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// SPDX-License-Identifier: GPL-2.0
/*
* 8253/PIT functions
*
*/
#include <linux/clockchips.h>
#include <linux/init.h>
#include <linux/timex.h>
#include <linux/i8253.h>
#include <asm/hypervisor.h>
#include <asm/apic.h>
#include <asm/hpet.h>
#include <asm/time.h>
#include <asm/smp.h>
/*
* HPET replaces the PIT, when enabled. So we need to know, which of
* the two timers is used
*/
struct clock_event_device *global_clock_event;
/*
* Modern chipsets can disable the PIT clock which makes it unusable. It
* would be possible to enable the clock but the registers are chipset
* specific and not discoverable. Avoid the whack a mole game.
*
* These platforms have discoverable TSC/CPU frequencies but this also
* requires to know the local APIC timer frequency as it normally is
* calibrated against the PIT interrupt.
*/
static bool __init use_pit(void)
{
if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
return true;
/* This also returns true when APIC is disabled */
return apic_needs_pit();
}
bool __init pit_timer_init(void)
{
if (!use_pit()) {
/*
* Don't just ignore the PIT. Ensure it's stopped, because
* VMMs otherwise steal CPU time just to pointlessly waggle
* the (masked) IRQ.
*/
clockevent_i8253_disable();
return false;
}
clockevent_i8253_init(true);
global_clock_event = &i8253_clockevent;
return true;
}
#ifndef CONFIG_X86_64
static int __init init_pit_clocksource(void)
{
/*
* Several reasons not to register PIT as a clocksource:
*
* - On SMP PIT does not scale due to i8253_lock
* - when HPET is enabled
* - when local APIC timer is active (PIT is switched off)
*/
if (num_possible_cpus() > 1 || is_hpet_enabled() ||
!clockevent_state_periodic(&i8253_clockevent))
return 0;
return clocksource_i8253_init();
}
arch_initcall(init_pit_clocksource);
#endif /* !CONFIG_X86_64 */
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