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
 18static struct hrtimer bctimer;
 19
 20static 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 */
 43static 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
 81static 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
 99static 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
106void 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}

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