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Diffstat (limited to 'Documentation/power/energy-model.rst')
| -rw-r--r-- | Documentation/power/energy-model.rst | 122 |
1 files changed, 102 insertions, 20 deletions
diff --git a/Documentation/power/energy-model.rst b/Documentation/power/energy-model.rst index a6fb986abe3c..13225965c9a4 100644 --- a/Documentation/power/energy-model.rst +++ b/Documentation/power/energy-model.rst @@ -20,6 +20,21 @@ 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:: @@ -69,11 +84,21 @@ 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 its 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 is 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); + 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 @@ -81,9 +106,54 @@ 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, and kernel/power/energy_model.c for further documentation on this -API. +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 @@ -104,12 +174,26 @@ 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. +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 @@ -117,8 +201,8 @@ EM framework:: -> drivers/cpufreq/foo_cpufreq.c - 01 static int est_power(unsigned long *mW, unsigned long *KHz, - 02 struct device *dev) + 01 static int est_power(struct device *dev, unsigned long *mW, + 02 unsigned long *KHz) 03 { 04 long freq, power; 05 @@ -139,24 +223,22 @@ EM framework:: 20 return 0; 21 } 22 - 23 static int foo_cpufreq_init(struct cpufreq_policy *policy) + 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, ret; + 27 int nr_opp; 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); + 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 /* 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 } + 39 static struct cpufreq_driver foo_cpufreq_driver = { + 40 .register_em = foo_cpufreq_register_em, + 41 }; |