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    SubjectRe: [PATCH 0/3] coupled cpuidle state support
    On 12/21/2011 1:09 AM, Colin Cross wrote:
    > On some ARM SMP SoCs (OMAP4460, Tegra 2, and probably more), the
    > cpus cannot be independently powered down, either due to
    > sequencing restrictions (on Tegra 2, cpu 0 must be the last to
    > power down), or due to HW bugs (on OMAP4460, a cpu powering up
    > will corrupt the gic state unless the other cpu runs a work
    > around). Each cpu has a power state that it can enter without
    > coordinating with the other cpu (usually Wait For Interrupt, or
    > WFI), and one or more "coupled" power states that affect blocks
    > shared between the cpus (L2 cache, interrupt controller, and
    > sometimes the whole SoC). Entering a coupled power state must
    > be tightly controlled on both cpus.
    >
    > The easiest solution to implementing coupled cpu power states is
    > to hotplug all but one cpu whenever possible, usually using a
    > cpufreq governor that looks at cpu load to determine when to
    > enable the secondary cpus. This causes problems, as hotplug is an
    > expensive operation, so the number of hotplug transitions must be
    > minimized, leading to very slow response to loads, often on the
    > order of seconds.
    >
    > This patch series implements an alternative solution, where each
    > cpu will wait in the WFI state until all cpus are ready to enter
    > a coupled state, at which point the coupled state function will
    > be called on all cpus at approximately the same time.
    >
    > Once all cpus are ready to enter idle, they are woken by an smp
    > cross call. At this point, there is a chance that one of the
    > cpus will find work to do, and choose not to enter suspend. A
    > final pass is needed to guarantee that all cpus will call the
    > power state enter function at the same time. During this pass,
    > each cpu will increment the ready counter, and continue once the
    > ready counter matches the number of online coupled cpus. If any
    > cpu exits idle, the other cpus will decrement their counter and
    > retry.

    this smells fundamentally racey to me; you can get an interrupt one
    cycle after you think you're done, but before the last guy enters WFI...

    how do you solve that issue ?




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