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/apic.h>
12 #include <asm/hpet.h>
13 #include <asm/time.h>
14 #include <asm/smp.h>
15 
16 /*
17  * HPET replaces the PIT, when enabled. So we need to know, which of
18  * the two timers is used
19  */
20 struct clock_event_device *global_clock_event;
21 
22 /*
23  * Modern chipsets can disable the PIT clock which makes it unusable. It
24  * would be possible to enable the clock but the registers are chipset
25  * specific and not discoverable. Avoid the whack a mole game.
26  *
27  * These platforms have discoverable TSC/CPU frequencies but this also
28  * requires to know the local APIC timer frequency as it normally is
29  * calibrated against the PIT interrupt.
30  */
use_pit(void)31 static bool __init use_pit(void)
32 {
33 	if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
34 		return true;
35 
36 	/* This also returns true when APIC is disabled */
37 	return apic_needs_pit();
38 }
39 
pit_timer_init(void)40 bool __init pit_timer_init(void)
41 {
42 	if (!use_pit())
43 		return false;
44 
45 	clockevent_i8253_init(true);
46 	global_clock_event = &i8253_clockevent;
47 	return true;
48 }
49 
50 #ifndef CONFIG_X86_64
init_pit_clocksource(void)51 static int __init init_pit_clocksource(void)
52 {
53 	 /*
54 	  * Several reasons not to register PIT as a clocksource:
55 	  *
56 	  * - On SMP PIT does not scale due to i8253_lock
57 	  * - when HPET is enabled
58 	  * - when local APIC timer is active (PIT is switched off)
59 	  */
60 	if (num_possible_cpus() > 1 || is_hpet_enabled() ||
61 	    !clockevent_state_periodic(&i8253_clockevent))
62 		return 0;
63 
64 	return clocksource_i8253_init();
65 }
66 arch_initcall(init_pit_clocksource);
67 #endif /* !CONFIG_X86_64 */
68