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 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 milliwatts); 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 'milliwatts' 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 'simple' EM 117~~~~~~~~~~~~~~~~~~~~~~~~~~~ 118 119The 'simple' EM is registered using the framework helper function 120cpufreq_register_em_with_opp(). It implements a power model which is tight to 121math formula:: 122 123 Power = C * V^2 * f 124 125The EM which is registered using this method might not reflect correctly the 126physics of a real device, e.g. when static power (leakage) is important. 127 128 1292.3 Accessing performance domains 130^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 131 132There are two API functions which provide the access to the energy model: 133em_cpu_get() which takes CPU id as an argument and em_pd_get() with device 134pointer as an argument. It depends on the subsystem which interface it is 135going to use, but in case of CPU devices both functions return the same 136performance domain. 137 138Subsystems interested in the energy model of a CPU can retrieve it using the 139em_cpu_get() API. The energy model tables are allocated once upon creation of 140the performance domains, and kept in memory untouched. 141 142The energy consumed by a performance domain can be estimated using the 143em_cpu_energy() API. The estimation is performed assuming that the schedutil 144CPUfreq governor is in use in case of CPU device. Currently this calculation is 145not provided for other type of devices. 146 147More details about the above APIs can be found in ``<linux/energy_model.h>`` 148or in Section 2.4 149 150 1512.4 Description details of this API 152^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 153.. kernel-doc:: include/linux/energy_model.h 154 :internal: 155 156.. kernel-doc:: kernel/power/energy_model.c 157 :export: 158 159 1603. Example driver 161----------------- 162 163The CPUFreq framework supports dedicated callback for registering 164the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em(). 165That callback has to be implemented properly for a given driver, 166because the framework would call it at the right time during setup. 167This section provides a simple example of a CPUFreq driver registering a 168performance domain in the Energy Model framework using the (fake) 'foo' 169protocol. The driver implements an est_power() function to be provided to the 170EM framework:: 171 172 -> drivers/cpufreq/foo_cpufreq.c 173 174 01 static int est_power(unsigned long *mW, unsigned long *KHz, 175 02 struct device *dev) 176 03 { 177 04 long freq, power; 178 05 179 06 /* Use the 'foo' protocol to ceil the frequency */ 180 07 freq = foo_get_freq_ceil(dev, *KHz); 181 08 if (freq < 0); 182 09 return freq; 183 10 184 11 /* Estimate the power cost for the dev at the relevant freq. */ 185 12 power = foo_estimate_power(dev, freq); 186 13 if (power < 0); 187 14 return power; 188 15 189 16 /* Return the values to the EM framework */ 190 17 *mW = power; 191 18 *KHz = freq; 192 19 193 20 return 0; 194 21 } 195 22 196 23 static void foo_cpufreq_register_em(struct cpufreq_policy *policy) 197 24 { 198 25 struct em_data_callback em_cb = EM_DATA_CB(est_power); 199 26 struct device *cpu_dev; 200 27 int nr_opp; 201 28 202 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus)); 203 30 204 31 /* Find the number of OPPs for this policy */ 205 32 nr_opp = foo_get_nr_opp(policy); 206 33 207 34 /* And register the new performance domain */ 208 35 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus, 209 36 true); 210 37 } 211 38 212 39 static struct cpufreq_driver foo_cpufreq_driver = { 213 40 .register_em = foo_cpufreq_register_em, 214 41 }; 215