On Tue, 4 May 2010, Matthew Garrett wrote:

quoted text
> On Mon, May 03, 2010 at 04:37:22PM -0700, Kevin Hilman wrote:
> 
> > Please forgive the ignorance of ACPI (in embedded, we thankfully live
> > in magical world without ACPI) but doesn't that already happen with
> > CPUidle and C-states?  I think of CPUidle as basically runtime PM for
> > the CPU.  IOW, runtime PM manages the devices, CPUidle manages the CPU
> > (via C-states), resulting in dynaimc PM for the entire system.  What
> > am I missing?
> 
> ACPI doesn't provide any functionality for cutting power to most devices 
> other than shifting into full system suspend. The number of wakeup 
> events available to us on a given machine is usually small and the 
> wakeup latency large, so it's not terribly practical to do this 
> transparently on most hardware.

Another thing that Kevin is missing: There is more to the system than
the devices and the CPU.  For example: RAM, an embedded controller (on
modern desktop/laptop systems), a power supply, and so on.  Dynamic PM
for the CPU and the devices won't power-down these things, but system
PM will.

Alan Stern

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