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