1.. SPDX-License-Identifier: GPL-2.0
2
3=======================
4Energy Model of devices
5=======================
6
71. Overview
8-----------
9
10The Energy Model (EM) framework serves as an interface between drivers knowing
11the power consumed by devices at various performance levels, and the kernel
12subsystems willing to use that information to make energy-aware decisions.
13
14The source of the information about the power consumed by devices can vary greatly
15from one platform to another. These power costs can be estimated using
16devicetree data in some cases. In others, the firmware will know better.
17Alternatively, userspace might be best positioned. And so on. In order to avoid
18each and every client subsystem to re-implement support for each and every
19possible source of information on its own, the EM framework intervenes as an
20abstraction layer which standardizes the format of power cost tables in the
21kernel, hence enabling to avoid redundant work.
22
23The power values might be expressed in micro-Watts or in an 'abstract scale'.
24Multiple subsystems might use the EM and it is up to the system integrator to
25check that the requirements for the power value scale types are met. An example
26can be found in the Energy-Aware Scheduler documentation
27Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or
28powercap power values expressed in an 'abstract scale' might cause issues.
29These subsystems are more interested in estimation of power used in the past,
30thus the real micro-Watts might be needed. An example of these requirements can
31be found in the Intelligent Power Allocation in
32Documentation/driver-api/thermal/power_allocator.rst.
33Kernel subsystems might implement automatic detection to check whether EM
34registered devices have inconsistent scale (based on EM internal flag).
35Important thing to keep in mind is that when the power values are expressed in
36an 'abstract scale' deriving real energy in micro-Joules would not be possible.
37
38The figure below depicts an example of drivers (Arm-specific here, but the
39approach is applicable to any architecture) providing power costs to the EM
40framework, and interested clients reading the data from it::
41
42       +---------------+  +-----------------+  +---------------+
43       | Thermal (IPA) |  | Scheduler (EAS) |  |     Other     |
44       +---------------+  +-----------------+  +---------------+
45               |                   | em_cpu_energy()   |
46               |                   | em_cpu_get()      |
47               +---------+         |         +---------+
48                         |         |         |
49                         v         v         v
50                        +---------------------+
51                        |    Energy Model     |
52                        |     Framework       |
53                        +---------------------+
54                           ^       ^       ^
55                           |       |       | em_dev_register_perf_domain()
56                +----------+       |       +---------+
57                |                  |                 |
58        +---------------+  +---------------+  +--------------+
59        |  cpufreq-dt   |  |   arm_scmi    |  |    Other     |
60        +---------------+  +---------------+  +--------------+
61                ^                  ^                 ^
62                |                  |                 |
63        +--------------+   +---------------+  +--------------+
64        | Device Tree  |   |   Firmware    |  |      ?       |
65        +--------------+   +---------------+  +--------------+
66
67In case of CPU devices the EM framework manages power cost tables per
68'performance domain' in the system. A performance domain is a group of CPUs
69whose performance is scaled together. Performance domains generally have a
701-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are
71required to have the same micro-architecture. CPUs in different performance
72domains can have different micro-architectures.
73
74
752. Core APIs
76------------
77
782.1 Config options
79^^^^^^^^^^^^^^^^^^
80
81CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
82
83
842.2 Registration of performance domains
85^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
86
87Registration of 'advanced' EM
88~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
89
90The 'advanced' EM gets it's name due to the fact that the driver is allowed
91to provide more precised power model. It's not limited to some implemented math
92formula in the framework (like it's in 'simple' EM case). It can better reflect
93the real power measurements performed for each performance state. Thus, this
94registration method should be preferred in case considering EM static power
95(leakage) is important.
96
97Drivers are expected to register performance domains into the EM framework by
98calling the following API::
99
100  int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
101		struct em_data_callback *cb, cpumask_t *cpus, bool microwatts);
102
103Drivers must provide a callback function returning <frequency, power> tuples
104for each performance state. The callback function provided by the driver is free
105to fetch data from any relevant location (DT, firmware, ...), and by any mean
106deemed necessary. Only for CPU devices, drivers must specify the CPUs of the
107performance domains using cpumask. For other devices than CPUs the last
108argument must be set to NULL.
109The last argument 'microwatts' is important to set with correct value. Kernel
110subsystems which use EM might rely on this flag to check if all EM devices use
111the same scale. If there are different scales, these subsystems might decide
112to return warning/error, stop working or panic.
