1.. SPDX-License-Identifier: GPL-2.0
2
3==========================================
4Dynamic Thermal Power Management framework
5==========================================
6
7On the embedded world, the complexity of the SoC leads to an
8increasing number of hotspots which need to be monitored and mitigated
9as a whole in order to prevent the temperature to go above the
10normative and legally stated 'skin temperature'.
11
12Another aspect is to sustain the performance for a given power budget,
13for example virtual reality where the user can feel dizziness if the
14performance is capped while a big CPU is processing something else. Or
15reduce the battery charging because the dissipated power is too high
16compared with the power consumed by other devices.
17
18The user space is the most adequate place to dynamically act on the
19different devices by limiting their power given an application
20profile: it has the knowledge of the platform.
21
22The Dynamic Thermal Power Management (DTPM) is a technique acting on
23the device power by limiting and/or balancing a power budget among
24different devices.
25
26The DTPM framework provides an unified interface to act on the
27device power.
28
29Overview
30========
31
32The DTPM framework relies on the powercap framework to create the
33powercap entries in the sysfs directory and implement the backend
34driver to do the connection with the power manageable device.
35
36The DTPM is a tree representation describing the power constraints
37shared between devices, not their physical positions.
38
39The nodes of the tree are a virtual description aggregating the power
40characteristics of the children nodes and their power limitations.
41
42The leaves of the tree are the real power manageable devices.
43
44For instance::
45
46  SoC
47   |
48   `-- pkg
49	|
50	|-- pd0 (cpu0-3)
51	|
52	`-- pd1 (cpu4-5)
53
54The 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
64When 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
76Each 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
90When 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
103Flat description
104----------------
105
106A root node is created and it is the parent of all the nodes. This
107description is the simplest one and it is supposed to give to user
108space a flat representation of all the devices supporting the power
109limitation without any power limitation distribution.
110
111Hierarchical description
112------------------------
113
114The different devices supporting the power limitation are represented
115hierarchically. There is one root node, all intermediate nodes are
116grouping the child nodes which can be intermediate nodes also or real
117devices.
118
119The intermediate nodes aggregate the power information and allows to
120set the power limit given the weight of the nodes.
121
122User space API
123==============
124
125As stated in the overview, the DTPM framework is built on top of the
126powercap framework. Thus the sysfs interface is the same, please refer
127to 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
152Constraints
153-----------
154
155 * Constraint 0: The power limitation is immediately applied, without
156   limitation in time.
157
158Kernel API
159==========
160
161Overview
162--------
163
164The DTPM framework has no power limiting backend support. It is
165generic and provides a set of API to let the different drivers to
166implement the backend part for the power limitation and create the
167power constraints tree.
168
169It is up to the platform to provide the initialization function to
170allocate and link the different nodes of the tree.
171
172A special macro has the role of declaring a node and the corresponding
173initialization function via a description structure. This one contains
174an optional parent field allowing to hook different devices to an
175already existing tree at boot time.
176
177For 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
186The nodes of the DTPM tree are described with dtpm structure. The
187steps 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
193The registration of the dtpm node is done with the powercap
194ops. Basically, it must implements the callbacks to get and set the
195power and the limit.
196
197Alternatively, if the node to be inserted is an intermediate one, then
198a simple function to insert it as a future parent is available.
199
200If a device has its power characteristics changing, then the tree must
201be updated with the new power numbers and weights.
202
203Nomenclature
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
213