68 lines
1.6 KiB
C
68 lines
1.6 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* 8253/PIT functions
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*
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*/
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#include <linux/clockchips.h>
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#include <linux/init.h>
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#include <linux/timex.h>
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#include <linux/i8253.h>
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#include <asm/apic.h>
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#include <asm/hpet.h>
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#include <asm/time.h>
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#include <asm/smp.h>
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/*
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* HPET replaces the PIT, when enabled. So we need to know, which of
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* the two timers is used
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*/
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struct clock_event_device *global_clock_event;
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/*
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* Modern chipsets can disable the PIT clock which makes it unusable. It
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* would be possible to enable the clock but the registers are chipset
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* specific and not discoverable. Avoid the whack a mole game.
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*
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* These platforms have discoverable TSC/CPU frequencies but this also
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* requires to know the local APIC timer frequency as it normally is
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* calibrated against the PIT interrupt.
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*/
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static bool __init use_pit(void)
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{
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if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
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return true;
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/* This also returns true when APIC is disabled */
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return apic_needs_pit();
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}
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bool __init pit_timer_init(void)
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{
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if (!use_pit())
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return false;
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clockevent_i8253_init(true);
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global_clock_event = &i8253_clockevent;
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return true;
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}
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#ifndef CONFIG_X86_64
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static int __init init_pit_clocksource(void)
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{
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/*
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* Several reasons not to register PIT as a clocksource:
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*
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* - On SMP PIT does not scale due to i8253_lock
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* - when HPET is enabled
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* - when local APIC timer is active (PIT is switched off)
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*/
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if (num_possible_cpus() > 1 || is_hpet_enabled() ||
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!clockevent_state_periodic(&i8253_clockevent))
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return 0;
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return clocksource_i8253_init();
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}
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arch_initcall(init_pit_clocksource);
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#endif /* !CONFIG_X86_64 */
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