245 lines
9.6 KiB
Plaintext
245 lines
9.6 KiB
Plaintext
CPU frequency and voltage scaling code in the Linux(TM) kernel
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L i n u x C P U F r e q
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C P U F r e q G o v e r n o r s
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- information for users and developers -
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Dominik Brodowski <linux@brodo.de>
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some additions and corrections by Nico Golde <nico@ngolde.de>
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Clock scaling allows you to change the clock speed of the CPUs on the
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fly. This is a nice method to save battery power, because the lower
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the clock speed, the less power the CPU consumes.
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Contents:
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---------
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1. What is a CPUFreq Governor?
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2. Governors In the Linux Kernel
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2.1 Performance
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2.2 Powersave
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2.3 Userspace
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2.4 Ondemand
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2.5 Conservative
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3. The Governor Interface in the CPUfreq Core
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1. What Is A CPUFreq Governor?
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==============================
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Most cpufreq drivers (in fact, all except one, longrun) or even most
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cpu frequency scaling algorithms only offer the CPU to be set to one
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frequency. In order to offer dynamic frequency scaling, the cpufreq
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core must be able to tell these drivers of a "target frequency". So
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these specific drivers will be transformed to offer a "->target"
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call instead of the existing "->setpolicy" call. For "longrun", all
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stays the same, though.
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How to decide what frequency within the CPUfreq policy should be used?
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That's done using "cpufreq governors". Two are already in this patch
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-- they're the already existing "powersave" and "performance" which
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set the frequency statically to the lowest or highest frequency,
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respectively. At least two more such governors will be ready for
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addition in the near future, but likely many more as there are various
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different theories and models about dynamic frequency scaling
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around. Using such a generic interface as cpufreq offers to scaling
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governors, these can be tested extensively, and the best one can be
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selected for each specific use.
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Basically, it's the following flow graph:
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CPU can be set to switch independently | CPU can only be set
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within specific "limits" | to specific frequencies
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"CPUfreq policy"
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consists of frequency limits (policy->{min,max})
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and CPUfreq governor to be used
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/ \
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/ \
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/ the cpufreq governor decides
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/ (dynamically or statically)
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/ what target_freq to set within
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/ the limits of policy->{min,max}
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/ \
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/ \
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Using the ->setpolicy call, Using the ->target call,
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the limits and the the frequency closest
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"policy" is set. to target_freq is set.
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It is assured that it
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is within policy->{min,max}
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2. Governors In the Linux Kernel
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================================
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2.1 Performance
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---------------
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The CPUfreq governor "performance" sets the CPU statically to the
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highest frequency within the borders of scaling_min_freq and
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scaling_max_freq.
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2.2 Powersave
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-------------
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The CPUfreq governor "powersave" sets the CPU statically to the
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lowest frequency within the borders of scaling_min_freq and
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scaling_max_freq.
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2.3 Userspace
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-------------
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The CPUfreq governor "userspace" allows the user, or any userspace
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program running with UID "root", to set the CPU to a specific frequency
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by making a sysfs file "scaling_setspeed" available in the CPU-device
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directory.
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2.4 Ondemand
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------------
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The CPUfreq governor "ondemand" sets the CPU depending on the
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current usage. To do this the CPU must have the capability to
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switch the frequency very quickly. There are a number of sysfs file
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accessible parameters:
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sampling_rate: measured in uS (10^-6 seconds), this is how often you
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want the kernel to look at the CPU usage and to make decisions on
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what to do about the frequency. Typically this is set to values of
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around '10000' or more. It's default value is (cmp. with users-guide.txt):
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transition_latency * 1000
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Be aware that transition latency is in ns and sampling_rate is in us, so you
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get the same sysfs value by default.
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Sampling rate should always get adjusted considering the transition latency
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To set the sampling rate 750 times as high as the transition latency
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in the bash (as said, 1000 is default), do:
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echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
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>ondemand/sampling_rate
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show_sampling_rate_min:
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The sampling rate is limited by the HW transition latency:
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transition_latency * 100
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Or by kernel restrictions:
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If CONFIG_NO_HZ is set, the limit is 10ms fixed.
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If CONFIG_NO_HZ is not set or nohz=off boot parameter is used, the
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limits depend on the CONFIG_HZ option:
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HZ=1000: min=20000us (20ms)
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HZ=250: min=80000us (80ms)
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HZ=100: min=200000us (200ms)
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The highest value of kernel and HW latency restrictions is shown and
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used as the minimum sampling rate.
