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)31static 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)40bool __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)51static 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