| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================= |
| Energy Model of devices |
| ======================= |
| |
| 1. Overview |
| ----------- |
| |
| The Energy Model (EM) framework serves as an interface between drivers knowing |
| the power consumed by devices at various performance levels, and the kernel |
| subsystems willing to use that information to make energy-aware decisions. |
| |
| The source of the information about the power consumed by devices can vary greatly |
| from one platform to another. These power costs can be estimated using |
| devicetree data in some cases. In others, the firmware will know better. |
| Alternatively, userspace might be best positioned. And so on. In order to avoid |
| each and every client subsystem to re-implement support for each and every |
| possible source of information on its own, the EM framework intervenes as an |
| abstraction layer which standardizes the format of power cost tables in the |
| kernel, hence enabling to avoid redundant work. |
| |
| The power values might be expressed in milli-Watts or in an 'abstract scale'. |
| Multiple subsystems might use the EM and it is up to the system integrator to |
| check that the requirements for the power value scale types are met. An example |
| can be found in the Energy-Aware Scheduler documentation |
| Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or |
| powercap power values expressed in an 'abstract scale' might cause issues. |
| These subsystems are more interested in estimation of power used in the past, |
| thus the real milli-Watts might be needed. An example of these requirements can |
| be found in the Intelligent Power Allocation in |
| Documentation/driver-api/thermal/power_allocator.rst. |
| Kernel subsystems might implement automatic detection to check whether EM |
| registered devices have inconsistent scale (based on EM internal flag). |
| Important thing to keep in mind is that when the power values are expressed in |
| an 'abstract scale' deriving real energy in milli-Joules would not be possible. |
| |
| The figure below depicts an example of drivers (Arm-specific here, but the |
| approach is applicable to any architecture) providing power costs to the EM |
| framework, and interested clients reading the data from it:: |
| |
| +---------------+ +-----------------+ +---------------+ |
| | Thermal (IPA) | | Scheduler (EAS) | | Other | |
| +---------------+ +-----------------+ +---------------+ |
| | | em_cpu_energy() | |
| | | em_cpu_get() | |
| +---------+ | +---------+ |
| | | | |
| v v v |
| +---------------------+ |
| | Energy Model | |
| | Framework | |
| +---------------------+ |
| ^ ^ ^ |
| | | | em_dev_register_perf_domain() |
| +----------+ | +---------+ |
| | | | |
| +---------------+ +---------------+ +--------------+ |
| | cpufreq-dt | | arm_scmi | | Other | |
| +---------------+ +---------------+ +--------------+ |
| ^ ^ ^ |
| | | | |
| +--------------+ +---------------+ +--------------+ |
| | Device Tree | | Firmware | | ? | |
| +--------------+ +---------------+ +--------------+ |
| |
| In case of CPU devices the EM framework manages power cost tables per |
| 'performance domain' in the system. A performance domain is a group of CPUs |
| whose performance is scaled together. Performance domains generally have a |
| 1-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are |
| required to have the same micro-architecture. CPUs in different performance |
| domains can have different micro-architectures. |
| |
| |
| 2. Core APIs |
| ------------ |
| |
| 2.1 Config options |
| ^^^^^^^^^^^^^^^^^^ |
| |
| CONFIG_ENERGY_MODEL must be enabled to use the EM framework. |
| |
| |
| 2.2 Registration of performance domains |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Registration of 'advanced' EM |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The 'advanced' EM gets it's name due to the fact that the driver is allowed |
| to provide more precised power model. It's not limited to some implemented math |
| formula in the framework (like it's in 'simple' EM case). It can better reflect |
| the real power measurements performed for each performance state. Thus, this |
| registration method should be preferred in case considering EM static power |
| (leakage) is important. |
| |
| Drivers are expected to register performance domains into the EM framework by |
| calling the following API:: |
| |
| int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states, |
