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. 33Important thing to keep in mind is that when the power values are expressed in 34an 'abstract scale' deriving real energy in milli-Joules would not be possible. 35 36The figure below depicts an example of drivers (Arm-specific here, but the 37approach is applicable to any architecture) providing power costs to the EM 38framework, and interested clients reading the data from it:: 39 40 +---------------+ +-----------------+ +---------------+ 41 | Thermal (IPA) | | Scheduler (EAS) | | Other | 42 +---------------+ +-----------------+ +---------------+ 43 | | em_cpu_energy() | 44 | | em_cpu_get() | 45 +---------+ | +---------+ 46 | | | 47 v v v 48 +---------------------+ 49 | Energy Model | 50 | Framework | 51 +---------------------+ 52 ^ ^ ^ 53 | | | em_dev_register_perf_domain() 54 +----------+ | +---------+ 55 | | | 56 +---------------+ +---------------+ +--------------+ 57 | cpufreq-dt | | arm_scmi | | Other | 58 +---------------+ +---------------+ +--------------+ 59 ^ ^ ^ 60 | | | 61 +--------------+ +---------------+ +--------------+ 62 | Device Tree | | Firmware | | ? | 63 +--------------+ +---------------+ +--------------+ 64 65In 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 67whose performance is scaled together. Performance domains generally have a 681-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are 69required to have the same micro-architecture. CPUs in different performance 70domains can have different micro-architectures. 71 72 732. Core APIs 74------------ 75 762.1 Config options 77^^^^^^^^^^^^^^^^^^ 78 79CONFIG_ENERGY_MODEL must be enabled to use the EM framework. 80 81 822.2 Registration of performance domains 83^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 84 85Drivers are expected to register performance domains into the EM framework by 86calling the following API:: 87 88 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states, 89 struct em_data_callback *cb, cpumask_t *cpus); 90 91Drivers must provide a callback function returning <frequency, power> tuples 92for each performance state. The callback function provided by the driver is free 93to fetch data from any relevant location (DT, firmware, ...), and by any mean 94deemed necessary. Only for CPU devices, drivers must specify the CPUs of the 95performance domains using cpumask. For other devices than CPUs the last 96argument must be set to NULL. 97See Section 3. for an example of driver implementing this 98callback, and kernel/power/energy_model.c for further documentation on this 99API. 100 101 1022.3 Accessing performance domains 103^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 104 105There are two API functions which provide the access to the energy model: 106em_cpu_get() which takes CPU id as an argument and em_pd_get() with device 107pointer as an argument. It depends on the subsystem which interface it is 108going to use, but in case of CPU devices both functions return the same 109performance domain. 110 111Subsystems interested in the energy model of a CPU can retrieve it using the 112em_cpu_get() API. The energy model tables are allocated once upon creation of 113the performance domains, and kept in memory untouched. 114 115The energy consumed by a performance domain can be estimated using the 116em_cpu_energy() API. The estimation is performed assuming that the schedutil 117CPUfreq governor is in use in case of CPU device. Currently this calculation is 118not provided for other type of devices. 119 120More details about the above APIs can be found in include/linux/energy_model.h. 121 122 1233. Example driver 124----------------- 125 126This section provides a simple example of a CPUFreq driver registering a 127performance domain in the Energy Model framework using the (fake) 'foo' 128protocol. The driver implements an est_power() function to be provided to the 129EM framework:: 130 131 -> drivers/cpufreq/foo_cpufreq.c 132 133 01 static int est_power(unsigned long *mW, unsigned long *KHz, 134 02 struct device *dev) 135 03 { 136 04 long freq, power; 137 05 138 06 /* Use the 'foo' protocol to ceil the frequency */ 139 07 freq = foo_get_freq_ceil(dev, *KHz); 140 08 if (freq < 0); 141 09 return freq; 142 10 143 11 /* Estimate the power cost for the dev at the relevant freq. */ 144 12 power = foo_estimate_power(dev, freq); 145 13 if (power < 0); 146 14 return power; 147 15 148 16 /* Return the values to the EM framework */ 149 17 *mW = power; 150 18 *KHz = freq; 151 19 152 20 return 0; 153 21 } 154 22 155 23 static int foo_cpufreq_init(struct cpufreq_policy *policy) 156 24 { 157 25 struct em_data_callback em_cb = EM_DATA_CB(est_power); 158 26 struct device *cpu_dev; 159 27 int nr_opp, ret; 160 28 161 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus)); 162 30 163 31 /* Do the actual CPUFreq init work ... */ 164 32 ret = do_foo_cpufreq_init(policy); 165 33 if (ret) 166 34 return ret; 167 35 168 36 /* Find the number of OPPs for this policy */ 169 37 nr_opp = foo_get_nr_opp(policy); 170 38 171 39 /* And register the new performance domain */ 172 40 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus); 173 41 174 42 return 0; 175 43 } 176