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    SubjectRe: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power_off
    On Mon, 13 Feb 2012, Lin Ming wrote:

    > From: Zhang Rui <>
    > 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

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