sctp: add routing output fallback

[cascardo/linux.git] / drivers / cpuidle / coupled.c

/*
 * coupled.c - helper functions to enter the same idle state on multiple cpus
 *
 * Copyright (c) 2011 Google, Inc.
 *
 * Author: Colin Cross <ccross@android.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 */

#include <linux/kernel.h>
#include <linux/cpu.h>
#include <linux/cpuidle.h>
#include <linux/mutex.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/spinlock.h>

#include "cpuidle.h"

/**
 * DOC: Coupled cpuidle states
 *
 * 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.
 *
 * This file implements a 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 idle.  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.
 *
 * requested_state stores the deepest coupled idle state each cpu
 * is ready for.  It is assumed that the states are indexed from
 * shallowest (highest power, lowest exit latency) to deepest
 * (lowest power, highest exit latency).  The requested_state
 * variable is not locked.  It is only written from the cpu that
 * it stores (or by the on/offlining cpu if that cpu is offline),
 * and only read after all the cpus are ready for the coupled idle
 * state are are no longer updating it.
 *
 * Three atomic counters are used.  alive_count tracks the number
 * of cpus in the coupled set that are currently or soon will be
 * online.  waiting_count tracks the number of cpus that are in
 * the waiting loop, in the ready loop, or in the coupled idle state.
 * ready_count tracks the number of cpus that are in the ready loop
 * or in the coupled idle state.
 *
 * 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.  The function should return
 *    with interrupts still disabled.
 */

/**
 * struct cpuidle_coupled - data for set of cpus that share a coupled idle state
 * @coupled_cpus: mask of cpus that are part of the coupled set
 * @requested_state: array of requested states for cpus in the coupled set
 * @ready_waiting_counts: combined count of cpus  in ready or waiting loops
 * @online_count: count of cpus that are online
 * @refcnt: reference count of cpuidle devices that are using this struct
 * @prevent: flag to prevent coupled idle while a cpu is hotplugging
 */
struct cpuidle_coupled {
        cpumask_t coupled_cpus;
        int requested_state[NR_CPUS];
        atomic_t ready_waiting_counts;
        atomic_t abort_barrier;
        int online_count;
        int refcnt;
        int prevent;
};

#define WAITING_BITS 16
#define MAX_WAITING_CPUS (1 << WAITING_BITS)
#define WAITING_MASK (MAX_WAITING_CPUS - 1)
#define READY_MASK (~WAITING_MASK)

#define CPUIDLE_COUPLED_NOT_IDLE        (-1)

static DEFINE_MUTEX(cpuidle_coupled_lock);
static DEFINE_PER_CPU(struct call_single_data, cpuidle_coupled_poke_cb);

/*
 * The cpuidle_coupled_poke_pending mask is used to avoid calling
 * __smp_call_function_single with the per cpu call_single_data struct already
 * in use.  This prevents a deadlock where two cpus are waiting for each others
 * call_single_data struct to be available
 */
static cpumask_t cpuidle_coupled_poke_pending;

/*
 * The cpuidle_coupled_poked mask is used to ensure that each cpu has been poked
 * once to minimize entering the ready loop with a poke pending, which would
 * require aborting and retrying.
 */
static cpumask_t cpuidle_coupled_poked;

/**
 * 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 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();
        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();
}

/**
 * cpuidle_state_is_coupled - check if a state is part of a coupled set
 * @drv: struct cpuidle_driver for the platform
 * @state: index of the target state in drv->states
 *
 * Returns true if the target state is coupled with cpus besides this one
 */
bool cpuidle_state_is_coupled(struct cpuidle_driver *drv, int state)
{
        return drv->states[state].flags & CPUIDLE_FLAG_COUPLED;
}

/**
 * cpuidle_coupled_set_ready - mark a cpu as ready
 * @coupled: the struct coupled that contains the current cpu
 */
static inline void cpuidle_coupled_set_ready(struct cpuidle_coupled *coupled)
{
        atomic_add(MAX_WAITING_CPUS, &coupled->ready_waiting_counts);
}

