.. 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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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, 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					    true);
  42
  43	        return 0;
  44	}