On Mon, 13 Feb 2012, Lin Ming wrote:

From: Zhang Rui <rui.zhang@intel.com>

Introduce flag can_power_off in device structure to support runtime
power off/on.

Note that, for a specific device driver,
"support runtime power off/on" means that the driver .runtime_suspend
callback needs to
1) save all the context so that it can restore the device back to the previous
   working state after powered on.
2) set can_power_off flag to tell the driver model that it's ready for power off.

The following example shows how this works.

device A
 |---------|
 v         v
device B  device C

A is the parent of device B and device C, and device A/B/C shares the
same power logic
(Only device A knows how to turn on/off the power).

In order to power off A, B, C at runtime,
1) device B and device C should support runtime power off
   (runtime suspended with can_power_off flag set)
2) pm idle request for device A is fired by runtime PM core.
3) in device A .runtime_suspend callback, it tries to set can_power_off flag.
4) if succeed, it means all its children have been ready for power off
   and it can turn off the power at any time.
5) if failed, it means at least one of its children does not support runtime
   power off, thus the power can not be turned off.

I'm not sure if this is really the right approach.  What you're trying 
to do is implement two different low-power states, basically D3hot and 
D3cold.  Currently the runtime PM core doesn't support such things; all 
it knows about is low power and full power.

Before doing an ad-hoc implementation, it would be best to step back
and think about other subsystems.  Other sorts of devices may well have
multiple low-power states.  What's the best way for this to be
supported by the PM core?

Alan Stern