xref: /linux/kernel/time/tick-broadcast-hrtimer.c (revision e8d235d4d8fb8957bae5f6ed4521115203a00d8b)
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 void bc_set_mode(enum clock_event_mode mode,
22 			struct clock_event_device *bc)
23 {
24 	switch (mode) {
25 	case CLOCK_EVT_MODE_SHUTDOWN:
26 		/*
27 		 * Note, we cannot cancel the timer here as we might
28 		 * run into the following live lock scenario:
29 		 *
30 		 * cpu 0		cpu1
31 		 * lock(broadcast_lock);
32 		 *			hrtimer_interrupt()
33 		 *			bc_handler()
34 		 *			   tick_handle_oneshot_broadcast();
35 		 *			    lock(broadcast_lock);
36 		 * hrtimer_cancel()
37 		 *  wait_for_callback()
38 		 */
39 		hrtimer_try_to_cancel(&bctimer);
40 		break;
41 	default:
42 		break;
43 	}
44 }
45 
46 /*
47  * This is called from the guts of the broadcast code when the cpu
48  * which is about to enter idle has the earliest broadcast timer event.
49  */
50 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
51 {
52 	int bc_moved;
53 	/*
54 	 * We try to cancel the timer first. If the callback is on
55 	 * flight on some other cpu then we let it handle it. If we
56 	 * were able to cancel the timer nothing can rearm it as we
57 	 * own broadcast_lock.
58 	 *
59 	 * However we can also be called from the event handler of
60 	 * ce_broadcast_hrtimer itself when it expires. We cannot
61 	 * restart the timer because we are in the callback, but we
62 	 * can set the expiry time and let the callback return
63 	 * HRTIMER_RESTART.
64 	 *
65 	 * Since we are in the idle loop at this point and because
66 	 * hrtimer_{start/cancel} functions call into tracing,
67 	 * calls to these functions must be bound within RCU_NONIDLE.
68 	 */
69 	RCU_NONIDLE(bc_moved = (hrtimer_try_to_cancel(&bctimer) >= 0) ?
70 		!hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED) :
71 			0);
72 	if (bc_moved) {
73 		/* Bind the "device" to the cpu */
74 		bc->bound_on = smp_processor_id();
75 	} else if (bc->bound_on == smp_processor_id()) {
76 		hrtimer_set_expires(&bctimer, expires);
77 	}
78 	return 0;
79 }
80 
81 static struct clock_event_device ce_broadcast_hrtimer = {
82 	.set_mode		= bc_set_mode,
83 	.set_next_ktime		= bc_set_next,
84 	.features		= CLOCK_EVT_FEAT_ONESHOT |
85 				  CLOCK_EVT_FEAT_KTIME |
86 				  CLOCK_EVT_FEAT_HRTIMER,
87 	.rating			= 0,
88 	.bound_on		= -1,
89 	.min_delta_ns		= 1,
90 	.max_delta_ns		= KTIME_MAX,
91 	.min_delta_ticks	= 1,
92 	.max_delta_ticks	= ULONG_MAX,
93 	.mult			= 1,
94 	.shift			= 0,
95 	.cpumask		= cpu_all_mask,
96 };
97 
98 static enum hrtimer_restart bc_handler(struct hrtimer *t)
99 {
100 	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
101 
102 	if (ce_broadcast_hrtimer.next_event.tv64 == KTIME_MAX)
103 		return HRTIMER_NORESTART;
104 
105 	return HRTIMER_RESTART;
106 }
107 
108 void tick_setup_hrtimer_broadcast(void)
109 {
110 	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
111 	bctimer.function = bc_handler;
112 	clockevents_register_device(&ce_broadcast_hrtimer);
113 }
114