kernel/time/tick-broadcast-hrtimer.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Emulate a local clock event device via a pseudo clock device.
   4  */
   5 #include <linux/cpu.h>
   6 #include <linux/err.h>
   7 #include <linux/hrtimer.h>
   8 #include <linux/interrupt.h>
   9 #include <linux/percpu.h>
  10 #include <linux/profile.h>
  11 #include <linux/clockchips.h>
  12 #include <linux/sched.h>
  13 #include <linux/smp.h>
  14 #include <linux/module.h>
  15
  16 #include "tick-internal.h"
  17
  18 static struct hrtimer bctimer;
  19
  20 static int bc_shutdown(struct clock_event_device *evt)
  21 {
  22         /*
  23          * Note, we cannot cancel the timer here as we might
  24          * run into the following live lock scenario:
  25          *
  26          * cpu 0                cpu1
  27          * lock(broadcast_lock);
  28          *                      hrtimer_interrupt()
  29          *                      bc_handler()
  30          *                         tick_handle_oneshot_broadcast();
  31          *                          lock(broadcast_lock);
  32          * hrtimer_cancel()
  33          *  wait_for_callback()
  34          */
  35         hrtimer_try_to_cancel(&bctimer);
  36         return 0;
  37 }
  38
  39 /*
  40  * This is called from the guts of the broadcast code when the cpu
  41  * which is about to enter idle has the earliest broadcast timer event.
  42  */
  43 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  44 {
  45         int bc_moved;
  46         /*
  47          * We try to cancel the timer first. If the callback is on
  48          * flight on some other cpu then we let it handle it. If we
  49          * were able to cancel the timer nothing can rearm it as we
  50          * own broadcast_lock.
  51          *
  52          * However we can also be called from the event handler of
  53          * ce_broadcast_hrtimer itself when it expires. We cannot
  54          * restart the timer because we are in the callback, but we
  55          * can set the expiry time and let the callback return
  56          * HRTIMER_RESTART.
  57          *
  58          * Since we are in the idle loop at this point and because
  59          * hrtimer_{start/cancel} functions call into tracing,
  60          * calls to these functions must be bound within RCU_NONIDLE.
  61          */
  62         RCU_NONIDLE({
  63                         bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
  64                         if (bc_moved)
  65                                 hrtimer_start(&bctimer, expires,
  66                                               HRTIMER_MODE_ABS_PINNED);});
  67         if (bc_moved) {
  68                 /* Bind the "device" to the cpu */
  69                 bc->bound_on = smp_processor_id();
  70         } else if (bc->bound_on == smp_processor_id()) {
  71                 hrtimer_set_expires(&bctimer, expires);
  72         }
  73         return 0;
  74 }
  75
  76 static struct clock_event_device ce_broadcast_hrtimer = {
  77         .name                   = "bc_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_possible_mask,
  92 };
  93
  94 static 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 != KTIME_MAX)
 100                         return HRTIMER_RESTART;
 101
 102         return HRTIMER_NORESTART;
 103 }
 104
 105 void 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 }