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/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 */