cpu_cooling: Remove static-power related documentation
commit 84fe2cab48 ("cpu_cooling: Drop static-power related stuff")
removed support for static-power in kernel, but it missed reflecting the
same in documentation. Remove the static power related documentation
bits as well.
Reported-by: Javi Merino <javi.merino@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Viresh Kumar
Rafael J. Wysocki
parent
84fe2cab48
commit
ac89c400eb
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@ -44,16 +44,14 @@ the user. The registration APIs returns the cooling device pointer.
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2. Power models
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The power API registration functions provide a simple power model for
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CPUs. The current power is calculated as dynamic + (optionally)
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static power. This power model requires that the operating-points of
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CPUs. The current power is calculated as dynamic power (static power isn't
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supported currently). This power model requires that the operating-points of
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the CPUs are registered using the kernel's opp library and the
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`cpufreq_frequency_table` is assigned to the `struct device` of the
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cpu. If you are using CONFIG_CPUFREQ_DT then the
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`cpufreq_frequency_table` should already be assigned to the cpu
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device.
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2.1 Dynamic power
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The dynamic power consumption of a processor depends on many factors.
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For a given processor implementation the primary factors are:
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@ -92,79 +90,3 @@ mW/MHz/uVolt^2. Typical values for mobile CPUs might lie in range
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from 100 to 500. For reference, the approximate values for the SoC in
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ARM's Juno Development Platform are 530 for the Cortex-A57 cluster and
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140 for the Cortex-A53 cluster.
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2.2 Static power
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Static leakage power consumption depends on a number of factors. For a
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given circuit implementation the primary factors are:
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- Time the circuit spends in each 'power state'
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- Temperature
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- Operating voltage
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- Process grade
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The time the circuit spends in each 'power state' for a given
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evaluation period at first order means OFF or ON. However,
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'retention' states can also be supported that reduce power during
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inactive periods without loss of context.
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Note: The visibility of state entries to the OS can vary, according to
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platform specifics, and this can then impact the accuracy of a model
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based on OS state information alone. It might be possible in some
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cases to extract more accurate information from system resources.
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The temperature, operating voltage and process 'grade' (slow to fast)
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of the circuit are all significant factors in static leakage power
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consumption. All of these have complex relationships to static power.
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Circuit implementation specific factors include the chosen silicon
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process as well as the type, number and size of transistors in both
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the logic gates and any RAM elements included.
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The static power consumption modelling must take into account the
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power managed regions that are implemented. Taking the example of an
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ARM processor cluster, the modelling would take into account whether
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each CPU can be powered OFF separately or if only a single power
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region is implemented for the complete cluster.
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In one view, there are others, a static power consumption model can
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then start from a set of reference values for each power managed
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region (e.g. CPU, Cluster/L2) in each state (e.g. ON, OFF) at an
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arbitrary process grade, voltage and temperature point. These values
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are then scaled for all of the following: the time in each state, the
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process grade, the current temperature and the operating voltage.
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However, since both implementation specific and complex relationships
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dominate the estimate, the appropriate interface to the model from the
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cpu cooling device is to provide a function callback that calculates
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the static power in this platform. When registering the cpu cooling
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device pass a function pointer that follows the `get_static_t`
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prototype:
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int plat_get_static(cpumask_t *cpumask, int interval,
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unsigned long voltage, u32 &power);
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`cpumask` is the cpumask of the cpus involved in the calculation.
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`voltage` is the voltage at which they are operating. The function
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should calculate the average static power for the last `interval`
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milliseconds. It returns 0 on success, -E* on error. If it
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succeeds, it should store the static power in `power`. Reading the
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temperature of the cpus described by `cpumask` is left for
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plat_get_static() to do as the platform knows best which thermal
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sensor is closest to the cpu.
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If `plat_static_func` is NULL, static power is considered to be
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negligible for this platform and only dynamic power is considered.
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The platform specific callback can then use any combination of tables
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and/or equations to permute the estimated value. Process grade
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information is not passed to the model since access to such data, from
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on-chip measurement capability or manufacture time data, is platform
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specific.
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Note: the significance of static power for CPUs in comparison to
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dynamic power is highly dependent on implementation. Given the
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potential complexity in implementation, the importance and accuracy of
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its inclusion when using cpu cooling devices should be assessed on a
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case by case basis.
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