LXR linux/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
  20static struct hrtimer bctimer;
  21
  22static 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 */
  45static 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
  78static 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_possible_mask,
  94};
  95
  96static 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
 107void 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}
 113