272 lines
10 KiB
ReStructuredText
272 lines
10 KiB
ReStructuredText
=================================
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Power allocator governor tunables
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=================================
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Trip points
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-----------
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The governor works optimally with the following two passive trip points:
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1. "switch on" trip point: temperature above which the governor
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control loop starts operating. This is the first passive trip
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point of the thermal zone.
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2. "desired temperature" trip point: it should be higher than the
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"switch on" trip point. This the target temperature the governor
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is controlling for. This is the last passive trip point of the
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thermal zone.
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PID Controller
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--------------
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The power allocator governor implements a
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Proportional-Integral-Derivative controller (PID controller) with
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temperature as the control input and power as the controlled output:
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P_max = k_p * e + k_i * err_integral + k_d * diff_err + sustainable_power
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where
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- e = desired_temperature - current_temperature
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- err_integral is the sum of previous errors
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- diff_err = e - previous_error
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It is similar to the one depicted below::
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k_d
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current_temp |
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| v
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| +----------+ +---+
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| +----->| diff_err |-->| X |------+
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| | +----------+ +---+ |
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| | | tdp actor
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| | k_i | | get_requested_power()
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| | | | | | |
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| | | | | | | ...
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v | v v v v v
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+---+ | +-------+ +---+ +---+ +---+ +----------+
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| S |-----+----->| sum e |----->| X |--->| S |-->| S |-->|power |
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+---+ | +-------+ +---+ +---+ +---+ |allocation|
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^ | ^ +----------+
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| | | | |
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| | +---+ | | |
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| +------->| X |-------------------+ v v
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| +---+ granted performance
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desired_temperature ^
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k_po/k_pu
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Sustainable power
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-----------------
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An estimate of the sustainable dissipatable power (in mW) should be
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provided while registering the thermal zone. This estimates the
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sustained power that can be dissipated at the desired control
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temperature. This is the maximum sustained power for allocation at
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the desired maximum temperature. The actual sustained power can vary
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for a number of reasons. The closed loop controller will take care of
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variations such as environmental conditions, and some factors related
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to the speed-grade of the silicon. `sustainable_power` is therefore
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simply an estimate, and may be tuned to affect the aggressiveness of
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the thermal ramp. For reference, the sustainable power of a 4" phone
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is typically 2000mW, while on a 10" tablet is around 4500mW (may vary
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depending on screen size).
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If you are using device tree, do add it as a property of the
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thermal-zone. For example::
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thermal-zones {
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soc_thermal {
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polling-delay = <1000>;
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polling-delay-passive = <100>;
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sustainable-power = <2500>;
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...
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Instead, if the thermal zone is registered from the platform code, pass a
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`thermal_zone_params` that has a `sustainable_power`. If no
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`thermal_zone_params` were being passed, then something like below
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will suffice::
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static const struct thermal_zone_params tz_params = {
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.sustainable_power = 3500,
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};
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and then pass `tz_params` as the 5th parameter to
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`thermal_zone_device_register()`
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k_po and k_pu
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-------------
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The implementation of the PID controller in the power allocator
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thermal governor allows the configuration of two proportional term
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constants: `k_po` and `k_pu`. `k_po` is the proportional term
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constant during temperature overshoot periods (current temperature is
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above "desired temperature" trip point). Conversely, `k_pu` is the
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proportional term constant during temperature undershoot periods
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(current temperature below "desired temperature" trip point).
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These controls are intended as the primary mechanism for configuring
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the permitted thermal "ramp" of the system. For instance, a lower
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`k_pu` value will provide a slower ramp, at the cost of capping
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available capacity at a low temperature. On the other hand, a high
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value of `k_pu` will result in the governor granting very high power
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while temperature is low, and may lead to temperature overshooting.
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The default value for `k_pu` is::
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2 * sustainable_power / (desired_temperature - switch_on_temp)
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This means that at `switch_on_temp` the output of the controller's
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proportional term will be 2 * `sustainable_power`. The default value
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for `k_po` is::
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sustainable_power / (desired_temperature - switch_on_temp)
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Focusing on the proportional and feed forward values of the PID
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controller equation we have::
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P_max = k_p * e + sustainable_power
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The proportional term is proportional to the difference between the
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desired temperature and the current one. When the current temperature
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is the desired one, then the proportional component is zero and
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`P_max` = `sustainable_power`. That is, the system should operate in
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thermal equilibrium under constant load. `sustainable_power` is only
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an estimate, which is the reason for closed-loop control such as this.
