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

1 .. SPDX-License-Identifier: GPL-2.0
4 CPU Idle Cooling
8 ----------
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
33 ----------
37 decrease. Acting on the idle state duration or the idle cycle
44 that will result in loss of performance during thermal control and
47 At a specific OPP, we can assume that injecting idle cycle on all CPUs
49 idle state target residency, we lead to dropping the static and the
51 this state). So the sustainable power with idle cycles has a linear
57 Idle Injection:
58 ---------------
60 The base concept of the idle injection is to force the CPU to go to an
61 idle state for a specified time each control cycle, it provides
64 their idle cycles synchronously, the cluster can reach its power down
66 to almost zero.  However, these idle cycles injection will add extra
69 We use a fixed duration of idle injection that gives an acceptable
71 or decreased by modulating the duty cycle of the idle injection.
78      |-------                         -------
81      <------>
82        idle  <---------------------->
85       <----------------------------->
94 starting state is selected. With a fixed idle duration and the duty
106      |-------                 -------
109      <------>
110        idle  <-------------->
113       <--------------------->
120      |-------         -------
123      <------>
124        idle  <------>
127       <------------->
130 The idle injection duration value must comply with the constraints:
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.
141 --------------------
143 When we reach the thermal trip point, we have to sustain a specified
159 tree. So with the idle injection mechanism, we want an average power
161 specific OPP and idle another amount of time. That could be put in a
164  P(opp)target = ((Trunning x (P(opp)running) + (Tidle x P(opp)idle)) /
169  Tidle = Trunning x ((P(opp)running / P(opp)target) - 1)
172 the idle injection we need. Alternatively if we have the idle
175  Trunning = Tidle / ((P(opp)running / P(opp)target) - 1)
188  * The idle state wake up latency (or entry + exit latency) is not
190    rigorously compute the idle injection
192  * The injected idle duration must be greater than the idle state
198  Trunning = (Tidle - Twakeup ) x
199 		(((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target )