113See Section 3. for an example of driver implementing this
114callback, or Section 2.4 for further documentation on this API
115
116Registration of EM using DT
117~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
118
119The  EM can also be registered using OPP framework and information in DT
120"operating-points-v2". Each OPP entry in DT can be extended with a property
121"opp-microwatt" containing micro-Watts power value. This OPP DT property
122allows a platform to register EM power values which are reflecting total power
123(static + dynamic). These power values might be coming directly from
124experiments and measurements.
125
126Registration of 'artificial' EM
127~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
128
129There is an option to provide a custom callback for drivers missing detailed
130knowledge about power value for each performance state. The callback
131.get_cost() is optional and provides the 'cost' values used by the EAS.
132This is useful for platforms that only provide information on relative
133efficiency between CPU types, where one could use the information to
134create an abstract power model. But even an abstract power model can
135sometimes be hard to fit in, given the input power value size restrictions.
136The .get_cost() allows to provide the 'cost' values which reflect the
137efficiency of the CPUs. This would allow to provide EAS information which
138has different relation than what would be forced by the EM internal
139formulas calculating 'cost' values. To register an EM for such platform, the
140driver must set the flag 'microwatts' to 0, provide .get_power() callback
141and provide .get_cost() callback. The EM framework would handle such platform
142properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such
143platform. Special care should be taken by other frameworks which are using EM
144to test and treat this flag properly.
145
146Registration of 'simple' EM
147~~~~~~~~~~~~~~~~~~~~~~~~~~~
148
149The 'simple' EM is registered using the framework helper function
150cpufreq_register_em_with_opp(). It implements a power model which is tight to
151math formula::
152
153	Power = C * V^2 * f
154
155The EM which is registered using this method might not reflect correctly the
156physics of a real device, e.g. when static power (leakage) is important.
157
158
1592.3 Accessing performance domains
160^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
161
162There are two API functions which provide the access to the energy model:
163em_cpu_get() which takes CPU id as an argument and em_pd_get() with device
164pointer as an argument. It depends on the subsystem which interface it is
165going to use, but in case of CPU devices both functions return the same
166performance domain.
167
168Subsystems interested in the energy model of a CPU can retrieve it using the
169em_cpu_get() API. The energy model tables are allocated once upon creation of
170the performance domains, and kept in memory untouched.
171
172The energy consumed by a performance domain can be estimated using the
173em_cpu_energy() API. The estimation is performed assuming that the schedutil
174CPUfreq governor is in use in case of CPU device. Currently this calculation is
175not provided for other type of devices.
176
177More details about the above APIs can be found in ``<linux/energy_model.h>``
178or in Section 2.4
179
180
1812.4 Description details of this API
182^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
183.. kernel-doc:: include/linux/energy_model.h
184   :internal:
185
186.. kernel-doc:: kernel/power/energy_model.c
187   :export:
188
189
1903. Example driver
191-----------------
192
193The CPUFreq framework supports dedicated callback for registering
194the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em().
195That callback has to be implemented properly for a given driver,
196because the framework would call it at the right time during setup.
197This section provides a simple example of a CPUFreq driver registering a
198performance domain in the Energy Model framework using the (fake) 'foo'
199protocol. The driver implements an est_power() function to be provided to the
200EM framework::
201
202  -> drivers/cpufreq/foo_cpufreq.c
203
204  01	static int est_power(struct device *dev, unsigned long *mW,
205  02			unsigned long *KHz)
206  03	{
207  04		long freq, power;
208  05
209  06		/* Use the 'foo' protocol to ceil the frequency */
210  07		freq = foo_get_freq_ceil(dev, *KHz);
211  08		if (freq < 0);
212  09			return freq;
213  10
214  11		/* Estimate the power cost for the dev at the relevant freq. */
215  12		power = foo_estimate_power(dev, freq);
216  13		if (power < 0);
217  14			return power;
218  15
219  16		/* Return the values to the EM framework */
220  17		*mW = power;
221  18		*KHz = freq;
222  19
223  20		return 0;
224  21	}
225  22
226  23	static void foo_cpufreq_register_em(struct cpufreq_policy *policy)
227  24	{
228  25		struct em_data_callback em_cb = EM_DATA_CB(est_power);
229  26		struct device *cpu_dev;
230  27		int nr_opp;
231  28
232  29		cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
233  30
234  31     	/* Find the number of OPPs for this policy */
235  32     	nr_opp = foo_get_nr_opp(policy);
236  33
237  34     	/* And register the new performance domain */
238  35     	em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus,
239  36					    true);
240  37	}
241  38
242  39	static struct cpufreq_driver foo_cpufreq_driver = {
243  40		.register_em = foo_cpufreq_register_em,
244  41	};
245