From d704aa0d44ade12660d4d7220b2a8d785b7b4247 Mon Sep 17 00:00:00 2001 From: Lukasz Luba Date: Tue, 2 Nov 2021 18:01:43 +0000 Subject: Documentation: power: Add description about new callback for EM registration The Energy Model (EM) registration for CPUs should now be done using a dedicated callback added recently into CPUFreq framework and drivers. Commit c17495b01b72 ("cpufreq: Add callback to register with energy model") The callback guaranties that the EM registration is called at the right time during driver setup. To avoid mistakes update the documentation to align with the existing code implementation. Signed-off-by: Lukasz Luba Signed-off-by: Rafael J. Wysocki --- Documentation/power/energy-model.rst | 31 ++++++++++++++++--------------- 1 file changed, 16 insertions(+), 15 deletions(-) (limited to 'Documentation/power') diff --git a/Documentation/power/energy-model.rst b/Documentation/power/energy-model.rst index 8a2788afe89b..7af0e1760962 100644 --- a/Documentation/power/energy-model.rst +++ b/Documentation/power/energy-model.rst @@ -138,6 +138,10 @@ or in Section 2.4 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 @@ -167,25 +171,22 @@ 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 true); - 42 - 43 return 0; - 44 } + 39 static struct cpufreq_driver foo_cpufreq_driver = { + 40 .register_em = foo_cpufreq_register_em, + 41 }; -- cgit v1.2.3-59-g8ed1b From 08374410a5ea3ff2fa9a87edd7d7ab15375b1c31 Mon Sep 17 00:00:00 2001 From: Lukasz Luba Date: Tue, 2 Nov 2021 18:01:44 +0000 Subject: Documentation: power: Describe 'advanced' and 'simple' EM models The Energy Model (EM) can be registered in two ways: 1) Using a helper function, which under the hood relies on OPP framework and DT entry in CPU node: 'dynamic-power-coefficient'. This is a 'simple' EM because it's tied to the math formula: Power = dynamic-power-coefficient * V^2 * f 2) Using em_dev_register_perf_domain() API function with a driver custom callback which provides power for each performance state. This is 'advanced' EM, since it can better reflect real power measurements for each performance state. It's not limited to any math formula and can better reflect real physics of the device. Add description of these two methods to the documentation, so developers could choose the suitable registration method (option). Signed-off-by: Lukasz Luba [ rjw: Changelog edits ] Signed-off-by: Rafael J. Wysocki --- Documentation/power/energy-model.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) (limited to 'Documentation/power') diff --git a/Documentation/power/energy-model.rst b/Documentation/power/energy-model.rst index 7af0e1760962..5ac62a7b4b7c 100644 --- a/Documentation/power/energy-model.rst +++ b/Documentation/power/energy-model.rst @@ -84,6 +84,16 @@ 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:: @@ -103,6 +113,18 @@ 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 '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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -- cgit v1.2.3-59-g8ed1b