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diff --git a/Documentation/cpu-freq/governors.txt b/Documentation/cpu-freq/governors.txt new file mode 100644 index 00000000000..b85481acd0c --- /dev/null +++ b/Documentation/cpu-freq/governors.txt @@ -0,0 +1,155 @@ + 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 specific 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: + +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. + |