arch/x86/kernel/i8253.c

   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/apic.h>
  12 #include <asm/hpet.h>
  13 #include <asm/time.h>
  14 #include <asm/smp.h>
  15
  16 /*
  17  * HPET replaces the PIT, when enabled. So we need to know, which of
  18  * the two timers is used
  19  */
  20 struct clock_event_device *global_clock_event;
  21
  22 /*
  23  * Modern chipsets can disable the PIT clock which makes it unusable. It
  24  * would be possible to enable the clock but the registers are chipset
  25  * specific and not discoverable. Avoid the whack a mole game.
  26  *
  27  * These platforms have discoverable TSC/CPU frequencies but this also
  28  * requires to know the local APIC timer frequency as it normally is
  29  * calibrated against the PIT interrupt.
  30  */
  31 static bool __init use_pit(void)
  32 {
  33         if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
  34                 return true;
  35
  36         /* This also returns true when APIC is disabled */
  37         return apic_needs_pit();
  38 }
  39
  40 bool __init pit_timer_init(void)
  41 {
  42         if (!use_pit())
  43                 return false;
  44
  45         clockevent_i8253_init(true);
  46         global_clock_event = &i8253_clockevent;
  47         return true;
  48 }
  49
  50 #ifndef CONFIG_X86_64
  51 static int __init init_pit_clocksource(void)
  52 {
  53          /*
  54           * Several reasons not to register PIT as a clocksource:
  55           *
  56           * - On SMP PIT does not scale due to i8253_lock
  57           * - when HPET is enabled
  58           * - when local APIC timer is active (PIT is switched off)
  59           */
  60         if (num_possible_cpus() > 1 || is_hpet_enabled() ||
  61             !clockevent_state_periodic(&i8253_clockevent))
  62                 return 0;
  63
  64         return clocksource_i8253_init();
  65 }
  66 arch_initcall(init_pit_clocksource);
  67 #endif /* !CONFIG_X86_64 */