1======================= 2CPU cooling APIs How To 3======================= 4 5Written by Amit Daniel Kachhap <amit.kachhap@linaro.org> 6 7Updated: 6 Jan 2015 8 9Copyright (c) 2012 Samsung Electronics Co., Ltd(http://www.samsung.com) 10 110. Introduction 12=============== 13 14The generic cpu cooling(freq clipping) provides registration/unregistration APIs 15to the caller. The binding of the cooling devices to the trip point is left for 16the user. The registration APIs returns the cooling device pointer. 17 181. cpu cooling APIs 19=================== 20 211.1 cpufreq registration/unregistration APIs 22-------------------------------------------- 23 24 :: 25 26 struct thermal_cooling_device 27 *cpufreq_cooling_register(struct cpumask *clip_cpus) 28 29 This interface function registers the cpufreq cooling device with the name 30 "thermal-cpufreq-%x". This api can support multiple instances of cpufreq 31 cooling devices. 32 33 clip_cpus: 34 cpumask of cpus where the frequency constraints will happen. 35 36 :: 37 38 struct thermal_cooling_device 39 *of_cpufreq_cooling_register(struct cpufreq_policy *policy) 40 41 This interface function registers the cpufreq cooling device with 42 the name "thermal-cpufreq-%x" linking it with a device tree node, in 43 order to bind it via the thermal DT code. This api can support multiple 44 instances of cpufreq cooling devices. 45 46 policy: 47 CPUFreq policy. 48 49 50 :: 51 52 void cpufreq_cooling_unregister(struct thermal_cooling_device *cdev) 53 54 This interface function unregisters the "thermal-cpufreq-%x" cooling device. 55 56 cdev: Cooling device pointer which has to be unregistered. 57 582. Power models 59=============== 60 61The power API registration functions provide a simple power model for 62CPUs. The current power is calculated as dynamic power (static power isn't 63supported currently). This power model requires that the operating-points of 64the CPUs are registered using the kernel's opp library and the 65`cpufreq_frequency_table` is assigned to the `struct device` of the 66cpu. If you are using CONFIG_CPUFREQ_DT then the 67`cpufreq_frequency_table` should already be assigned to the cpu 68device. 69 70The dynamic power consumption of a processor depends on many factors. 71For a given processor implementation the primary factors are: 72 73- The time the processor spends running, consuming dynamic power, as 74 compared to the time in idle states where dynamic consumption is 75 negligible. Herein we refer to this as 'utilisation'. 76- The voltage and frequency levels as a result of DVFS. The DVFS 77 level is a dominant factor governing power consumption. 78- In running time the 'execution' behaviour (instruction types, memory 79 access patterns and so forth) causes, in most cases, a second order 80 variation. In pathological cases this variation can be significant, 81 but typically it is of a much lesser impact than the factors above. 82 83A high level dynamic power consumption model may then be represented as:: 84 85 Pdyn = f(run) * Voltage^2 * Frequency * Utilisation 86 87f(run) here represents the described execution behaviour and its 88result has a units of Watts/Hz/Volt^2 (this often expressed in 89mW/MHz/uVolt^2) 90 91The detailed behaviour for f(run) could be modelled on-line. However, 92in practice, such an on-line model has dependencies on a number of 93implementation specific processor support and characterisation 94factors. Therefore, in initial implementation that contribution is 95represented as a constant coefficient. This is a simplification 96consistent with the relative contribution to overall power variation. 97 98In this simplified representation our model becomes:: 99 100 Pdyn = Capacitance * Voltage^2 * Frequency * Utilisation 101 102Where `capacitance` is a constant that represents an indicative 103running time dynamic power coefficient in fundamental units of 104mW/MHz/uVolt^2. Typical values for mobile CPUs might lie in range 105from 100 to 500. For reference, the approximate values for the SoC in 106ARM's Juno Development Platform are 530 for the Cortex-A57 cluster and 107140 for the Cortex-A53 cluster. 108