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SubjectRe: [PATCH 0/8] Suspend block api (version 6)
On Tue, 4 May 2010, Matthew Garrett wrote:

> 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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