1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==========================================
4 Dynamic Thermal Power Management framework
5 ==========================================
6
7 On the embedded world, the complexity of the SoC leads to an
8 increasing number of hotspots which need to be monitored and mitigated
9 as a whole in order to prevent the temperature to go above the
10 normative and legally stated 'skin temperature'.
11
12 Another aspect is to sustain the performance for a given power budget,
13 for example virtual reality where the user can feel dizziness if the
14 performance is capped while a big CPU is processing something else. Or
15 reduce the battery charging because the dissipated power is too high
16 compared with the power consumed by other devices.
17
18 The user space is the most adequate place to dynamically act on the
19 different devices by limiting their power given an application
20 profile: it has the knowledge of the platform.
21
22 The Dynamic Thermal Power Management (DTPM) is a technique acting on
23 the device power by limiting and/or balancing a power budget among
24 different devices.
25
26 The DTPM framework provides an unified interface to act on the
27 device power.
28
29 Overview
30 ========
31
32 The DTPM framework relies on the powercap framework to create the
33 powercap entries in the sysfs directory and implement the backend
34 driver to do the connection with the power manageable device.
35
36 The DTPM is a tree representation describing the power constraints
37 shared between devices, not their physical positions.
38
39 The nodes of the tree are a virtual description aggregating the power
40 characteristics of the children nodes and their power limitations.
41
42 The leaves of the tree are the real power manageable devices.
43
44 For instance::
45
46 SoC
47 |
48 `-- pkg
49 |
50 |-- pd0 (cpu0-3)
51 |
52 `-- pd1 (cpu4-5)
53
54 The pkg power will be the sum of pd0 and pd1 power numbers::
55
56 SoC (400mW - 3100mW)
57 |
58 `-- pkg (400mW - 3100mW)
59 |
60 |-- pd0 (100mW - 700mW)
61 |
62 `-- pd1 (300mW - 2400mW)
63
64 When the nodes are inserted in the tree, their power characteristics are propagated to the parents::
65
66 SoC (600mW - 5900mW)
67 |
68 |-- pkg (400mW - 3100mW)
69 | |
70 | |-- pd0 (100mW - 700mW)
71 | |
72 | `-- pd1 (300mW - 2400mW)
73 |
74 `-- pd2 (200mW - 2800mW)
75
76 Each node have a weight on a 2^10 basis reflecting the percentage of power consumption along the siblings::
77
78 SoC (w=1024)
79 |
80 |-- pkg (w=538)
81 | |
82 | |-- pd0 (w=231)
83 | |
84 | `-- pd1 (w=794)
85 |
86 `-- pd2 (w=486)
87
88 Note the sum of weights at the same level are equal to 1024.
89
90 When a power limitation is applied to a node, then it is distributed along the children given their weights. For example, if we set a power limitation of 3200mW at the 'SoC' root node, the resulting tree will be::
91
92 SoC (w=1024) <--- power_limit = 3200mW
93 |
94 |-- pkg (w=538) --> power_limit = 1681mW
95 | |
96 | |-- pd0 (w=231) --> power_limit = 378mW
97 | |
98 | `-- pd1 (w=794) --> power_limit = 1303mW
99 |
100 `-- pd2 (w=486) --> power_limit = 1519mW
101
102
103 Flat description
104 ----------------
105
106 A root node is created and it is the parent of all the nodes. This
107 description is the simplest one and it is supposed to give to user
108 space a flat representation of all the devices supporting the power
109 limitation without any power limitation distribution.
110
111 Hierarchical description
112 ------------------------
113
114 The different devices supporting the power limitation are represented
115 hierarchically. There is one root node, all intermediate nodes are
116 grouping the child nodes which can be intermediate nodes also or real
117 devices.
118
119 The intermediate nodes aggregate the power information and allows to
120 set the power limit given the weight of the nodes.
121
122 User space API
123 ==============
124
125 As stated in the overview, the DTPM framework is built on top of the
126 powercap framework. Thus the sysfs interface is the same, please refer
127 to the powercap documentation for further details.
128
129 * power_uw: Instantaneous power consumption. If the node is an
130 intermediate node, then the power consumption will be the sum of all
131 children power consumption.
132
133 * max_power_range_uw: The power range resulting of the maximum power
134 minus the minimum power.
135
136 * name: The name of the node. This is implementation dependent. Even
137 if it is not recommended for the user space, several nodes can have
138 the same name.
139
140 * constraint_X_name: The name of the constraint.
141
142 * constraint_X_max_power_uw: The maximum power limit to be applicable
143 to the node.
144
145 * constraint_X_power_limit_uw: The power limit to be applied to the
146 node. If the value contained in constraint_X_max_power_uw is set,
147 the constraint will be removed.
148
149 * constraint_X_time_window_us: The meaning of this file will depend
150 on the constraint number.
151
152 Constraints
153 -----------
154
155 * Constraint 0: The power limitation is immediately applied, without
156 limitation in time.
157
158 Kernel API
159 ==========
160
161 Overview
162 --------
163
164 The DTPM framework has no power limiting backend support. It is
165 generic and provides a set of API to let the different drivers to
166 implement the backend part for the power limitation and create the
167 power constraints tree.
168
169 It is up to the platform to provide the initialization function to
170 allocate and link the different nodes of the tree.
171
172 A special macro has the role of declaring a node and the corresponding
173 initialization function via a description structure. This one contains
174 an optional parent field allowing to hook different devices to an
175 already existing tree at boot time.
176
177 For instance::
178
179 struct dtpm_descr my_descr = {
180 .name = "my_name",
181 .init = my_init_func,
182 };
183
184 DTPM_DECLARE(my_descr);
185
186 The nodes of the DTPM tree are described with dtpm structure. The
187 steps to add a new power limitable device is done in three steps:
188
189 * Allocate the dtpm node
190 * Set the power number of the dtpm node
191 * Register the dtpm node
192
193 The registration of the dtpm node is done with the powercap
194 ops. Basically, it must implements the callbacks to get and set the
195 power and the limit.
196
197 Alternatively, if the node to be inserted is an intermediate one, then
198 a simple function to insert it as a future parent is available.
199
200 If a device has its power characteristics changing, then the tree must
201 be updated with the new power numbers and weights.
202
203 Nomenclature
204 ------------
205
206 * dtpm_alloc() : Allocate and initialize a dtpm structure
207
208 * dtpm_register() : Add the dtpm node to the tree
209
210 * dtpm_unregister() : Remove the dtpm node from the tree
211
212 * dtpm_update_power() : Update the power characteristics of the dtpm node
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