xref: /openbmc/linux/Documentation/power/energy-model.rst (revision d704aa0d44ade12660d4d7220b2a8d785b7b4247)
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 milli-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 milli-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 milli-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
87Drivers are expected to register performance domains into the EM framework by
88calling the following API::
89
90  int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
91		struct em_data_callback *cb, cpumask_t *cpus, bool milliwatts);
92
93Drivers must provide a callback function returning <frequency, power> tuples
94for each performance state. The callback function provided by the driver is free
95to fetch data from any relevant location (DT, firmware, ...), and by any mean
96deemed necessary. Only for CPU devices, drivers must specify the CPUs of the
97performance domains using cpumask. For other devices than CPUs the last
98argument must be set to NULL.
99The last argument 'milliwatts' is important to set with correct value. Kernel
100subsystems which use EM might rely on this flag to check if all EM devices use
101the same scale. If there are different scales, these subsystems might decide
102to: return warning/error, stop working or panic.
103See Section 3. for an example of driver implementing this
104callback, or Section 2.4 for further documentation on this API
105
106
1072.3 Accessing performance domains
108^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
109
110There are two API functions which provide the access to the energy model:
111em_cpu_get() which takes CPU id as an argument and em_pd_get() with device
112pointer as an argument. It depends on the subsystem which interface it is
113going to use, but in case of CPU devices both functions return the same
114performance domain.
115
116Subsystems interested in the energy model of a CPU can retrieve it using the
117em_cpu_get() API. The energy model tables are allocated once upon creation of
118the performance domains, and kept in memory untouched.
119
120The energy consumed by a performance domain can be estimated using the
121em_cpu_energy() API. The estimation is performed assuming that the schedutil
122CPUfreq governor is in use in case of CPU device. Currently this calculation is
123not provided for other type of devices.
124
125More details about the above APIs can be found in ``<linux/energy_model.h>``
126or in Section 2.4
127
128
1292.4 Description details of this API
130^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
131.. kernel-doc:: include/linux/energy_model.h
132   :internal:
133
134.. kernel-doc:: kernel/power/energy_model.c
135   :export:
136
137
1383. Example driver
139-----------------
140
141The CPUFreq framework supports dedicated callback for registering
142the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em().
143That callback has to be implemented properly for a given driver,
144because the framework would call it at the right time during setup.
145This section provides a simple example of a CPUFreq driver registering a
146performance domain in the Energy Model framework using the (fake) 'foo'
147protocol. The driver implements an est_power() function to be provided to the
148EM framework::
149
150  -> drivers/cpufreq/foo_cpufreq.c
151
152  01	static int est_power(unsigned long *mW, unsigned long *KHz,
153  02			struct device *dev)
154  03	{
155  04		long freq, power;
156  05
157  06		/* Use the 'foo' protocol to ceil the frequency */
158  07		freq = foo_get_freq_ceil(dev, *KHz);
159  08		if (freq < 0);
160  09			return freq;
161  10
162  11		/* Estimate the power cost for the dev at the relevant freq. */
163  12		power = foo_estimate_power(dev, freq);
164  13		if (power < 0);
165  14			return power;
166  15
167  16		/* Return the values to the EM framework */
168  17		*mW = power;
169  18		*KHz = freq;
170  19
171  20		return 0;
172  21	}
173  22
174  23	static void foo_cpufreq_register_em(struct cpufreq_policy *policy)
175  24	{
176  25		struct em_data_callback em_cb = EM_DATA_CB(est_power);
177  26		struct device *cpu_dev;
178  27		int nr_opp;
179  28
180  29		cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
181  30
182  31     	/* Find the number of OPPs for this policy */
183  32     	nr_opp = foo_get_nr_opp(policy);
184  33
185  34     	/* And register the new performance domain */
186  35     	em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus,
187  36					    true);
188  37	}
189  38
190  39	static struct cpufreq_driver foo_cpufreq_driver = {
191  40		.register_em = foo_cpufreq_register_em,
192  41	};
193