1 .. SPDX-License-Identifier: GPL-2.0
3 =======================
4 Energy Model of devices
5 =======================
10 The Energy Model (EM) framework serves as an interface between drivers knowing
11 the power consumed by devices at various performance levels, and the kernel
12 subsystems willing to use that information to make energy-aware decisions.
14 The source of the information about the power consumed by devices can vary greatly
15 from one platform to another. These power costs can be estimated using
16 devicetree data in some cases. In others, the firmware will know better.
17 Alternatively, userspace might be best positioned. And so on. In order to avoid
18 each and every client subsystem to re-implement support for each and every
19 possible source of information on its own, the EM framework intervenes as an
20 abstraction layer which standardizes the format of power cost tables in the
21 kernel, hence enabling to avoid redundant work.
23 The power values might be expressed in milli-Watts or in an 'abstract scale'.
24 Multiple subsystems might use the EM and it is up to the system integrator to
25 check that the requirements for the power value scale types are met. An example
26 can be found in the Energy-Aware Scheduler documentation
27 Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or
28 powercap power values expressed in an 'abstract scale' might cause issues.
29 These subsystems are more interested in estimation of power used in the past,
30 thus the real milli-Watts might be needed. An example of these requirements can
31 be found in the Intelligent Power Allocation in
32 Documentation/driver-api/thermal/power_allocator.rst.
33 Important thing to keep in mind is that when the power values are expressed in
34 an 'abstract scale' deriving real energy in milli-Joules would not be possible.
36 The figure below depicts an example of drivers (Arm-specific here, but the
37 approach is applicable to any architecture) providing power costs to the EM
38 framework, and interested clients reading the data from it::
40 +---------------+ +-----------------+ +---------------+
41 | Thermal (IPA) | | Scheduler (EAS) | | Other |
42 +---------------+ +-----------------+ +---------------+
45 +---------+ | +---------+
48 +---------------------+
51 +---------------------+
53 | | | em_dev_register_perf_domain()
54 +----------+ | +---------+
56 +---------------+ +---------------+ +--------------+
57 | cpufreq-dt | | arm_scmi | | Other |
58 +---------------+ +---------------+ +--------------+
61 +--------------+ +---------------+ +--------------+
62 | Device Tree | | Firmware | | ? |
63 +--------------+ +---------------+ +--------------+
65 In case of CPU devices the EM framework manages power cost tables per
66 'performance domain' in the system. A performance domain is a group of CPUs
67 whose performance is scaled together. Performance domains generally have a
68 1-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are
69 required to have the same micro-architecture. CPUs in different performance
70 domains can have different micro-architectures.
79 CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
82 2.2 Registration of performance domains
83 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
85 Drivers are expected to register performance domains into the EM framework by
86 calling the following API::
88 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
89 struct em_data_callback *cb, cpumask_t *cpus);
91 Drivers must provide a callback function returning <frequency, power> tuples
92 for each performance state. The callback function provided by the driver is free
93 to fetch data from any relevant location (DT, firmware, ...), and by any mean
94 deemed necessary. Only for CPU devices, drivers must specify the CPUs of the
95 performance domains using cpumask. For other devices than CPUs the last
96 argument must be set to NULL.
97 See Section 3. for an example of driver implementing this
98 callback, and kernel/power/energy_model.c for further documentation on this
102 2.3 Accessing performance domains
103 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
105 There are two API functions which provide the access to the energy model:
106 em_cpu_get() which takes CPU id as an argument and em_pd_get() with device
107 pointer as an argument. It depends on the subsystem which interface it is
108 going to use, but in case of CPU devices both functions return the same
111 Subsystems interested in the energy model of a CPU can retrieve it using the
112 em_cpu_get() API. The energy model tables are allocated once upon creation of
113 the performance domains, and kept in memory untouched.
115 The energy consumed by a performance domain can be estimated using the
116 em_cpu_energy() API. The estimation is performed assuming that the schedutil
117 CPUfreq governor is in use in case of CPU device. Currently this calculation is
118 not provided for other type of devices.
120 More details about the above APIs can be found in include/linux/energy_model.h.
126 This section provides a simple example of a CPUFreq driver registering a
127 performance domain in the Energy Model framework using the (fake) 'foo'
128 protocol. The driver implements an est_power() function to be provided to the
131 -> drivers/cpufreq/foo_cpufreq.c
133 01 static int est_power(unsigned long *mW, unsigned long *KHz,
134 02 struct device *dev)
138 06 /* Use the 'foo' protocol to ceil the frequency */
139 07 freq = foo_get_freq_ceil(dev, *KHz);
143 11 /* Estimate the power cost for the dev at the relevant freq. */
144 12 power = foo_estimate_power(dev, freq);
148 16 /* Return the values to the EM framework */
155 23 static int foo_cpufreq_init(struct cpufreq_policy *policy)
157 25 struct em_data_callback em_cb = EM_DATA_CB(est_power);
158 26 struct device *cpu_dev;
161 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
163 31 /* Do the actual CPUFreq init work ... */
164 32 ret = do_foo_cpufreq_init(policy);
168 36 /* Find the number of OPPs for this policy */
169 37 nr_opp = foo_get_nr_opp(policy);
171 39 /* And register the new performance domain */
172 40 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus);