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 */ 21 struct 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 */ use_pit(void)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 pit_timer_init(void)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 init_pit_clocksource(void)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 } 74 arch_initcall(init_pit_clocksource); 75 #endif /* !CONFIG_X86_64 */ 76