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.