i8253.c

 1// SPDX-License-Identifier: GPL-2.0
 2/*
 3 * 8253/PIT functions
 4 *
 5 */
 6#include <linux/clockchips.h>
 7#include <linux/init.h>
 8#include <linux/timex.h>
 9#include <linux/i8253.h>
10
11#include <asm/hypervisor.h>
12#include <asm/apic.h>
13#include <asm/hpet.h>
14#include <asm/time.h>
15#include <asm/smp.h>
16
17/*
18 * HPET replaces the PIT, when enabled. So we need to know, which of
19 * the two timers is used
20 */
21struct clock_event_device *global_clock_event;
22
23/*
24 * Modern chipsets can disable the PIT clock which makes it unusable. It
25 * would be possible to enable the clock but the registers are chipset
26 * specific and not discoverable. Avoid the whack a mole game.
27 *
28 * These platforms have discoverable TSC/CPU frequencies but this also
29 * requires to know the local APIC timer frequency as it normally is
30 * calibrated against the PIT interrupt.
31 */
32static bool __init use_pit(void)
33{
34	if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
35		return true;
36
37	/* This also returns true when APIC is disabled */
38	return apic_needs_pit();
39}
40
41bool __init pit_timer_init(void)
42{
43	if (!use_pit()) {
44		/*
45		 * Don't just ignore the PIT. Ensure it's stopped, because
46		 * VMMs otherwise steal CPU time just to pointlessly waggle
47		 * the (masked) IRQ.
48		 */
49		scoped_guard(irq)
50			clockevent_i8253_disable();
51		return false;
52	}
53	clockevent_i8253_init(true);
54	global_clock_event = &i8253_clockevent;
55	return true;
56}
57
58#ifndef CONFIG_X86_64
59static int __init init_pit_clocksource(void)
60{
61	 /*
62	  * Several reasons not to register PIT as a clocksource:
63	  *
64	  * - On SMP PIT does not scale due to i8253_lock
65	  * - when HPET is enabled
66	  * - when local APIC timer is active (PIT is switched off)
67	  */
68	if (num_possible_cpus() > 1 || is_hpet_enabled() ||
69	    !clockevent_state_periodic(&i8253_clockevent))
70		return 0;
71
72	return clocksource_i8253_init();
73}
74arch_initcall(init_pit_clocksource);
75#endif /* !CONFIG_X86_64 */