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    SubjectRe: [PATCH 0/3] coupled cpuidle state support
    On 12/21/2011 01:09 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.
    >
    > To use coupled cpuidle states, a cpuidle driver must:
    >
    > Set struct cpuidle_device.coupled_cpus to the mask of all
    > coupled cpus, usually the same as cpu_possible_mask if all cpus
    > are part of the same cluster. The coupled_cpus mask must be
    > set in the struct cpuidle_device for each cpu.
    >
    > Set struct cpuidle_device.safe_state to a state that is not a
    > coupled state. This is usually WFI.
    >
    > Set CPUIDLE_FLAG_COUPLED in struct cpuidle_state.flags for each
    > state that affects multiple cpus.
    >
    > Provide a struct cpuidle_state.enter function for each state
    > that affects multiple cpus. This function is guaranteed to be
    > called on all cpus at approximately the same time. The driver
    > should ensure that the cpus all abort together if any cpu tries
    > to abort once the function is called.
    >
    > This series was functionally tested on v3.0, but has only been
    > compile-tested on v3.2 after the removal of per-cpu state fields.

    Hi Colin,

    this patchset could be interesting to resolve in a generic way the cpu
    dependencies.
    What is the status of this patchset ?

    Did you have the opportunity to measure the power consumption with and
    without this patchset ?

    Thanks
    -- Daniel

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