68 lines
1.6 KiB
C
68 lines
1.6 KiB
C
|
// 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/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())
|
||
|
|
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 */
|