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 */ 32 static 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 41 bool __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 clockevent_i8253_disable(); 50 return false; 51 } 52 clockevent_i8253_init(true); 53 global_clock_event = &i8253_clockevent; 54 return true; 55 } 56 57 #ifndef CONFIG_X86_64 58 static int __init init_pit_clocksource(void) 59 { 60 /* 61 * Several reasons not to register PIT as a clocksource: 62 * 63 * - On SMP PIT does not scale due to i8253_lock 64 * - when HPET is enabled 65 * - when local APIC timer is active (PIT is switched off) 66 */ 67 if (num_possible_cpus() > 1 || is_hpet_enabled() || 68 !clockevent_state_periodic(&i8253_clockevent)) 69 return 0; 70 71 return clocksource_i8253_init(); 72 } 73 arch_initcall(init_pit_clocksource); 74 #endif /* !CONFIG_X86_64 */ 75