kernel/time/tick-broadcast-hrtimer.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * linux/kernel/time/tick-broadcast-hrtimer.c
   4  * This file emulates a local clock event device
   5  * via a pseudo clock device.
   6  */
   7 #include <linux/cpu.h>
   8 #include <linux/err.h>
   9 #include <linux/hrtimer.h>
  10 #include <linux/interrupt.h>
  11 #include <linux/percpu.h>
  12 #include <linux/profile.h>
  13 #include <linux/clockchips.h>
  14 #include <linux/sched.h>
  15 #include <linux/smp.h>
  16 #include <linux/module.h>
  17
  18 #include "tick-internal.h"
  19
  20 static struct hrtimer bctimer;
  21
  22 static int bc_shutdown(struct clock_event_device *evt)
  23 {
  24         /*
  25          * Note, we cannot cancel the timer here as we might
  26          * run into the following live lock scenario:
  27          *
  28          * cpu 0                cpu1
  29          * lock(broadcast_lock);
  30          *                      hrtimer_interrupt()
  31          *                      bc_handler()
  32          *                         tick_handle_oneshot_broadcast();
  33          *                          lock(broadcast_lock);
  34          * hrtimer_cancel()
  35          *  wait_for_callback()
  36          */
  37         hrtimer_try_to_cancel(&bctimer);
  38         return 0;
  39 }
  40
  41 /*
  42  * This is called from the guts of the broadcast code when the cpu
  43  * which is about to enter idle has the earliest broadcast timer event.
  44  */
  45 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  46 {
  47         int bc_moved;
  48         /*
  49          * We try to cancel the timer first. If the callback is on
  50          * flight on some other cpu then we let it handle it. If we
  51          * were able to cancel the timer nothing can rearm it as we
  52          * own broadcast_lock.
  53          *
  54          * However we can also be called from the event handler of
  55          * ce_broadcast_hrtimer itself when it expires. We cannot
  56          * restart the timer because we are in the callback, but we
  57          * can set the expiry time and let the callback return
  58          * HRTIMER_RESTART.
  59          *
  60          * Since we are in the idle loop at this point and because
  61          * hrtimer_{start/cancel} functions call into tracing,
  62          * calls to these functions must be bound within RCU_NONIDLE.
  63          */
  64         RCU_NONIDLE({
  65                         bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
  66                         if (bc_moved)
  67                                 hrtimer_start(&bctimer, expires,
  68                                               HRTIMER_MODE_ABS_PINNED);});
  69         if (bc_moved) {
  70                 /* Bind the "device" to the cpu */
  71                 bc->bound_on = smp_processor_id();
  72         } else if (bc->bound_on == smp_processor_id()) {
  73                 hrtimer_set_expires(&bctimer, expires);
  74         }
  75         return 0;
  76 }
  77
  78 static struct clock_event_device ce_broadcast_hrtimer = {
  79         .name                   = "bc_hrtimer",
  80         .set_state_shutdown     = bc_shutdown,
  81         .set_next_ktime         = bc_set_next,
  82         .features               = CLOCK_EVT_FEAT_ONESHOT |
  83                                   CLOCK_EVT_FEAT_KTIME |
  84                                   CLOCK_EVT_FEAT_HRTIMER,
  85         .rating                 = 0,
  86         .bound_on               = -1,
  87         .min_delta_ns           = 1,
  88         .max_delta_ns           = KTIME_MAX,
  89         .min_delta_ticks        = 1,
  90         .max_delta_ticks        = ULONG_MAX,
  91         .mult                   = 1,
  92         .shift                  = 0,
  93         .cpumask                = cpu_all_mask,
  94 };
  95
  96 static enum hrtimer_restart bc_handler(struct hrtimer *t)
  97 {
  98         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
  99
 100         if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
 101                 if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
 102                         return HRTIMER_RESTART;
 103
 104         return HRTIMER_NORESTART;
 105 }
 106
 107 void tick_setup_hrtimer_broadcast(void)
 108 {
 109         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 110         bctimer.function = bc_handler;
 111         clockevents_register_device(&ce_broadcast_hrtimer);
 112 }