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    SubjectRe: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power_off
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    On 一, 2012-02-13 at 10:01 -0500, Alan Stern wrote:
    > 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.
    >
    Exactly.
    what I'm trying to fix here is to add a "special" runtime low power
    state, aka, power off.

    > 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?
    >
    I thought about this before, e.g. introduce support for multiple runtime
    low power states in runtime PM core, like suspend/hibernate for system
    low power states. But I'm not sure if this is workable because the low
    power states varies between devices/buses/platforms.

    So I decided to introduce a special low power state, aka, runtime power
    off, first, which means the same thing to different
    devices/buses/platforms.

    thanks,
    rui


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