thermal/drivers/cpu_cooling: Add idle cooling device documentation

Situation:

----------

Under certain circumstances a SoC can reach a critical temperature

limit and is unable to stabilize the temperature around a temperature

control. When the SoC has to stabilize the temperature, the kernel can

act on a cooling device to mitigate the dissipated power. When the

critical temperature is reached, a decision must be taken to reduce

the temperature, that, in turn impacts performance.

Another situation is when the silicon temperature continues to

increase even after the dynamic leakage is reduced to its minimum by

clock gating the component. This runaway phenomenon can continue due

to the static leakage. The only solution is to power down the

component, thus dropping the dynamic and static leakage that will

allow the component to cool down.

Last but not least, the system can ask for a specific power budget but

because of the OPP density, we can only choose an OPP with a power

budget lower than the requested one and under-utilize the CPU, thus

losing performance. In other words, one OPP under-utilizes the CPU

with a power less than the requested power budget and the next OPP

exceeds the power budget. An intermediate OPP could have been used if

it were present.

Solutions:

----------

If we can remove the static and the dynamic leakage for a specific

duration in a controlled period, the SoC temperature will

decrease. Acting on the idle state duration or the idle cycle

injection period, we can mitigate the temperature by modulating the

power budget.

The Operating Performance Point (OPP) density has a great influence on

the control precision of cpufreq, however different vendors have a

plethora of OPP density, and some have large power gap between OPPs,

that will result in loss of performance during thermal control and

loss of power in other scenarios.

At a specific OPP, we can assume that injecting idle cycle on all CPUs

belong to the same cluster, with a duration greater than the cluster

idle state target residency, we lead to dropping the static and the

dynamic leakage for this period (modulo the energy needed to enter

this state). So the sustainable power with idle cycles has a linear

relation with the OPP’s sustainable power and can be computed with a

coefficient similar to:

	    Power(IdleCycle) = Coef x Power(OPP)

Idle Injection:

---------------

The base concept of the idle injection is to force the CPU to go to an

idle state for a specified time each control cycle, it provides

another way to control CPU power and heat in addition to

cpufreq. Ideally, if all CPUs belonging to the same cluster, inject

their idle cycles synchronously, the cluster can reach its power down

state with a minimum power consumption and reduce the static leakage

to almost zero.  However, these idle cycles injection will add extra

latencies as the CPUs will have to wakeup from a deep sleep state.

We use a fixed duration of idle injection that gives an acceptable

performance penalty and a fixed latency. Mitigation can be increased

or decreased by modulating the duty cycle of the idle injection.

     ^

     |

     |

     |-------                         -------

     |_______|_______________________|_______|___________

     <------>

       idle  <---------------------->

                    running

      <----------------------------->

              duty cycle 25%

The implementation of the cooling device bases the number of states on

the duty cycle percentage. When no mitigation is happening the cooling

device state is zero, meaning the duty cycle is 0%.

When the mitigation begins, depending on the governor's policy, a

starting state is selected. With a fixed idle duration and the duty

cycle (aka the cooling device state), the running duration can be

computed.

The governor will change the cooling device state thus the duty cycle

and this variation will modulate the cooling effect.

     ^

     |

     |

     |-------                 -------

     |_______|_______________|_______|___________

     <------>

       idle  <-------------->

                running

      <----------------------------->

              duty cycle 33%

     ^

     |

     |

     |-------         -------

     |_______|_______|_______|___________

     <------>

       idle  <------>

              running

      <------------->

       duty cycle 50%

The idle injection duration value must comply with the constraints:

- It is less than or equal to the latency we tolerate when the

  mitigation begins. It is platform dependent and will depend on the

  user experience, reactivity vs performance trade off we want. This

  value should be specified.

- It is greater than the idle state’s target residency we want to go

  for thermal mitigation, otherwise we end up consuming more energy.

Power considerations

--------------------

When we reach the thermal trip point, we have to sustain a specified

power for a specific temperature but at this time we consume:

 Power = Capacitance x Voltage^2 x Frequency x Utilisation

... which is more than the sustainable power (or there is something

wrong in the system setup). The ‘Capacitance’ and ‘Utilisation’ are a

fixed value, ‘Voltage’ and the ‘Frequency’ are fixed artificially

because we don’t want to change the OPP. We can group the

‘Capacitance’ and the ‘Utilisation’ into a single term which is the

‘Dynamic Power Coefficient (Cdyn)’ Simplifying the above, we have:

 Pdyn = Cdyn x Voltage^2 x Frequency

The power allocator governor will ask us somehow to reduce our power

in order to target the sustainable power defined in the device

tree. So with the idle injection mechanism, we want an average power

(Ptarget) resulting in an amount of time running at full power on a

specific OPP and idle another amount of time. That could be put in a

equation:

 P(opp)target = ((Trunning x (P(opp)running) + (Tidle x P(opp)idle)) /

			(Trunning + Tidle)

  ...

 Tidle = Trunning x ((P(opp)running / P(opp)target) - 1)

At this point if we know the running period for the CPU, that gives us

the idle injection we need. Alternatively if we have the idle

injection duration, we can compute the running duration with:

 Trunning = Tidle / ((P(opp)running / P(opp)target) - 1)

Practically, if the running power is less than the targeted power, we

end up with a negative time value, so obviously the equation usage is

bound to a power reduction, hence a higher OPP is needed to have the

running power greater than the targeted power.

However, in this demonstration we ignore three aspects:

 * The static leakage is not defined here, we can introduce it in the

   equation but assuming it will be zero most of the time as it is

   difficult to get the values from the SoC vendors

 * The idle state wake up latency (or entry + exit latency) is not

   taken into account, it must be added in the equation in order to

   rigorously compute the idle injection

 * The injected idle duration must be greater than the idle state

   target residency, otherwise we end up consuming more energy and

   potentially invert the mitigation effect

So the final equation is:

 Trunning = (Tidle - Twakeup ) x

		(((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target )