/**
 * cpuidle_coupled_set_not_ready - mark a cpu as not ready
 * @coupled: the struct coupled that contains the current cpu
 *
 * Decrements the ready counter, unless the ready (and thus the waiting) counter
 * is equal to the number of online cpus.  Prevents a race where one cpu
 * decrements the waiting counter and then re-increments it just before another
 * cpu has decremented its ready counter, leading to the ready counter going
 * down from the number of online cpus without going through the coupled idle
 * state.
 *
 * Returns 0 if the counter was decremented successfully, -EINVAL if the ready
 * counter was equal to the number of online cpus.
 */
static
inline int cpuidle_coupled_set_not_ready(struct cpuidle_coupled *coupled)
{
        int all;
        int ret;

        all = coupled->online_count | (coupled->online_count << WAITING_BITS);
        ret = atomic_add_unless(&coupled->ready_waiting_counts,
                -MAX_WAITING_CPUS, all);

        return ret ? 0 : -EINVAL;
}

/**
 * cpuidle_coupled_no_cpus_ready - check if no cpus in a coupled set are ready
 * @coupled: the struct coupled that contains the current cpu
 *
 * Returns true if all of the cpus in a coupled set are out of the ready loop.
 */
static inline int cpuidle_coupled_no_cpus_ready(struct cpuidle_coupled *coupled)
{
        int r = atomic_read(&coupled->ready_waiting_counts) >> WAITING_BITS;
        return r == 0;
}

/**
 * cpuidle_coupled_cpus_ready - check if all cpus in a coupled set are ready
 * @coupled: the struct coupled that contains the current cpu
 *
 * Returns true if all cpus coupled to this target state are in the ready loop
 */
static inline bool cpuidle_coupled_cpus_ready(struct cpuidle_coupled *coupled)
{
        int r = atomic_read(&coupled->ready_waiting_counts) >> WAITING_BITS;
        return r == coupled->online_count;
}

/**
 * cpuidle_coupled_cpus_waiting - check if all cpus in a coupled set are waiting
 * @coupled: the struct coupled that contains the current cpu
 *
 * Returns true if all cpus coupled to this target state are in the wait loop
 */
static inline bool cpuidle_coupled_cpus_waiting(struct cpuidle_coupled *coupled)
{
        int w = atomic_read(&coupled->ready_waiting_counts) & WAITING_MASK;
        return w == coupled->online_count;
}

/**
 * cpuidle_coupled_no_cpus_waiting - check if no cpus in coupled set are waiting
 * @coupled: the struct coupled that contains the current cpu
 *
 * Returns true if all of the cpus in a coupled set are out of the waiting loop.
 */
static inline int cpuidle_coupled_no_cpus_waiting(struct cpuidle_coupled *coupled)
{
        int w = atomic_read(&coupled->ready_waiting_counts) & WAITING_MASK;
        return w == 0;
}

/**
 * cpuidle_coupled_get_state - determine the deepest idle state
 * @dev: struct cpuidle_device for this cpu
 * @coupled: the struct coupled that contains the current cpu
 *
 * Returns the deepest idle state that all coupled cpus can enter
 */
static inline int cpuidle_coupled_get_state(struct cpuidle_device *dev,
                struct cpuidle_coupled *coupled)
{
        int i;
        int state = INT_MAX;

        /*
         * Read barrier ensures that read of requested_state is ordered after
         * reads of ready_count.  Matches the write barriers
         * cpuidle_set_state_waiting.
         */
        smp_rmb();

        for_each_cpu(i, &coupled->coupled_cpus)
                if (cpu_online(i) && coupled->requested_state[i] < state)
                        state = coupled->requested_state[i];

        return state;
}

static void cpuidle_coupled_handle_poke(void *info)
{
        int cpu = (unsigned long)info;
        cpumask_set_cpu(cpu, &cpuidle_coupled_poked);
        cpumask_clear_cpu(cpu, &cpuidle_coupled_poke_pending);
}

/**
 * cpuidle_coupled_poke - wake up a cpu that may be waiting
 * @cpu: target cpu
 *
 * Ensures that the target cpu exits it's waiting idle state (if it is in it)
 * and will see updates to waiting_count before it re-enters it's waiting idle
 * state.
 *
 * If cpuidle_coupled_poked_mask is already set for the target cpu, that cpu
 * either has or will soon have a pending IPI that will wake it out of idle,
 * or it is currently processing the IPI and is not in idle.
 */
static void cpuidle_coupled_poke(int cpu)
{
        struct call_single_data *csd = &per_cpu(cpuidle_coupled_poke_cb, cpu);

        if (!cpumask_test_and_set_cpu(cpu, &cpuidle_coupled_poke_pending))
                smp_call_function_single_async(cpu, csd);
}

