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diff --git a/Documentation/power/energy-model.rst b/Documentation/power/energy-model.rst new file mode 100644 index 000000000..a6fb986ab --- /dev/null +++ b/Documentation/power/energy-model.rst @@ -0,0 +1,162 @@ +.. 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 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 +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +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); + +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. +See Section 3. for an example of driver implementing this +callback, and kernel/power/energy_model.c for further documentation on this +API. + + +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 include/linux/energy_model.h. + + +3. Example driver +----------------- + +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 int foo_cpufreq_init(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, ret; + 28 + 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus)); + 30 + 31 /* Do the actual CPUFreq init work ... */ + 32 ret = do_foo_cpufreq_init(policy); + 33 if (ret) + 34 return ret; + 35 + 36 /* Find the number of OPPs for this policy */ + 37 nr_opp = foo_get_nr_opp(policy); + 38 + 39 /* And register the new performance domain */ + 40 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus); + 41 + 42 return 0; + 43 } |