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
  19 static struct hrtimer bctimer;
  20
  21 static void bc_set_mode(enum clock_event_mode mode,
  22                         struct clock_event_device *bc)
  23 {
  24         switch (mode) {
  25         case CLOCK_EVT_MODE_SHUTDOWN:
  26                 /*
  27                  * Note, we cannot cancel the timer here as we might
  28                  * run into the following live lock scenario:
  29                  *
  30                  * cpu 0                cpu1
  31                  * lock(broadcast_lock);
  32                  *                      hrtimer_interrupt()
  33                  *                      bc_handler()
  34                  *                         tick_handle_oneshot_broadcast();
  35                  *                          lock(broadcast_lock);
  36                  * hrtimer_cancel()
  37                  *  wait_for_callback()
  38                  */
  39                 hrtimer_try_to_cancel(&bctimer);
  40                 break;
  41         default:
  42                 break;
  43         }
  44 }
  45
  46 /*
  47  * This is called from the guts of the broadcast code when the cpu
  48  * which is about to enter idle has the earliest broadcast timer event.
  49  */
  50 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  51 {
  52         int bc_moved;
  53         /*
  54          * We try to cancel the timer first. If the callback is on
  55          * flight on some other cpu then we let it handle it. If we
  56          * were able to cancel the timer nothing can rearm it as we
  57          * own broadcast_lock.
  58          *
  59          * However we can also be called from the event handler of
  60          * ce_broadcast_hrtimer itself when it expires. We cannot
  61          * restart the timer because we are in the callback, but we
  62          * can set the expiry time and let the callback return
  63          * HRTIMER_RESTART.
  64          *
  65          * Since we are in the idle loop at this point and because
  66          * hrtimer_{start/cancel} functions call into tracing,
  67          * calls to these functions must be bound within RCU_NONIDLE.
  68          */
  69         RCU_NONIDLE(bc_moved = (hrtimer_try_to_cancel(&bctimer) >= 0) ?
  70                 !hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED) :
  71                         0);
  72         if (bc_moved) {
  73                 /* Bind the "device" to the cpu */
  74                 bc->bound_on = smp_processor_id();
  75         } else if (bc->bound_on == smp_processor_id()) {
  76                 hrtimer_set_expires(&bctimer, expires);
  77         }
  78         return 0;
  79 }
  80
  81 static struct clock_event_device ce_broadcast_hrtimer = {
  82         .set_mode               = bc_set_mode,
  83         .set_next_ktime         = bc_set_next,
  84         .features               = CLOCK_EVT_FEAT_ONESHOT |
  85                                   CLOCK_EVT_FEAT_KTIME |
  86                                   CLOCK_EVT_FEAT_HRTIMER,
  87         .rating                 = 0,
  88         .bound_on               = -1,
  89         .min_delta_ns           = 1,
  90         .max_delta_ns           = KTIME_MAX,
  91         .min_delta_ticks        = 1,
  92         .max_delta_ticks        = ULONG_MAX,
  93         .mult                   = 1,
  94         .shift                  = 0,
  95         .cpumask                = cpu_all_mask,
  96 };
  97
  98 static enum hrtimer_restart bc_handler(struct hrtimer *t)
  99 {
 100         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
 101
 102         if (ce_broadcast_hrtimer.next_event.tv64 == KTIME_MAX)
 103                 return HRTIMER_NORESTART;
 104
 105         return HRTIMER_RESTART;
 106 }
 107
 108 void tick_setup_hrtimer_broadcast(void)
 109 {
 110         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 111         bctimer.function = bc_handler;
 112         clockevents_register_device(&ce_broadcast_hrtimer);
 113 }