156 lines
5.3 KiB
Plaintext
156 lines
5.3 KiB
Plaintext
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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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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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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 independetly | 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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2.1 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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2.2 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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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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