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 	/*
46 	 * This is called either from enter/exit idle code or from the
47 	 * broadcast handler. In all cases tick_broadcast_lock is held.
48 	 *
49 	 * hrtimer_cancel() cannot be called here neither from the
50 	 * broadcast handler nor from the enter/exit idle code. The idle
51 	 * code can run into the problem described in bc_shutdown() and the
52 	 * broadcast handler cannot wait for itself to complete for obvious
53 	 * reasons.
54 	 *
55 	 * Each caller tries to arm the hrtimer on its own CPU, but if the
56 	 * hrtimer callback function is currently running, then
57 	 * hrtimer_start() cannot move it and the timer stays on the CPU on
58 	 * which it is assigned at the moment.
59 	 *
60 	 * As this can be called from idle code, the hrtimer_start()
61 	 * invocation has to be wrapped with RCU_NONIDLE() as
62 	 * hrtimer_start() can call into tracing.
63 	 */
64 	RCU_NONIDLE( {
65 		hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
66 		/*
67 		 * The core tick broadcast mode expects bc->bound_on to be set
68 		 * correctly to prevent a CPU which has the broadcast hrtimer
69 		 * armed from going deep idle.
70 		 *
71 		 * As tick_broadcast_lock is held, nothing can change the cpu
72 		 * base which was just established in hrtimer_start() above. So
73 		 * the below access is safe even without holding the hrtimer
74 		 * base lock.
75 		 */
76 		bc->bound_on = bctimer.base->cpu_base->cpu;
77 	} );
78 	return 0;
79 }
80 
81 static struct clock_event_device ce_broadcast_hrtimer = {
82 	.name			= "bc_hrtimer",
83 	.set_state_shutdown	= bc_shutdown,
84 	.set_next_ktime		= bc_set_next,
85 	.features		= CLOCK_EVT_FEAT_ONESHOT |
86 				  CLOCK_EVT_FEAT_KTIME |
87 				  CLOCK_EVT_FEAT_HRTIMER,
88 	.rating			= 0,
89 	.bound_on		= -1,
90 	.min_delta_ns		= 1,
91 	.max_delta_ns		= KTIME_MAX,
92 	.min_delta_ticks	= 1,
93 	.max_delta_ticks	= ULONG_MAX,
94 	.mult			= 1,
95 	.shift			= 0,
96 	.cpumask		= cpu_possible_mask,
97 };
98 
99 static enum hrtimer_restart bc_handler(struct hrtimer *t)
100 {
101 	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
102 
103 	return HRTIMER_NORESTART;
104 }
105 
106 void tick_setup_hrtimer_broadcast(void)
107 {
108 	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
109 	bctimer.function = bc_handler;
110 	clockevents_register_device(&ce_broadcast_hrtimer);
111 }
112