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 */
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
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
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 */