arch/x86/kernel/tsc.c

   1 #include <linux/sched.h>
   2 #include <linux/init.h>
   3 #include <linux/module.h>
   4 #include <linux/timer.h>
   5
   6 unsigned int cpu_khz;           /* TSC clocks / usec, not used here */
   7 EXPORT_SYMBOL(cpu_khz);
   8 unsigned int tsc_khz;
   9 EXPORT_SYMBOL(tsc_khz);
  10
  11 /*
  12  * TSC can be unstable due to cpufreq or due to unsynced TSCs
  13  */
  14 int tsc_unstable;
  15
  16 /* native_sched_clock() is called before tsc_init(), so
  17    we must start with the TSC soft disabled to prevent
  18    erroneous rdtsc usage on !cpu_has_tsc processors */
  19 int tsc_disabled = -1;
  20
  21 /*
  22  * Scheduler clock - returns current time in nanosec units.
  23  */
  24 u64 native_sched_clock(void)
  25 {
  26         u64 this_offset;
  27
  28         /*
  29          * Fall back to jiffies if there's no TSC available:
  30          * ( But note that we still use it if the TSC is marked
  31          *   unstable. We do this because unlike Time Of Day,
  32          *   the scheduler clock tolerates small errors and it's
  33          *   very important for it to be as fast as the platform
  34          *   can achive it. )
  35          */
  36         if (unlikely(tsc_disabled)) {
  37                 /* No locking but a rare wrong value is not a big deal: */
  38                 return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
  39         }
  40
  41         /* read the Time Stamp Counter: */
  42         rdtscll(this_offset);
  43
  44         /* return the value in ns */
  45         return cycles_2_ns(this_offset);
  46 }
  47
  48 /* We need to define a real function for sched_clock, to override the
  49    weak default version */
  50 #ifdef CONFIG_PARAVIRT
  51 unsigned long long sched_clock(void)
  52 {
  53         return paravirt_sched_clock();
  54 }
  55 #else
  56 unsigned long long
  57 sched_clock(void) __attribute__((alias("native_sched_clock")));
  58 #endif
  59
  60 int check_tsc_unstable(void)
  61 {
  62         return tsc_unstable;
  63 }
  64 EXPORT_SYMBOL_GPL(check_tsc_unstable);
  65
  66 #ifdef CONFIG_X86_TSC
  67 int __init notsc_setup(char *str)
  68 {
  69         printk(KERN_WARNING "notsc: Kernel compiled with CONFIG_X86_TSC, "
  70                         "cannot disable TSC completely.\n");
  71         tsc_disabled = 1;
  72         return 1;
  73 }
  74 #else
  75 /*
  76  * disable flag for tsc. Takes effect by clearing the TSC cpu flag
  77  * in cpu/common.c
  78  */
  79 int __init notsc_setup(char *str)
  80 {
  81         setup_clear_cpu_cap(X86_FEATURE_TSC);
  82         return 1;
  83 }
  84 #endif
  85
  86 __setup("notsc", notsc_setup);