1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Emulate a local clock event device via a pseudo clock device.
4  */
5 #include <linux/cpu.h>
6 #include <linux/err.h>
7 #include <linux/hrtimer.h>
8 #include <linux/interrupt.h>
9 #include <linux/percpu.h>
10 #include <linux/profile.h>
11 #include <linux/clockchips.h>
12 #include <linux/sched.h>
13 #include <linux/smp.h>
14 #include <linux/module.h>
15 
16 #include "tick-internal.h"
17 
18 static struct hrtimer bctimer;
19 
20 static int bc_shutdown(struct clock_event_device *evt)
21 {
22 	/*
23 	 * Note, we cannot cancel the timer here as we might
24 	 * run into the following live lock scenario:
25 	 *
26 	 * cpu 0		cpu1
27 	 * lock(broadcast_lock);
28 	 *			hrtimer_interrupt()
29 	 *			bc_handler()
30 	 *			   tick_handle_oneshot_broadcast();
31 	 *			    lock(broadcast_lock);
32 	 * hrtimer_cancel()
33 	 *  wait_for_callback()
34 	 */
35 	hrtimer_try_to_cancel(&bctimer);
36 	return 0;
37 }
38 
39 /*
40  * This is called from the guts of the broadcast code when the cpu
41  * which is about to enter idle has the earliest broadcast timer event.
42  */
43 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
44 {
45 	int bc_moved;
46 	/*
47 	 * We try to cancel the timer first. If the callback is on
48 	 * flight on some other cpu then we let it handle it. If we
49 	 * were able to cancel the timer nothing can rearm it as we
50 	 * own broadcast_lock.
51 	 *
52 	 * However we can also be called from the event handler of
53 	 * ce_broadcast_hrtimer itself when it expires. We cannot
54 	 * restart the timer because we are in the callback, but we
55 	 * can set the expiry time and let the callback return
56 	 * HRTIMER_RESTART.
57 	 *
58 	 * Since we are in the idle loop at this point and because
59 	 * hrtimer_{start/cancel} functions call into tracing,
60 	 * calls to these functions must be bound within RCU_NONIDLE.
61 	 */
62 	RCU_NONIDLE({
63 			bc_moved = hrtimer_try_to_cancel(&bctimer) >= 0;
64 			if (bc_moved)
65 				hrtimer_start(&bctimer, expires,
66 					      HRTIMER_MODE_ABS_PINNED);});
67 	if (bc_moved) {
68 		/* Bind the "device" to the cpu */
69 		bc->bound_on = smp_processor_id();
70 	} else if (bc->bound_on == smp_processor_id()) {
71 		hrtimer_set_expires(&bctimer, expires);
72 	}
73 	return 0;
74 }
75 
76 static struct clock_event_device ce_broadcast_hrtimer = {
77 	.name			= "bc_hrtimer",
78 	.set_state_shutdown	= bc_shutdown,
79 	.set_next_ktime		= bc_set_next,
80 	.features		= CLOCK_EVT_FEAT_ONESHOT |
81 				  CLOCK_EVT_FEAT_KTIME |
82 				  CLOCK_EVT_FEAT_HRTIMER,
83 	.rating			= 0,
84 	.bound_on		= -1,
85 	.min_delta_ns		= 1,
86 	.max_delta_ns		= KTIME_MAX,
87 	.min_delta_ticks	= 1,
88 	.max_delta_ticks	= ULONG_MAX,
89 	.mult			= 1,
90 	.shift			= 0,
91 	.cpumask		= cpu_possible_mask,
92 };
93 
94 static enum hrtimer_restart bc_handler(struct hrtimer *t)
95 {
96 	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
97 
98 	if (clockevent_state_oneshot(&ce_broadcast_hrtimer))
99 		if (ce_broadcast_hrtimer.next_event != KTIME_MAX)
100 			return HRTIMER_RESTART;
101 
102 	return HRTIMER_NORESTART;
103 }
104 
105 void tick_setup_hrtimer_broadcast(void)
106 {
107 	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
108 	bctimer.function = bc_handler;
109 	clockevents_register_device(&ce_broadcast_hrtimer);
110 }
111