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    SubjectRe: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function
    Date
    On Tuesday, May 08, 2012, Colin Cross wrote:
    > Adds cpuidle_coupled_parallel_barrier, which can be used by coupled
    > cpuidle state enter functions to handle resynchronization after
    > determining if any cpu needs to abort. The normal use case will
    > be:
    >
    > static bool abort_flag;
    > static atomic_t abort_barrier;
    >
    > int arch_cpuidle_enter(struct cpuidle_device *dev, ...)
    > {
    > if (arch_turn_off_irq_controller()) {
    > /* returns an error if an irq is pending and would be lost
    > if idle continued and turned off power */
    > abort_flag = true;
    > }
    >
    > cpuidle_coupled_parallel_barrier(dev, &abort_barrier);
    >
    > if (abort_flag) {
    > /* One of the cpus didn't turn off it's irq controller */
    > arch_turn_on_irq_controller();
    > return -EINTR;
    > }
    >
    > /* continue with idle */
    > ...
    > }
    >
    > This will cause all cpus to abort idle together if one of them needs
    > to abort.
    >
    > Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
    > Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
    > Reviewed-by: Kevin Hilman <khilman@ti.com>
    > Tested-by: Kevin Hilman <khilman@ti.com>
    > Signed-off-by: Colin Cross <ccross@android.com>
    > ---
    > drivers/cpuidle/coupled.c | 37 +++++++++++++++++++++++++++++++++++++
    > include/linux/cpuidle.h | 4 ++++
    > 2 files changed, 41 insertions(+), 0 deletions(-)
    >
    > diff --git a/drivers/cpuidle/coupled.c b/drivers/cpuidle/coupled.c
    > index 93101fb..3e65de1 100644
    > --- a/drivers/cpuidle/coupled.c
    > +++ b/drivers/cpuidle/coupled.c
    > @@ -130,6 +130,43 @@ struct cpuidle_coupled {
    > static cpumask_t cpuidle_coupled_poked_mask;
    >
    > /**
    > + * cpuidle_coupled_parallel_barrier - synchronize all online coupled cpus
    > + * @dev: cpuidle_device of the calling cpu
    > + * @a: atomic variable to hold the barrier
    > + *
    > + * No caller to this function will return from this function until all online
    > + * cpus in the same coupled group have called this function. Once any caller
    > + * has returned from this function, the barrier is immediately available for
    > + * reuse.
    > + *
    > + * The atomic variable a must be initialized to 0 before any cpu calls
    > + * this function, will be reset to 0 before any cpu returns from this function.
    > + *
    > + * Must only be called from within a coupled idle state handler
    > + * (state.enter when state.flags has CPUIDLE_FLAG_COUPLED set).
    > + *
    > + * Provides full smp barrier semantics before and after calling.
    > + */
    > +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a)
    > +{
    > + int n = dev->coupled->online_count;
    > +
    > + smp_mb__before_atomic_inc();
    > + atomic_inc(a);
    > +
    > + while (atomic_read(a) < n)
    > + cpu_relax();
    > +
    > + if (atomic_inc_return(a) == n * 2) {
    > + atomic_set(a, 0);
    > + return;
    > + }
    > +
    > + while (atomic_read(a) > n)
    > + cpu_relax();
    > +}

    Well, this looks like "wait until all CPUs execute this code". Don't we have
    anything like this already somewhere?

    > +
    > +/**
    > * cpuidle_state_is_coupled - check if a state is part of a coupled set
    > * @dev: struct cpuidle_device for the current cpu
    > * @drv: struct cpuidle_driver for the platform
    > diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
    > index 6038448..5ab7183 100644
    > --- a/include/linux/cpuidle.h
    > +++ b/include/linux/cpuidle.h
    > @@ -183,6 +183,10 @@ static inline int cpuidle_wrap_enter(struct cpuidle_device *dev,
    >
    > #endif
    >
    > +#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
    > +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a);
    > +#endif

    Why exactly is the extra Kconfig option necessary?

    > +
    > /******************************
    > * CPUIDLE GOVERNOR INTERFACE *
    > ******************************/

    Rafael


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