Line data    Source code

       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           1 : static bool __init use_pit(void)
      32             : {
      33           1 :         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           1 :         return apic_needs_pit();
      38             : }
      39             : 
      40           1 : bool __init pit_timer_init(void)
      41             : {
      42           1 :         if (!use_pit())
      43             :                 return false;
      44             : 
      45           0 :         clockevent_i8253_init(true);
      46           0 :         global_clock_event = &i8253_clockevent;
      47           0 :         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 */