/**
 * cpuidle_coupled_poke_others - wake up all other cpus that may be waiting
 * @dev: struct cpuidle_device for this cpu
 * @coupled: the struct coupled that contains the current cpu
 *
 * Calls cpuidle_coupled_poke on all other online cpus.
 */
static void cpuidle_coupled_poke_others(int this_cpu,
                struct cpuidle_coupled *coupled)
{
        int cpu;

        for_each_cpu(cpu, &coupled->coupled_cpus)
                if (cpu != this_cpu && cpu_online(cpu))
                        cpuidle_coupled_poke(cpu);
}

/**
 * cpuidle_coupled_set_waiting - mark this cpu as in the wait loop
 * @dev: struct cpuidle_device for this cpu
 * @coupled: the struct coupled that contains the current cpu
 * @next_state: the index in drv->states of the requested state for this cpu
 *
 * Updates the requested idle state for the specified cpuidle device.
 * Returns the number of waiting cpus.
 */
static int cpuidle_coupled_set_waiting(int cpu,
                struct cpuidle_coupled *coupled, int next_state)
{
        coupled->requested_state[cpu] = next_state;

        /*
         * The atomic_inc_return provides a write barrier to order the write
         * to requested_state with the later write that increments ready_count.
         */
        return atomic_inc_return(&coupled->ready_waiting_counts) & WAITING_MASK;
}

/**
 * cpuidle_coupled_set_not_waiting - mark this cpu as leaving the wait loop
 * @dev: struct cpuidle_device for this cpu
 * @coupled: the struct coupled that contains the current cpu
 *
 * Removes the requested idle state for the specified cpuidle device.
 */
static void cpuidle_coupled_set_not_waiting(int cpu,
                struct cpuidle_coupled *coupled)
{
        /*
         * Decrementing waiting count can race with incrementing it in
         * cpuidle_coupled_set_waiting, but that's OK.  Worst case, some
         * cpus will increment ready_count and then spin until they
         * notice that this cpu has cleared it's requested_state.
         */
        atomic_dec(&coupled->ready_waiting_counts);

        coupled->requested_state[cpu] = CPUIDLE_COUPLED_NOT_IDLE;
}

/**
 * cpuidle_coupled_set_done - mark this cpu as leaving the ready loop
 * @cpu: the current cpu
 * @coupled: the struct coupled that contains the current cpu
 *
 * Marks this cpu as no longer in the ready and waiting loops.  Decrements
 * the waiting count first to prevent another cpu looping back in and seeing
 * this cpu as waiting just before it exits idle.
 */
static void cpuidle_coupled_set_done(int cpu, struct cpuidle_coupled *coupled)
{
        cpuidle_coupled_set_not_waiting(cpu, coupled);
        atomic_sub(MAX_WAITING_CPUS, &coupled->ready_waiting_counts);
}

/**
 * cpuidle_coupled_clear_pokes - spin until the poke interrupt is processed
 * @cpu - this cpu
 *
 * Turns on interrupts and spins until any outstanding poke interrupts have
 * been processed and the poke bit has been cleared.
 *
 * Other interrupts may also be processed while interrupts are enabled, so
 * need_resched() must be tested after this function returns to make sure
 * the interrupt didn't schedule work that should take the cpu out of idle.
 *
 * Returns 0 if no poke was pending, 1 if a poke was cleared.
 */
static int cpuidle_coupled_clear_pokes(int cpu)
{
        if (!cpumask_test_cpu(cpu, &cpuidle_coupled_poke_pending))
                return 0;

        local_irq_enable();
        while (cpumask_test_cpu(cpu, &cpuidle_coupled_poke_pending))
                cpu_relax();
        local_irq_disable();

        return 1;
}

static bool cpuidle_coupled_any_pokes_pending(struct cpuidle_coupled *coupled)
{
        cpumask_t cpus;
        int ret;

