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 callbback 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 }