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