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
while 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.