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 
20 static struct hrtimer bctimer;
21 
22 static 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  */
45 static 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 
78 static 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_all_mask,
94 };
95 
96 static 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 
107 void 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