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 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  */
  44 static 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
  77 static struct clock_event_device ce_broadcast_hrtimer = {
  78         .name                   = "bc_hrtimer",
  79         .set_state_shutdown     = bc_shutdown,
  80         .set_next_ktime         = bc_set_next,
  81         .features               = CLOCK_EVT_FEAT_ONESHOT |
  82                                   CLOCK_EVT_FEAT_KTIME |
  83                                   CLOCK_EVT_FEAT_HRTIMER,
  84         .rating                 = 0,
  85         .bound_on               = -1,
  86         .min_delta_ns           = 1,
  87         .max_delta_ns           = KTIME_MAX,
  88         .min_delta_ticks        = 1,
  89         .max_delta_ticks        = ULONG_MAX,
  90         .mult                   = 1,
  91         .shift                  = 0,
  92         .cpumask                = cpu_all_mask,
  93 };
  94
  95 static enum hrtimer_restart bc_handler(struct hrtimer *t)
  96 {
  97         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
  98
  99         if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
 100                 if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
 101                         return HRTIMER_RESTART;
 102
 103         return HRTIMER_NORESTART;
 104 }
 105
 106 void tick_setup_hrtimer_broadcast(void)
 107 {
 108         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 109         bctimer.function = bc_handler;
 110         clockevents_register_device(&ce_broadcast_hrtimer);
 111 }