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show_sampling_rate_max: THIS INTERFACE IS DEPRECATED, DON'T USE IT.
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up_threshold: defines what the average CPU usage between the samplings
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of 'sampling_rate' needs to be for the kernel to make a decision on
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whether it should increase the frequency. For example when it is set
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to its default value of '95' it means that between the checking
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intervals the CPU needs to be on average more than 95% in use to then
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decide that the CPU frequency needs to be increased.
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ignore_nice_load: this parameter takes a value of '0' or '1'. When
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set to '0' (its default), all processes are counted towards the
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'cpu utilisation' value. When set to '1', the processes that are
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run with a 'nice' value will not count (and thus be ignored) in the
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overall usage calculation. This is useful if you are running a CPU
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intensive calculation on your laptop that you do not care how long it
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takes to complete as you can 'nice' it and prevent it from taking part
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in the deciding process of whether to increase your CPU frequency.
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sampling_down_factor: this parameter controls the rate at which the
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kernel makes a decision on when to decrease the frequency while running
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at top speed. When set to 1 (the default) decisions to reevaluate load
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are made at the same interval regardless of current clock speed. But
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when set to greater than 1 (e.g. 100) it acts as a multiplier for the
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scheduling interval for reevaluating load when the CPU is at its top
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speed due to high load. This improves performance by reducing the overhead
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of load evaluation and helping the CPU stay at its top speed when truly
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busy, rather than shifting back and forth in speed. This tunable has no
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effect on behavior at lower speeds/lower CPU loads.
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2.5 Conservative
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----------------
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The CPUfreq governor "conservative", much like the "ondemand"
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governor, sets the CPU depending on the current usage. It differs in
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behaviour in that it gracefully increases and decreases the CPU speed
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rather than jumping to max speed the moment there is any load on the
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CPU. This behaviour more suitable in a battery powered environment.
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The governor is tweaked in the same manner as the "ondemand" governor
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through sysfs with the addition of:
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freq_step: this describes what percentage steps the cpu freq should be
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increased and decreased smoothly by. By default the cpu frequency will
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increase in 5% chunks of your maximum cpu frequency. You can change this
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value to anywhere between 0 and 100 where '0' will effectively lock your
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CPU at a speed regardless of its load whilst '100' will, in theory, make
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it behave identically to the "ondemand" governor.
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down_threshold: same as the 'up_threshold' found for the "ondemand"
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governor but for the opposite direction. For example when set to its
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default value of '20' it means that if the CPU usage needs to be below
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20% between samples to have the frequency decreased.
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3. The Governor Interface in the CPUfreq Core
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=============================================
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A new governor must register itself with the CPUfreq core using
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"cpufreq_register_governor". The struct cpufreq_governor, which has to
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be passed to that function, must contain the following values:
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governor->name - A unique name for this governor
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governor->governor - The governor callback function
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governor->owner - .THIS_MODULE for the governor module (if
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appropriate)
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The governor->governor callback is called with the current (or to-be-set)
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cpufreq_policy struct for that CPU, and an unsigned int event. The
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following events are currently defined:
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CPUFREQ_GOV_START: This governor shall start its duty for the CPU
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policy->cpu
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CPUFREQ_GOV_STOP: This governor shall end its duty for the CPU
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policy->cpu
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CPUFREQ_GOV_LIMITS: The limits for CPU policy->cpu have changed to
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policy->min and policy->max.
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If you need other "events" externally of your driver, _only_ use the
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cpufreq_governor_l(unsigned int cpu, unsigned int event) call to the
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CPUfreq core to ensure proper locking.
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The CPUfreq governor may call the CPU processor driver using one of
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these two functions:
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int cpufreq_driver_target(struct cpufreq_policy *policy,
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unsigned int target_freq,
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unsigned int relation);
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int __cpufreq_driver_target(struct cpufreq_policy *policy,
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unsigned int target_freq,
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unsigned int relation);
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target_freq must be within policy->min and policy->max, of course.
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What's the difference between these two functions? When your governor
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still is in a direct code path of a call to governor->governor, the
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per-CPU cpufreq lock is still held in the cpufreq core, and there's
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no need to lock it again (in fact, this would cause a deadlock). So
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use __cpufreq_driver_target only in these cases. In all other cases
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(for example, when there's a "daemonized" function that wakes up
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every second), use cpufreq_driver_target to lock the cpufreq per-CPU
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lock before the command is passed to the cpufreq processor driver.
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