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(oneshot: 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 */ |
68 | |