        cpumask_and(&cpus, cpu_online_mask, &coupled->coupled_cpus);
        ret = cpumask_and(&cpus, &cpuidle_coupled_poke_pending, &cpus);

        return ret;
}

/**
 * cpuidle_enter_state_coupled - attempt to enter a state with coupled cpus
 * @dev: struct cpuidle_device for the current cpu
 * @drv: struct cpuidle_driver for the platform
 * @next_state: index of the requested state in drv->states
 *
 * Coordinate with coupled cpus to enter the target state.  This is a two
 * stage process.  In the first stage, the cpus are operating independently,
 * and may call into cpuidle_enter_state_coupled at completely different times.
 * To save as much power as possible, the first cpus to call this function will
 * go to an intermediate state (the cpuidle_device's safe state), and wait for
 * all the other cpus to call this function.  Once all coupled cpus are idle,
 * the second stage will start.  Each coupled cpu will spin until all cpus have
 * guaranteed that they will call the target_state.
 *
 * This function must be called with interrupts disabled.  It may enable
 * interrupts while preparing for idle, and it will always return with
 * interrupts enabled.
 */
int cpuidle_enter_state_coupled(struct cpuidle_device *dev,
                struct cpuidle_driver *drv, int next_state)
{
        int entered_state = -1;
        struct cpuidle_coupled *coupled = dev->coupled;
        int w;

        if (!coupled)
                return -EINVAL;

        while (coupled->prevent) {
                cpuidle_coupled_clear_pokes(dev->cpu);
                if (need_resched()) {
                        local_irq_enable();
                        return entered_state;
                }
                entered_state = cpuidle_enter_state(dev, drv,
                        drv->safe_state_index);
                local_irq_disable();
        }

        /* Read barrier ensures online_count is read after prevent is cleared */
        smp_rmb();

reset:
        cpumask_clear_cpu(dev->cpu, &cpuidle_coupled_poked);

        w = cpuidle_coupled_set_waiting(dev->cpu, coupled, next_state);
        /*
         * If this is the last cpu to enter the waiting state, poke
         * all the other cpus out of their waiting state so they can
         * enter a deeper state.  This can race with one of the cpus
         * exiting the waiting state due to an interrupt and
         * decrementing waiting_count, see comment below.
         */
        if (w == coupled->online_count) {
                cpumask_set_cpu(dev->cpu, &cpuidle_coupled_poked);
                cpuidle_coupled_poke_others(dev->cpu, coupled);
        }

retry:
        /*
         * Wait for all coupled cpus to be idle, using the deepest state
         * allowed for a single cpu.  If this was not the poking cpu, wait
         * for at least one poke before leaving to avoid a race where
         * two cpus could arrive at the waiting loop at the same time,
         * but the first of the two to arrive could skip the loop without
         * processing the pokes from the last to arrive.
         */
        while (!cpuidle_coupled_cpus_waiting(coupled) ||
                        !cpumask_test_cpu(dev->cpu, &cpuidle_coupled_poked)) {
                if (cpuidle_coupled_clear_pokes(dev->cpu))
                        continue;

                if (need_resched()) {
                        cpuidle_coupled_set_not_waiting(dev->cpu, coupled);
                        goto out;
                }

                if (coupled->prevent) {
                        cpuidle_coupled_set_not_waiting(dev->cpu, coupled);
                        goto out;
                }

                entered_state = cpuidle_enter_state(dev, drv,
                        drv->safe_state_index);
                local_irq_disable();
        }

        cpuidle_coupled_clear_pokes(dev->cpu);
        if (need_resched()) {
                cpuidle_coupled_set_not_waiting(dev->cpu, coupled);
                goto out;
        }

        /*
         * Make sure final poke status for this cpu is visible before setting
         * cpu as ready.
         */
        smp_wmb();

