[V4,2/4] thermal/drivers/cpu_cooling: Add idle cooling device documentation

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+
+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 )