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    SubjectRe: [RFC PATCH V3 2/3] cpuidle: list based cpuidle driver registration and selection
    * Peter Zijlstra <peterz@infradead.org> [2011-02-10 10:53:30]:

    > On Thu, 2011-02-10 at 12:30 +0530, Vaidyanathan Srinivasan wrote:
    >
    > > We discussed this in the previous posts. On ppc64 we would like to
    > > have single global registration, but for x86 Arjan recommended that we
    > > keep the per-cpu registration or else we may break legacy or buggy
    > > devices.
    > >
    > > Ref: http://lkml.org/lkml/2009/10/7/210
    > >
    > > One corner case that was against using lowest common C-State is that
    > > we could have cores/packages sporting a new lower C-State if they are
    > > at a thermal limit and want the OS to go to much lower C-State and
    > > sacrifice performance. Our global design will prevent that cpu from
    > > going to a state lower than the rest of the system.
    > >
    > > This is only a remote possibility, but could happen on battery
    > > operated devices, under low battery mode etc. Basically we would have
    > > to keep our design open to allow individual CPUs to goto 'their'
    > > deepest allowed sleep state even in a asymmetric case.
    >
    >
    > But but but, its a stupid ACPI bug if it reports different C states for
    > different CPUs.
    >
    > Len, does intel_idle also suffer this or does it simply ignore what ACPI
    > has to say?
    >
    > Also, suppose for some daft reason it doesn't report C2 as available on
    > one of the CPUs, what happens if we use it anyway? (ie, use the union of
    > the reported states).

    Using a union of available C-States on all CPUs will take care of the
    above mentioned (buggy) case and generally simplify the registration
    mechanism. This implies that a cpu is allowed to use any of the
    registered ACPI C-States across the system.

    --Vaidy



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