        /*
         * All coupled cpus are probably idle.  There is a small chance that
         * one of the other cpus just became active.  Increment the ready count,
         * and spin until all coupled cpus have incremented the counter. Once a
         * cpu has incremented the ready counter, it cannot abort idle and must
         * spin until either all cpus have incremented the ready counter, or
         * another cpu leaves idle and decrements the waiting counter.
         */

        cpuidle_coupled_set_ready(coupled);
        while (!cpuidle_coupled_cpus_ready(coupled)) {
                /* Check if any other cpus bailed out of idle. */
                if (!cpuidle_coupled_cpus_waiting(coupled))
                        if (!cpuidle_coupled_set_not_ready(coupled))
                                goto retry;

                cpu_relax();
        }

        /*
         * Make sure read of all cpus ready is done before reading pending pokes
         */
        smp_rmb();

        /*
         * There is a small chance that a cpu left and reentered idle after this
         * cpu saw that all cpus were waiting.  The cpu that reentered idle will
         * have sent this cpu a poke, which will still be pending after the
         * ready loop.  The pending interrupt may be lost by the interrupt
         * controller when entering the deep idle state.  It's not possible to
         * clear a pending interrupt without turning interrupts on and handling
         * it, and it's too late to turn on interrupts here, so reset the
         * coupled idle state of all cpus and retry.
         */
        if (cpuidle_coupled_any_pokes_pending(coupled)) {
                cpuidle_coupled_set_done(dev->cpu, coupled);
                /* Wait for all cpus to see the pending pokes */
                cpuidle_coupled_parallel_barrier(dev, &coupled->abort_barrier);
                goto reset;
        }

        /* all cpus have acked the coupled state */
        next_state = cpuidle_coupled_get_state(dev, coupled);

        entered_state = cpuidle_enter_state(dev, drv, next_state);

        cpuidle_coupled_set_done(dev->cpu, coupled);

out:
        /*
         * Normal cpuidle states are expected to return with irqs enabled.
         * That leads to an inefficiency where a cpu receiving an interrupt
         * that brings it out of idle will process that interrupt before
         * exiting the idle enter function and decrementing ready_count.  All
         * other cpus will need to spin waiting for the cpu that is processing
         * the interrupt.  If the driver returns with interrupts disabled,
         * all other cpus will loop back into the safe idle state instead of
         * spinning, saving power.
         *
         * Calling local_irq_enable here allows coupled states to return with
         * interrupts disabled, but won't cause problems for drivers that
         * exit with interrupts enabled.
         */
        local_irq_enable();

        /*
         * Wait until all coupled cpus have exited idle.  There is no risk that
         * a cpu exits and re-enters the ready state because this cpu has
         * already decremented its waiting_count.
         */
        while (!cpuidle_coupled_no_cpus_ready(coupled))
                cpu_relax();

        return entered_state;
}

static void cpuidle_coupled_update_online_cpus(struct cpuidle_coupled *coupled)
{
        cpumask_t cpus;
        cpumask_and(&cpus, cpu_online_mask, &coupled->coupled_cpus);
        coupled->online_count = cpumask_weight(&cpus);
}

/**
 * cpuidle_coupled_register_device - register a coupled cpuidle device
 * @dev: struct cpuidle_device for the current cpu
 *
 * Called from cpuidle_register_device to handle coupled idle init.  Finds the
 * cpuidle_coupled struct for this set of coupled cpus, or creates one if none
 * exists yet.
 */
int cpuidle_coupled_register_device(struct cpuidle_device *dev)
{
        int cpu;
        struct cpuidle_device *other_dev;
        struct call_single_data *csd;
        struct cpuidle_coupled *coupled;

        if (cpumask_empty(&dev->coupled_cpus))
                return 0;

        for_each_cpu(cpu, &dev->coupled_cpus) {
                other_dev = per_cpu(cpuidle_devices, cpu);
                if (other_dev && other_dev->coupled) {
                        coupled = other_dev->coupled;
                        goto have_coupled;
                }
        }

