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    SubjectRe: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power_off
    Date
    On Tuesday, February 14, 2012, Zhang Rui wrote:
    > On 一, 2012-02-13 at 20:38 +0100, Rafael J. Wysocki wrote:
    > > On Monday, February 13, 2012, 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.
    > >
    > > I'd rather say all it knows about is "suspended" and "active", which mean
    > > "the device is not processing I/O" and "the device may be processing I/O",
    > > respectively. A "suspended" device may or may not be in a low-power state,
    > > but the runtime PM core doesn't care about that.
    > >
    > yes, I know that.
    >
    > > > 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?
    > >
    > > Well, I honestly don't think there's any way they all can be covered at the
    > > same time and that's why we chose to support only "suspended" and "active"
    > > as defined above.
    >
    > > The handling of multiple low-power states must be
    > > implemented outside of the runtime PM core (like in the PCI core, for example).
    >
    > Surely I'd prefer to implement it in the bus code, :), but the problem
    > is that several buses maybe involved at the same time.
    > Let's take ZPODD for example,
    > ZPODD is attached to a SATA port. Only SATA port knows that it can be
    > runtime powered off, because its ACPI node has _PR3._OFF.
    > But when ATA layer code tries to put SATA port to D3_COLD at runtime,it
    > must make sure all the devices/drivers in the same power domain are
    > ready for power off, and in this case, we need to get this info from
    > SCSI layer.

    Then you need to get it from there. I know that this is a difficult problem,
    have been working on a similar one for several months now. :-)

    Thanks,
    Rafael
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