| struct em_data_callback *cb, cpumask_t *cpus, bool milliwatts); |
| |
| Drivers must provide a callback function returning <frequency, power> tuples |
| for each performance state. The callback function provided by the driver is free |
| to fetch data from any relevant location (DT, firmware, ...), and by any mean |
| deemed necessary. Only for CPU devices, drivers must specify the CPUs of the |
| performance domains using cpumask. For other devices than CPUs the last |
| argument must be set to NULL. |
| The last argument 'milliwatts' is important to set with correct value. Kernel |
| subsystems which use EM might rely on this flag to check if all EM devices use |
| the same scale. If there are different scales, these subsystems might decide |
| to: return warning/error, stop working or panic. |
| See Section 3. for an example of driver implementing this |
| callback, or Section 2.4 for further documentation on this API |
| |
| Registration of 'simple' EM |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The 'simple' EM is registered using the framework helper function |
| cpufreq_register_em_with_opp(). It implements a power model which is tight to |
| math formula:: |
| |
| Power = C * V^2 * f |
| |
| The EM which is registered using this method might not reflect correctly the |
| physics of a real device, e.g. when static power (leakage) is important. |
| |
| |
| 2.3 Accessing performance domains |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| There are two API functions which provide the access to the energy model: |
| em_cpu_get() which takes CPU id as an argument and em_pd_get() with device |
| pointer as an argument. It depends on the subsystem which interface it is |
| going to use, but in case of CPU devices both functions return the same |
| performance domain. |
| |
| Subsystems interested in the energy model of a CPU can retrieve it using the |
| em_cpu_get() API. The energy model tables are allocated once upon creation of |
| the performance domains, and kept in memory untouched. |
| |
| The energy consumed by a performance domain can be estimated using the |
| em_cpu_energy() API. The estimation is performed assuming that the schedutil |
| CPUfreq governor is in use in case of CPU device. Currently this calculation is |
| not provided for other type of devices. |
| |
| More details about the above APIs can be found in ``<linux/energy_model.h>`` |
| or in Section 2.4 |
| |
| |
| 2.4 Description details of this API |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| .. kernel-doc:: include/linux/energy_model.h |
| :internal: |
| |
| .. kernel-doc:: kernel/power/energy_model.c |
| :export: |
| |
| |
| 3. Example driver |
| ----------------- |
| |
| The CPUFreq framework supports dedicated callback for registering |
| the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em(). |
| That callback has to be implemented properly for a given driver, |
| because the framework would call it at the right time during setup. |
| This section provides a simple example of a CPUFreq driver registering a |
| performance domain in the Energy Model framework using the (fake) 'foo' |
| protocol. The driver implements an est_power() function to be provided to the |
| EM framework:: |
| |
| -> drivers/cpufreq/foo_cpufreq.c |
| |
| 01 static int est_power(unsigned long *mW, unsigned long *KHz, |
| 02 struct device *dev) |
| 03 { |
| 04 long freq, power; |
| 05 |
| 06 /* Use the 'foo' protocol to ceil the frequency */ |
| 07 freq = foo_get_freq_ceil(dev, *KHz); |
| 08 if (freq < 0); |
| 09 return freq; |
| 10 |
| 11 /* Estimate the power cost for the dev at the relevant freq. */ |
| 12 power = foo_estimate_power(dev, freq); |
| 13 if (power < 0); |
| 14 return power; |
| 15 |
| 16 /* Return the values to the EM framework */ |
| 17 *mW = power; |
| 18 *KHz = freq; |
| 19 |
| 20 return 0; |
| 21 } |
| 22 |
| 23 static void foo_cpufreq_register_em(struct cpufreq_policy *policy) |
| 24 { |
| 25 struct em_data_callback em_cb = EM_DATA_CB(est_power); |
| 26 struct device *cpu_dev; |
| 27 int nr_opp; |
| 28 |
| 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus)); |
| 30 |
| 31 /* Find the number of OPPs for this policy */ |
| 32 nr_opp = foo_get_nr_opp(policy); |
| 33 |
| 34 /* And register the new performance domain */ |
| 35 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus, |
| 36 true); |
| 37 } |
| 38 |
| 39 static struct cpufreq_driver foo_cpufreq_driver = { |
| 40 .register_em = foo_cpufreq_register_em, |
| 41 }; |