        /* No existing coupled info found, create a new one */
        coupled = kzalloc(sizeof(struct cpuidle_coupled), GFP_KERNEL);
        if (!coupled)
                return -ENOMEM;

        coupled->coupled_cpus = dev->coupled_cpus;

have_coupled:
        dev->coupled = coupled;
        if (WARN_ON(!cpumask_equal(&dev->coupled_cpus, &coupled->coupled_cpus)))
                coupled->prevent++;

        cpuidle_coupled_update_online_cpus(coupled);

        coupled->refcnt++;

        csd = &per_cpu(cpuidle_coupled_poke_cb, dev->cpu);
        csd->func = cpuidle_coupled_handle_poke;
        csd->info = (void *)(unsigned long)dev->cpu;

        return 0;
}

/**
 * cpuidle_coupled_unregister_device - unregister a coupled cpuidle device
 * @dev: struct cpuidle_device for the current cpu
 *
 * Called from cpuidle_unregister_device to tear down coupled idle.  Removes the
 * cpu from the coupled idle set, and frees the cpuidle_coupled_info struct if
 * this was the last cpu in the set.
 */
void cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
{
        struct cpuidle_coupled *coupled = dev->coupled;

        if (cpumask_empty(&dev->coupled_cpus))
                return;

        if (--coupled->refcnt)
                kfree(coupled);
        dev->coupled = NULL;
}

/**
 * cpuidle_coupled_prevent_idle - prevent cpus from entering a coupled state
 * @coupled: the struct coupled that contains the cpu that is changing state
 *
 * Disables coupled cpuidle on a coupled set of cpus.  Used to ensure that
 * cpu_online_mask doesn't change while cpus are coordinating coupled idle.
 */
static void cpuidle_coupled_prevent_idle(struct cpuidle_coupled *coupled)
{
        int cpu = get_cpu();

        /* Force all cpus out of the waiting loop. */
        coupled->prevent++;
        cpuidle_coupled_poke_others(cpu, coupled);
        put_cpu();
        while (!cpuidle_coupled_no_cpus_waiting(coupled))
                cpu_relax();
}

/**
 * cpuidle_coupled_allow_idle - allows cpus to enter a coupled state
 * @coupled: the struct coupled that contains the cpu that is changing state
 *
 * Enables coupled cpuidle on a coupled set of cpus.  Used to ensure that
 * cpu_online_mask doesn't change while cpus are coordinating coupled idle.
 */
static void cpuidle_coupled_allow_idle(struct cpuidle_coupled *coupled)
{
        int cpu = get_cpu();

        /*
         * Write barrier ensures readers see the new online_count when they
         * see prevent == 0.
         */
        smp_wmb();
        coupled->prevent--;
        /* Force cpus out of the prevent loop. */
        cpuidle_coupled_poke_others(cpu, coupled);
        put_cpu();
}

/**
 * cpuidle_coupled_cpu_notify - notifier called during hotplug transitions
 * @nb: notifier block
 * @action: hotplug transition
 * @hcpu: target cpu number
 *
 * Called when a cpu is brought on or offline using hotplug.  Updates the
 * coupled cpu set appropriately
 */
static int cpuidle_coupled_cpu_notify(struct notifier_block *nb,
                unsigned long action, void *hcpu)
{
        int cpu = (unsigned long)hcpu;
        struct cpuidle_device *dev;

        switch (action & ~CPU_TASKS_FROZEN) {
        case CPU_UP_PREPARE:
        case CPU_DOWN_PREPARE:
        case CPU_ONLINE:
        case CPU_DEAD:
        case CPU_UP_CANCELED:
        case CPU_DOWN_FAILED:
                break;
        default:
                return NOTIFY_OK;
        }

        mutex_lock(&cpuidle_lock);

        dev = per_cpu(cpuidle_devices, cpu);
        if (!dev || !dev->coupled)
                goto out;

        switch (action & ~CPU_TASKS_FROZEN) {
        case CPU_UP_PREPARE:
        case CPU_DOWN_PREPARE:
                cpuidle_coupled_prevent_idle(dev->coupled);
                break;
        case CPU_ONLINE:
        case CPU_DEAD:
                cpuidle_coupled_update_online_cpus(dev->coupled);
                /* Fall through */
        case CPU_UP_CANCELED:
        case CPU_DOWN_FAILED:
                cpuidle_coupled_allow_idle(dev->coupled);
                break;
        }

out:
        mutex_unlock(&cpuidle_lock);
        return NOTIFY_OK;
}

static struct notifier_block cpuidle_coupled_cpu_notifier = {
        .notifier_call = cpuidle_coupled_cpu_notify,
};

static int __init cpuidle_coupled_init(void)
{
        return register_cpu_notifier(&cpuidle_coupled_cpu_notifier);
}
core_initcall(cpuidle_coupled_init);