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diff --git a/Documentation/power/energy-model.txt b/Documentation/power/energy-model.txt new file mode 100644 index 000000000000..a2b0ae4c76bd --- /dev/null +++ b/Documentation/power/energy-model.txt @@ -0,0 +1,144 @@ + ==================== + Energy Model of CPUs + ==================== + +1. Overview +----------- + +The Energy Model (EM) framework serves as an interface between drivers knowing +the power consumed by CPUs 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 CPUs 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_pd_energy() | + | | em_cpu_get() | + +---------+ | +---------+ + | | | + v v v + +---------------------+ + | Energy Model | + | Framework | + +---------------------+ + ^ ^ ^ + | | | em_register_perf_domain() + +----------+ | +---------+ + | | | + +---------------+ +---------------+ +--------------+ + | cpufreq-dt | | arm_scmi | | Other | + +---------------+ +---------------+ +--------------+ + ^ ^ ^ + | | | + +--------------+ +---------------+ +--------------+ + | Device Tree | | Firmware | | ? | + +--------------+ +---------------+ +--------------+ + +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_register_perf_domain(cpumask_t *span, unsigned int nr_states, + struct em_data_callback *cb); + +Drivers must specify the CPUs of the performance domains using the cpumask +argument, and provide a callback function returning <frequency, power> tuples +for each capacity 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. 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 + +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_pd_energy() API. The estimation is performed assuming that the schedutil +CPUfreq governor is in use. + +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, int cpu) +02 { +03 long freq, power; +04 +05 /* Use the 'foo' protocol to ceil the frequency */ +06 freq = foo_get_freq_ceil(cpu, *KHz); +07 if (freq < 0); +08 return freq; +09 +10 /* Estimate the power cost for the CPU at the relevant freq. */ +11 power = foo_estimate_power(cpu, freq); +12 if (power < 0); +13 return power; +14 +15 /* Return the values to the EM framework */ +16 *mW = power; +17 *KHz = freq; +18 +19 return 0; +20 } +21 +22 static int foo_cpufreq_init(struct cpufreq_policy *policy) +23 { +24 struct em_data_callback em_cb = EM_DATA_CB(est_power); +25 int nr_opp, ret; +26 +27 /* Do the actual CPUFreq init work ... */ +28 ret = do_foo_cpufreq_init(policy); +29 if (ret) +30 return ret; +31 +32 /* Find the number of OPPs for this policy */ +33 nr_opp = foo_get_nr_opp(policy); +34 +35 /* And register the new performance domain */ +36 em_register_perf_domain(policy->cpus, nr_opp, &em_cb); +37 +38 return 0; +39 } |