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 .. 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 micro-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 micro-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 micro-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 microwatts);
 
 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 'microwatts' 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 EM using DT
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The  EM can also be registered using OPP framework and information in DT
 "operating-points-v2". Each OPP entry in DT can be extended with a property
 "opp-microwatt" containing micro-Watts power value. This OPP DT property
 allows a platform to register EM power values which are reflecting total power
 (static + dynamic). These power values might be coming directly from
 experiments and measurements.
 
 Registration of 'artificial' EM
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 There is an option to provide a custom callback for drivers missing detailed
 knowledge about power value for each performance state. The callback
 .get_cost() is optional and provides the 'cost' values used by the EAS.
 This is useful for platforms that only provide information on relative
 efficiency between CPU types, where one could use the information to
 create an abstract power model. But even an abstract power model can
 sometimes be hard to fit in, given the input power value size restrictions.
 The .get_cost() allows to provide the 'cost' values which reflect the
 efficiency of the CPUs. This would allow to provide EAS information which
 has different relation than what would be forced by the EM internal
 formulas calculating 'cost' values. To register an EM for such platform, the
 driver must set the flag 'microwatts' to 0, provide .get_power() callback
 and provide .get_cost() callback. The EM framework would handle such platform
 properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such
 platform. Special care should be taken by other frameworks which are using EM
 to test and treat this flag properly.
 
 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(struct device *dev, unsigned long *mW,
   02                    unsigned long *KHz)
   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    };

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