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
bc_shutdown(struct clock_event_device * evt)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 */
bc_set_next(ktime_t expires,struct clock_event_device * bc)45 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
46 {
47 /*
48 * This is called either from enter/exit idle code or from the
49 * broadcast handler. In all cases tick_broadcast_lock is held.
50 *
51 * hrtimer_cancel() cannot be called here neither from the
52 * broadcast handler nor from the enter/exit idle code. The idle
53 * code can run into the problem described in bc_shutdown() and the
54 * broadcast handler cannot wait for itself to complete for obvious
55 * reasons.
56 *
57 * Each caller tries to arm the hrtimer on its own CPU, but if the
58 * hrtimer callbback function is currently running, then
59 * hrtimer_start() cannot move it and the timer stays on the CPU on
60 * which it is assigned at the moment.
61 *
62 * As this can be called from idle code, the hrtimer_start()
63 * invocation has to be wrapped with RCU_NONIDLE() as
64 * hrtimer_start() can call into tracing.
65 */
66 RCU_NONIDLE( {
67 hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED);
68 /*
69 * The core tick broadcast mode expects bc->bound_on to be set
70 * correctly to prevent a CPU which has the broadcast hrtimer
71 * armed from going deep idle.
72 *
73 * As tick_broadcast_lock is held, nothing can change the cpu
74 * base which was just established in hrtimer_start() above. So
75 * the below access is safe even without holding the hrtimer
76 * base lock.
77 */
78 bc->bound_on = bctimer.base->cpu_base->cpu;
79 } );
80 return 0;
81 }
82
83 static struct clock_event_device ce_broadcast_hrtimer = {
84 .name = "bc_hrtimer",
85 .set_state_shutdown = bc_shutdown,
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
bc_handler(struct hrtimer * t)101 static enum hrtimer_restart bc_handler(struct hrtimer *t)
102 {
103 ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
104
105 return HRTIMER_NORESTART;
106 }
107
tick_setup_hrtimer_broadcast(void)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