Not the OP, but there are many (tens of) "power domains" on the system, and if you care about power/battery life then you want to power each of them down when t

Not the OP, but there are many (tens of) "power domains" on the system, and if you care about power/battery life then you want to power each of them down when they're unused, automatically, even while the rest of the system stays running. i.e. why pay the battery life cost for powering a USB hub when there are no USB devices plugged in, or for a DSP when there's no sound being played right now. But the calculation of when it's safe to power down any of the power domains can be very complicated -- you can't turn off a power supply unless everything that depends on it is unused, so there's a subtle and board-dependent graph/dependency problem to solve.