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Expanding `k_pu` we get::
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P_max = 2 * sustainable_power * (T_set - T) / (T_set - T_on) +
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sustainable_power
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where:
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- T_set is the desired temperature
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- T is the current temperature
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- T_on is the switch on temperature
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When the current temperature is the switch_on temperature, the above
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formula becomes::
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P_max = 2 * sustainable_power * (T_set - T_on) / (T_set - T_on) +
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sustainable_power = 2 * sustainable_power + sustainable_power =
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3 * sustainable_power
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Therefore, the proportional term alone linearly decreases power from
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3 * `sustainable_power` to `sustainable_power` as the temperature
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rises from the switch on temperature to the desired temperature.
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k_i and integral_cutoff
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-----------------------
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`k_i` configures the PID loop's integral term constant. This term
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allows the PID controller to compensate for long term drift and for
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the quantized nature of the output control: cooling devices can't set
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the exact power that the governor requests. When the temperature
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error is below `integral_cutoff`, errors are accumulated in the
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integral term. This term is then multiplied by `k_i` and the result
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added to the output of the controller. Typically `k_i` is set low (1
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or 2) and `integral_cutoff` is 0.
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k_d
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---
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`k_d` configures the PID loop's derivative term constant. It's
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recommended to leave it as the default: 0.
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Cooling device power API
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========================
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Cooling devices controlled by this governor must supply the additional
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"power" API in their `cooling_device_ops`. It consists on three ops:
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1. ::
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int get_requested_power(struct thermal_cooling_device *cdev,
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struct thermal_zone_device *tz, u32 *power);
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@cdev:
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The `struct thermal_cooling_device` pointer
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@tz:
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thermal zone in which we are currently operating
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@power:
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pointer in which to store the calculated power
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`get_requested_power()` calculates the power requested by the device
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in milliwatts and stores it in @power . It should return 0 on
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success, -E* on failure. This is currently used by the power
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allocator governor to calculate how much power to give to each cooling
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device.
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2. ::
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int state2power(struct thermal_cooling_device *cdev, struct
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thermal_zone_device *tz, unsigned long state,
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u32 *power);
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@cdev:
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The `struct thermal_cooling_device` pointer
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@tz:
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thermal zone in which we are currently operating
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@state:
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A cooling device state
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@power:
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pointer in which to store the equivalent power
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Convert cooling device state @state into power consumption in
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milliwatts and store it in @power. It should return 0 on success, -E*
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on failure. This is currently used by thermal core to calculate the
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maximum power that an actor can consume.
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3. ::
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int power2state(struct thermal_cooling_device *cdev, u32 power,
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unsigned long *state);
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@cdev:
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The `struct thermal_cooling_device` pointer
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@power:
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power in milliwatts
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@state:
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pointer in which to store the resulting state
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Calculate a cooling device state that would make the device consume at
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most @power mW and store it in @state. It should return 0 on success,
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-E* on failure. This is currently used by the thermal core to convert
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a given power set by the power allocator governor to a state that the
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cooling device can set. It is a function because this conversion may
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depend on external factors that may change so this function should the
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best conversion given "current circumstances".
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Cooling device weights
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----------------------
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Weights are a mechanism to bias the allocation among cooling
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devices. They express the relative power efficiency of different
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cooling devices. Higher weight can be used to express higher power
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efficiency. Weighting is relative such that if each cooling device
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has a weight of one they are considered equal. This is particularly
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useful in heterogeneous systems where two cooling devices may perform
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the same kind of compute, but with different efficiency. For example,
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a system with two different types of processors.
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If the thermal zone is registered using
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`thermal_zone_device_register()` (i.e., platform code), then weights
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are passed as part of the thermal zone's `thermal_bind_parameters`.
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If the platform is registered using device tree, then they are passed
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as the `contribution` property of each map in the `cooling-maps` node.
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Limitations of the power allocator governor
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===========================================
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The power allocator governor's PID controller works best if there is a
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periodic tick. If you have a driver that calls
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`thermal_zone_device_update()` (or anything that ends up calling the
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governor's `throttle()` function) repetitively, the governor response
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won't be very good. Note that this is not particular to this
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governor, step-wise will also misbehave if you call its throttle()
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faster than the normal thermal framework tick (due to interrupts for
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example) as it will overreact.
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