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_UNUSED:
26 	case CLOCK_EVT_MODE_SHUTDOWN:
27 		/*
28 		 * Note, we cannot cancel the timer here as we might
29 		 * run into the following live lock scenario:
30 		 *
31 		 * cpu 0		cpu1
32 		 * lock(broadcast_lock);
33 		 *			hrtimer_interrupt()
34 		 *			bc_handler()
35 		 *			   tick_handle_oneshot_broadcast();
36 		 *			    lock(broadcast_lock);
37 		 * hrtimer_cancel()
38 		 *  wait_for_callback()
39 		 */
40 		hrtimer_try_to_cancel(&bctimer);
41 		break;
42 	default:
43 		break;
44 	}
45 }
46 
47 /*
48  * This is called from the guts of the broadcast code when the cpu
49  * which is about to enter idle has the earliest broadcast timer event.
50  */
51 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
52 {
53 	int bc_moved;
54 	/*
55 	 * We try to cancel the timer first. If the callback is on
56 	 * flight on some other cpu then we let it handle it. If we
57 	 * were able to cancel the timer nothing can rearm it as we
58 	 * own broadcast_lock.
59 	 *
60 	 * However we can also be called from the event handler of
61 	 * ce_broadcast_hrtimer itself when it expires. We cannot
62 	 * restart the timer because we are in the callback, but we
63 	 * can set the expiry time and let the callback return
64 	 * HRTIMER_RESTART.
65 	 *
66 	 * Since we are in the idle loop at this point and because
67 	 * hrtimer_{start/cancel} functions call into tracing,
68 	 * calls to these functions must be bound within RCU_NONIDLE.
69 	 */
70 	RCU_NONIDLE({
71 			bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
72 			if (bc_moved)
73 				hrtimer_start(&bctimer, expires,
74 					      HRTIMER_MODE_ABS_PINNED);});
75 	if (bc_moved) {
76 		/* Bind the "device" to the cpu */
77 		bc->bound_on = smp_processor_id();
78 	} else if (bc->bound_on == smp_processor_id()) {
79 		hrtimer_set_expires(&bctimer, expires);
80 	}
81 	return 0;
82 }
83 
84 static struct clock_event_device ce_broadcast_hrtimer = {
85 	.set_mode		= bc_set_mode,
86 	.set_next_ktime		= bc_set_next,
87 	.features		= CLOCK_EVT_FEAT_ONESHOT |
88 				  CLOCK_EVT_FEAT_KTIME |
89 				  CLOCK_EVT_FEAT_HRTIMER,
90 	.rating			= 0,
91 	.bound_on		= -1,
92 	.min_delta_ns		= 1,
93 	.max_delta_ns		= KTIME_MAX,
94 	.min_delta_ticks	= 1,
95 	.max_delta_ticks	= ULONG_MAX,
96 	.mult			= 1,
97 	.shift			= 0,
98 	.cpumask		= cpu_all_mask,
99 };
100 
101 static enum hrtimer_restart bc_handler(struct hrtimer *t)
102 {
103 	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
104 
105 	switch (ce_broadcast_hrtimer.mode) {
106 	case CLOCK_EVT_MODE_ONESHOT:
107 		if (ce_broadcast_hrtimer.next_event.tv64 != KTIME_MAX)
108 			return HRTIMER_RESTART;
109 	default:
110 		return HRTIMER_NORESTART;
111 	}
112 }
113 
114 void tick_setup_hrtimer_broadcast(void)
115 {
116 	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
117 	bctimer.function = bc_handler;
118 	clockevents_register_device(&ce_broadcast_hrtimer);
119 }
120