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 }
112