statically to the
|
|
lowest frequency within the borders of scaling_min_freq and
|
|
scaling_max_freq.
|
|
|
|
|
|
2.3 Userspace
|
|
-------------
|
|
|
|
The CPUfreq governor "userspace" allows the user, or any userspace
|
|
program running with UID "root", to set the CPU to a specific frequency
|
|
by making a sysfs file "scaling_setspeed" available in the CPU-device
|
|
directory.
|
|
|
|
|
|
2.4 Ondemand
|
|
------------
|
|
|
|
The CPUfreq governor "ondemand" sets the CPU depending on the
|
|
current usage. To do this the CPU must have the capability to
|
|
switch the frequency very quickly. There are a number of sysfs file
|
|
accessible parameters:
|
|
|
|
sampling_rate: measured in uS (10^-6 seconds), this is how often you
|
|
want the kernel to look at the CPU usage and to make decisions on
|
|
what to do about the frequency. Typically this is set to values of
|
|
around '10000' or more.
|
|
|
|
show_sampling_rate_(min|max): the minimum and maximum sampling rates
|
|
available that you may set 'sampling_rate' to.
|
|
|
|
up_threshold: defines what the average CPU usaged between the samplings
|
|
of 'sampling_rate' needs to be for the kernel to make a decision on
|
|
whether it should increase the frequency. For example when it is set
|
|
to its default value of '80' it means that between the checking
|
|
intervals the CPU needs to be on average more than 80% in use to then
|
|
decide that the CPU frequency needs to be increased.
|
|
|
|
sampling_down_factor: this parameter controls the rate that the CPU
|
|
makes a decision on when to decrease the frequency. When set to its
|
|
default value of '5' it means that at 1/5 the sampling_rate the kernel
|
|
makes a decision to lower the frequency. Five "lower rate" decisions
|
|
have to be made in a row before the CPU frequency is actually lower.
|
|
If set to '1' then the frequency decreases as quickly as it increases,
|
|
if set to '2' it decreases at half the rate of the increase.
|
|
|
|
ignore_nice_load: this parameter takes a value of '0' or '1'. When
|
|
set to '0' (its default), all processes are counted towards the
|
|
'cpu utilisation' value. When set to '1', the processes that are
|
|
run with a 'nice' value will not count (and thus be ignored) in the
|
|
overall usage calculation. This is useful if you are running a CPU
|
|
intensive calculation on your laptop that you do not care how long it
|
|
takes to complete as you can 'nice' it and prevent it from taking part
|
|
in the deciding process of whether to increase your CPU frequency.
|
|
|
|
|
|
2.5 Conservative
|
|
----------------
|
|
|
|
The CPUfreq governor "conservative", much like the "ondemand"
|
|
governor, sets the CPU depending on the current usage. It differs in
|
|
behaviour in that it gracefully increases and decreases the CPU speed
|
|
rather than jumping to max speed the moment there is any load on the
|
|
CPU. This behaviour more suitable in a battery powered environment.
|
|
The governor is tweaked in the same manner as the "ondemand" governor
|
|
through sysfs with the addition of:
|
|
|
|
freq_step: this describes what percentage steps the cpu freq should be
|
|
increased and decreased smoothly by. By default the cpu frequency will
|
|
increase in 5% chunks of your maximum cpu frequency. You can change this
|
|
value to anywhere between 0 and 100 where '0' will effectively lock your
|
|
CPU at a speed regardless of its load whilst '100' will, in theory, make
|
|
it behave identically to the "ondemand" governor.
|
|
|
|
down_threshold: same as the 'up_threshold' found for the "ondemand"
|
|
governor but for the opposite direction. For example when set to its
|
|
default value of '20' it means that if the CPU usage needs to be below
|
|
20% between samples to have the frequency decreased.
|
|
|
|
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:
|
|
|
|
int cpufreq_driver_target(struct cpufreq_policy *policy,
|
|
unsigned int target_freq,
|
|
unsigned int relation);
|
|
|
|
int __cpufreq_driver_target(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
|
|
per-CPU cpufreq 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 to lock the cpufreq per-CPU
|
|
lock before the command is passed to the cpufreq processor driver.
|
|
|