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    SubjectRe: [RFC PATCH V1 1/2] cpuidle: Data structure changes for global cpuidle device
    I agree it is silly to allocate a cpuidle_device
    for every cpu in the system as we do today.

    Yes, splitting the counters out of cpuidle_device
    is a necessary part of fixing that.

    However, cpuidle_device.cpuidle_state[] is currently not per-driver,
    it is per-cpu, and it is writable.

    In particular, the cpuidle_device->prepare() mechanism
    causes updates to the cpuidle_state[].flags,
    setting and clearing CPUIDLE_FLAG_IGNORE to
    tell the governor not to chose a state
    on a per-cpu basis at run-time.

    I don't like that mechanism.
    I'd like to see it replaced, and when replaced,
    cpuidle_state[] can be per system-wide driver.

    I think the real problem that prepare() was trying to solve
    is that the driver today does not have the ability to over-rule
    the choice made by the governor. The driver may discover
    in the course of trying to satisfy the request of the governor
    that it needs to demote to a shallower state; or it may
    do its best to satisfy the governor's request, and the hardware
    may demote its request to a shallower state.

    Unfortunately, when this happens, the driver dutifully
    returns the time spent in the state to cpuidle_idle_call(),
    who then updates the wrong last_residency, time, and usage counters.

    Sure is ironic for the driver to allocate the data structures and
    then hand the timer to the uppper layer, just to have the upper layer
    update the wrong data structures...

    Surely the driver enter routine should update the counters
    that the driver was obligated to allocate, and it should return
    the state actually entered (for tracing), rather than the time spent
    there.

    The generic cpuidle code should simply handle where the counters live
    in the sysfs namespace, not updating the counters.

    This needs to be addressed before cpuidle_device.cpuidle_state[]
    can be made one/system.

    cheers,
    Len Brown, Intel Open Source Technology Center



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