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Documentation / thermal / cpu-cooling-api.txt


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	CPU cooling APIs How To
2	===================================
3	
4	Written by Amit Daniel Kachhap <amit.kachhap@linaro.org>
5	
6	Updated: 6 Jan 2015
7	
8	Copyright (c)  2012 Samsung Electronics Co., Ltd(http://www.samsung.com)
9	
10	0. Introduction
11	
12	The generic cpu cooling(freq clipping) provides registration/unregistration APIs
13	to the caller. The binding of the cooling devices to the trip point is left for
14	the user. The registration APIs returns the cooling device pointer.
15	
16	1. cpu cooling APIs
17	
18	1.1 cpufreq registration/unregistration APIs
19	1.1.1 struct thermal_cooling_device *cpufreq_cooling_register(
20		struct cpumask *clip_cpus)
21	
22	    This interface function registers the cpufreq cooling device with the name
23	    "thermal-cpufreq-%x". This api can support multiple instances of cpufreq
24	    cooling devices.
25	
26	   clip_cpus: cpumask of cpus where the frequency constraints will happen.
27	
28	1.1.2 struct thermal_cooling_device *of_cpufreq_cooling_register(
29						struct cpufreq_policy *policy)
30	
31	    This interface function registers the cpufreq cooling device with
32	    the name "thermal-cpufreq-%x" linking it with a device tree node, in
33	    order to bind it via the thermal DT code. This api can support multiple
34	    instances of cpufreq cooling devices.
35	
36	    policy: CPUFreq policy.
37	
38	1.1.3 void cpufreq_cooling_unregister(struct thermal_cooling_device *cdev)
39	
40	    This interface function unregisters the "thermal-cpufreq-%x" cooling device.
41	
42	    cdev: Cooling device pointer which has to be unregistered.
43	
44	2. Power models
45	
46	The power API registration functions provide a simple power model for
47	CPUs.  The current power is calculated as dynamic power (static power isn't
48	supported currently).  This power model requires that the operating-points of
49	the CPUs are registered using the kernel's opp library and the
50	`cpufreq_frequency_table` is assigned to the `struct device` of the
51	cpu.  If you are using CONFIG_CPUFREQ_DT then the
52	`cpufreq_frequency_table` should already be assigned to the cpu
53	device.
54	
55	The dynamic power consumption of a processor depends on many factors.
56	For a given processor implementation the primary factors are:
57	
58	- The time the processor spends running, consuming dynamic power, as
59	  compared to the time in idle states where dynamic consumption is
60	  negligible.  Herein we refer to this as 'utilisation'.
61	- The voltage and frequency levels as a result of DVFS.  The DVFS
62	  level is a dominant factor governing power consumption.
63	- In running time the 'execution' behaviour (instruction types, memory
64	  access patterns and so forth) causes, in most cases, a second order
65	  variation.  In pathological cases this variation can be significant,
66	  but typically it is of a much lesser impact than the factors above.
67	
68	A high level dynamic power consumption model may then be represented as:
69	
70	Pdyn = f(run) * Voltage^2 * Frequency * Utilisation
71	
72	f(run) here represents the described execution behaviour and its
73	result has a units of Watts/Hz/Volt^2 (this often expressed in
74	mW/MHz/uVolt^2)
75	
76	The detailed behaviour for f(run) could be modelled on-line.  However,
77	in practice, such an on-line model has dependencies on a number of
78	implementation specific processor support and characterisation
79	factors.  Therefore, in initial implementation that contribution is
80	represented as a constant coefficient.  This is a simplification
81	consistent with the relative contribution to overall power variation.
82	
83	In this simplified representation our model becomes:
84	
85	Pdyn = Capacitance * Voltage^2 * Frequency * Utilisation
86	
87	Where `capacitance` is a constant that represents an indicative
88	running time dynamic power coefficient in fundamental units of
89	mW/MHz/uVolt^2.  Typical values for mobile CPUs might lie in range
90	from 100 to 500.  For reference, the approximate values for the SoC in
91	ARM's Juno Development Platform are 530 for the Cortex-A57 cluster and
92	140 for the Cortex-A53 cluster.
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