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_UNUSED:
  26         case CLOCK_EVT_MODE_SHUTDOWN:
  27                 /*
  28                  * Note, we cannot cancel the timer here as we might
  29                  * run into the following live lock scenario:
  30                  *
  31                  * cpu 0                cpu1
  32                  * lock(broadcast_lock);
  33                  *                      hrtimer_interrupt()
  34                  *                      bc_handler()
  35                  *                         tick_handle_oneshot_broadcast();
  36                  *                          lock(broadcast_lock);
  37                  * hrtimer_cancel()
  38                  *  wait_for_callback()
  39                  */
  40                 hrtimer_try_to_cancel(&bctimer);
  41                 break;
  42         default:
  43                 break;
  44         }
  45 }
  46
  47 /*
  48  * This is called from the guts of the broadcast code when the cpu
  49  * which is about to enter idle has the earliest broadcast timer event.
  50  */
  51 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  52 {
  53         int bc_moved;
  54         /*
  55          * We try to cancel the timer first. If the callback is on
  56          * flight on some other cpu then we let it handle it. If we
  57          * were able to cancel the timer nothing can rearm it as we
  58          * own broadcast_lock.
  59          *
  60          * However we can also be called from the event handler of
  61          * ce_broadcast_hrtimer itself when it expires. We cannot
  62          * restart the timer because we are in the callback, but we
  63          * can set the expiry time and let the callback return
  64          * HRTIMER_RESTART.
  65          *
  66          * Since we are in the idle loop at this point and because
  67          * hrtimer_{start/cancel} functions call into tracing,
  68          * calls to these functions must be bound within RCU_NONIDLE.
  69          */
  70         RCU_NONIDLE({
  71                         bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
  72                         if (bc_moved)
  73                                 hrtimer_start(&bctimer, expires,
  74                                               HRTIMER_MODE_ABS_PINNED);});
  75         if (bc_moved) {
  76                 /* Bind the "device" to the cpu */
  77                 bc->bound_on = smp_processor_id();
  78         } else if (bc->bound_on == smp_processor_id()) {
  79                 hrtimer_set_expires(&bctimer, expires);
  80         }
  81         return 0;
  82 }
  83
  84 static struct clock_event_device ce_broadcast_hrtimer = {
  85         .set_mode               = bc_set_mode,
  86         .set_next_ktime         = bc_set_next,
  87         .features               = CLOCK_EVT_FEAT_ONESHOT |
  88                                   CLOCK_EVT_FEAT_KTIME |
  89                                   CLOCK_EVT_FEAT_HRTIMER,
  90         .rating                 = 0,
  91         .bound_on               = -1,
  92         .min_delta_ns           = 1,
  93         .max_delta_ns           = KTIME_MAX,
  94         .min_delta_ticks        = 1,
  95         .max_delta_ticks        = ULONG_MAX,
  96         .mult                   = 1,
  97         .shift                  = 0,
  98         .cpumask                = cpu_all_mask,
  99 };
 100
 101 static enum hrtimer_restart bc_handler(struct hrtimer *t)
 102 {
 103         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
 104
 105         switch (ce_broadcast_hrtimer.mode) {
 106         case CLOCK_EVT_MODE_ONESHOT:
 107                 if (ce_broadcast_hrtimer.next_event.tv64 != KTIME_MAX)
 108                         return HRTIMER_RESTART;
 109         default:
 110                 return HRTIMER_NORESTART;
 111         }
 112 }
 113
 114 void tick_setup_hrtimer_broadcast(void)
 115 {
 116         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 117         bctimer.function = bc_handler;
 118         clockevents_register_device(&ce_broadcast_hrtimer);
 119 }