LXR linux/kernel/time/tick-broadcast-hrtimer.c

   1/*
   2 * linux/kernel/time/tick-broadcast-hrtimer.c
   3 * This file emulates a local clock event device
   4 * via a pseudo clock device.
   5 */
   6#include <linux/cpu.h>
   7#include <linux/err.h>
   8#include <linux/hrtimer.h>
   9#include <linux/interrupt.h>
  10#include <linux/percpu.h>
  11#include <linux/profile.h>
  12#include <linux/clockchips.h>
  13#include <linux/sched.h>
  14#include <linux/smp.h>
  15#include <linux/module.h>
  16
  17#include "tick-internal.h"
  18
  19static struct hrtimer bctimer;
  20
  21static int bc_shutdown(struct clock_event_device *evt)
  22{
  23        /*
  24         * Note, we cannot cancel the timer here as we might
  25         * run into the following live lock scenario:
  26         *
  27         * cpu 0                cpu1
  28         * lock(broadcast_lock);
  29         *                      hrtimer_interrupt()
  30         *                      bc_handler()
  31         *                         tick_handle_oneshot_broadcast();
  32         *                          lock(broadcast_lock);
  33         * hrtimer_cancel()
  34         *  wait_for_callback()
  35         */
  36        hrtimer_try_to_cancel(&bctimer);
  37        return 0;
  38}
  39
  40/*
  41 * This is called from the guts of the broadcast code when the cpu
  42 * which is about to enter idle has the earliest broadcast timer event.
  43 */
  44static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  45{
  46        int bc_moved;
  47        /*
  48         * We try to cancel the timer first. If the callback is on
  49         * flight on some other cpu then we let it handle it. If we
  50         * were able to cancel the timer nothing can rearm it as we
  51         * own broadcast_lock.
  52         *
  53         * However we can also be called from the event handler of
  54         * ce_broadcast_hrtimer itself when it expires. We cannot
  55         * restart the timer because we are in the callback, but we
  56         * can set the expiry time and let the callback return
  57         * HRTIMER_RESTART.
  58         *
  59         * Since we are in the idle loop at this point and because
  60         * hrtimer_{start/cancel} functions call into tracing,
  61         * calls to these functions must be bound within RCU_NONIDLE.
  62         */
  63        RCU_NONIDLE({
  64                        bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
  65                        if (bc_moved)
  66                                hrtimer_start(&bctimer, expires,
  67                                              HRTIMER_MODE_ABS_PINNED);});
  68        if (bc_moved) {
  69                /* Bind the "device" to the cpu */
  70                bc->bound_on = smp_processor_id();
  71        } else if (bc->bound_on == smp_processor_id()) {
  72                hrtimer_set_expires(&bctimer, expires);
  73        }
  74        return 0;
  75}
  76
  77static struct clock_event_device ce_broadcast_hrtimer = {
  78        .set_state_shutdown     = bc_shutdown,
  79        .set_next_ktime         = bc_set_next,
  80        .features               = CLOCK_EVT_FEAT_ONESHOT |
  81                                  CLOCK_EVT_FEAT_KTIME |
  82                                  CLOCK_EVT_FEAT_HRTIMER,
  83        .rating                 = 0,
  84        .bound_on               = -1,
  85        .min_delta_ns           = 1,
  86        .max_delta_ns           = KTIME_MAX,
  87        .min_delta_ticks        = 1,
  88        .max_delta_ticks        = ULONG_MAX,
  89        .mult                   = 1,
  90        .shift                  = 0,
  91        .cpumask                = cpu_all_mask,
  92};
  93
  94static enum hrtimer_restart bc_handler(struct hrtimer *t)
  95{
  96        ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
  97
  98        if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
  99                if (ce_broadcast_hrtimer.next_event.tv64 != KTIME_MAX)
 100                        return HRTIMER_RESTART;
 101
 102        return HRTIMER_NORESTART;
 103}
 104
 105void tick_setup_hrtimer_broadcast(void)
 106{
 107        hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 108        bctimer.function = bc_handler;
 109        clockevents_register_device(&ce_broadcast_hrtimer);
 110}
 111