arch/i386/kernel/timers/timer.c

   1 #include <linux/init.h>
   2 #include <linux/kernel.h>
   3 #include <linux/string.h>
   4 #include <asm/timer.h>
   5
   6 #ifdef CONFIG_HPET_TIMER
   7 /*
   8  * HPET memory read is slower than tsc reads, but is more dependable as it
   9  * always runs at constant frequency and reduces complexity due to
  10  * cpufreq. So, we prefer HPET timer to tsc based one. Also, we cannot use
  11  * timer_pit when HPET is active. So, we default to timer_tsc.
  12  */
  13 #endif
  14 /* list of timers, ordered by preference, NULL terminated */
  15 static struct init_timer_opts* __initdata timers[] = {
  16 #ifdef CONFIG_X86_CYCLONE_TIMER
  17         &timer_cyclone_init,
  18 #endif
  19 #ifdef CONFIG_HPET_TIMER
  20         &timer_hpet_init,
  21 #endif
  22 #ifdef CONFIG_X86_PM_TIMER
  23         &timer_pmtmr_init,
  24 #endif
  25         &timer_tsc_init,
  26         &timer_pit_init,
  27         NULL,
  28 };
  29
  30 static char clock_override[10] __initdata;
  31
  32 static int __init clock_setup(char* str)
  33 {
  34         if (str)
  35                 strlcpy(clock_override, str, sizeof(clock_override));
  36         return 1;
  37 }
  38 __setup("clock=", clock_setup);
  39
  40
  41 /* The chosen timesource has been found to be bad.
  42  * Fall back to a known good timesource (the PIT)
  43  */
  44 void clock_fallback(void)
  45 {
  46         cur_timer = &timer_pit;
  47 }
  48
  49 /* iterates through the list of timers, returning the first
  50  * one that initializes successfully.
  51  */
  52 struct timer_opts* __init select_timer(void)
  53 {
  54         int i = 0;
  55
  56         /* find most preferred working timer */
  57         while (timers[i]) {
  58                 if (timers[i]->init)
  59                         if (timers[i]->init(clock_override) == 0)
  60                                 return timers[i]->opts;
  61                 ++i;
  62         }
  63
  64         panic("select_timer: Cannot find a suitable timer\n");
  65         return NULL;
  66 }