driver-api/thermal/cpu-idle-cooling.rst

1 .. SPDX-License-Identifier: GPL-2.0
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13 act on a cooling device to mitigate the dissipated power. When the
20 to the static leakage. The only solution is to power down the
24 Last but not least, the system can ask for a specific power budget but
25 because of the OPP density, we can only choose an OPP with a power
26 budget lower than the requested one and under-utilize the CPU, thus
27 losing performance. In other words, one OPP under-utilizes the CPU
28 with a power less than the requested power budget and the next OPP
29 exceeds the power budget. An intermediate OPP could have been used if
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39 power budget.
43 plethora of OPP density, and some have large power gap between OPPs,
45 loss of power in other scenarios.
51 this state). So the sustainable power with idle cycles has a linear
52 relation with the OPP’s sustainable power and can be computed with a
55 	    Power(IdleCycle) = Coef x Power(OPP)
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62 another way to control CPU power and heat in addition to
64 their idle cycles synchronously, the cluster can reach its power down
65 state with a minimum power consumption and reduce the static leakage
78      |-------                         -------
81      <------>
82        idle  <---------------------->
85       <----------------------------->
106      |-------                 -------
109      <------>
110        idle  <-------------->
113       <--------------------->
120      |-------         -------
123      <------>
124        idle  <------>
127       <------------->
132 - It is less than or equal to the latency we tolerate when the
137 - It is greater than the idle state’s target residency we want to go
138   for thermal mitigation, otherwise we end up consuming more energy.
140 Power considerations
141 --------------------
144 power for a specific temperature but at this time we consume::
146  Power = Capacitance x Voltage^2 x Frequency x Utilisation
148 ... which is more than the sustainable power (or there is something
153 ‘Dynamic Power Coefficient (Cdyn)’ Simplifying the above, we have::
157 The power allocator governor will ask us somehow to reduce our power
158 in order to target the sustainable power defined in the device
159 tree. So with the idle injection mechanism, we want an average power
160 (Ptarget) resulting in an amount of time running at full power on a
169  Tidle = Trunning x ((P(opp)running / P(opp)target) - 1)
175  Trunning = Tidle / ((P(opp)running / P(opp)target) - 1)
177 Practically, if the running power is less than the targeted power, we
178 end up with a negative time value, so obviously the equation usage is
179 bound to a power reduction, hence a higher OPP is needed to have the
180 running power greater than the targeted power.
188  * The idle state wake up latency (or entry + exit latency) is not
193    target residency, otherwise we end up consuming more energy and
198  Trunning = (Tidle - Twakeup ) x
199 		(((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target )