From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> > When we make a decision on what idle state to enter all we do, and > that's OS agnostic, is predicting (+checking the next event) the next IRQ and > see if it is worth entering a state or not. We have to compare it against > a baseline, which is the processor being in standbywfi and that's what > these bindings define. > > I do not understand why you want to define min-residency against the > previous shallower state. > > What this binding says is: standbywfi is the shallower idle state in > power consumption terms. Deeper idle states save more power than > standbywfi if the residency in that state is at least min-residency. > > I do not see where the problem is to be honest, maybe I need an example. > > Thanks! > Lorenzo Sorry, I should have explained myself more clearly. I've been pondering about these issues somewhat lately so I'm perhaps suffering from a bit of a tunnel vision. In short, when we choose an idle state based on expected idle duration we are not comparing wfi against all possible idle states in turn and making a decision between wfi and state X. Instead we want to choose among all states the one that gives minimum energy for the expected idle time. I'll try to elaborate.. Entering and exiting idle states takes time at nonzero power. To make up for this lost energy we indeed want the time in the idle state to be sufficiently long to make up for the lost energy. Now the important question here is "make up compared to what?". The energy over the idle time can be also interpreted as average power. When the idle time increases the average power for a state approaches the in-state power. A deeper idle state would be a state with lower in-state power and longer entry/exit time. Therefore the average power for a deeper idle state drops slower as function of idle time than the average power for a shallower idle state. If we'd plot the average power for a number of idle states as function of idle duration, we'd get a set of "constant over idle time plus constant" style curves. Average power for state 0 will drop fastest close to the in-state power of state 0. Average power for state 1 will drop slower and approach the in-state power of state 1, average power for state 2 will drop even slower and approach the in-state power of state 3. To define that the min-residency is the breakeven time against state 0 means that we are looking at the curves and asking "when does the average power for state X cross the average power for state 0?". But that would be the guideline for making a decision between state 0 and the state in question. Even if average power for state 2 is below the average power of state 0 it is not necessarily yet below the average power of state 1. To break even against state 1 the idle duration needs to be longer. Yet another way to look at this: for three states we can define three times of interest: - t1: the time when state1 breaks even against state0 - t2: the time when state2 breaks even against state0 - t3: the time when state2 breaks even against state1 and t3 would typically be larger than t2. --Antti -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
