jiffies.c 2.5 KB

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  1. /***********************************************************************
  2. * linux/kernel/time/jiffies.c
  3. *
  4. * This file contains the jiffies based clocksource.
  5. *
  6. * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
  7. *
  8. * This program is free software; you can redistribute it and/or modify
  9. * it under the terms of the GNU General Public License as published by
  10. * the Free Software Foundation; either version 2 of the License, or
  11. * (at your option) any later version.
  12. *
  13. * This program is distributed in the hope that it will be useful,
  14. * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15. * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  16. * GNU General Public License for more details.
  17. *
  18. * You should have received a copy of the GNU General Public License
  19. * along with this program; if not, write to the Free Software
  20. * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  21. *
  22. ************************************************************************/
  23. #include <linux/clocksource.h>
  24. #include <linux/jiffies.h>
  25. #include <linux/init.h>
  26. /* The Jiffies based clocksource is the lowest common
  27. * denominator clock source which should function on
  28. * all systems. It has the same coarse resolution as
  29. * the timer interrupt frequency HZ and it suffers
  30. * inaccuracies caused by missed or lost timer
  31. * interrupts and the inability for the timer
  32. * interrupt hardware to accuratly tick at the
  33. * requested HZ value. It is also not reccomended
  34. * for "tick-less" systems.
  35. */
  36. #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
  37. /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
  38. * conversion, the .shift value could be zero. However
  39. * this would make NTP adjustments impossible as they are
  40. * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
  41. * shift both the nominator and denominator the same
  42. * amount, and give ntp adjustments in units of 1/2^8
  43. *
  44. * The value 8 is somewhat carefully chosen, as anything
  45. * larger can result in overflows. NSEC_PER_JIFFY grows as
  46. * HZ shrinks, so values greater then 8 overflow 32bits when
  47. * HZ=100.
  48. */
  49. #define JIFFIES_SHIFT 8
  50. static cycle_t jiffies_read(void)
  51. {
  52. return (cycle_t) jiffies;
  53. }
  54. struct clocksource clocksource_jiffies = {
  55. .name = "jiffies",
  56. .rating = 1, /* lowest valid rating*/
  57. .read = jiffies_read,
  58. .mask = 0xffffffff, /*32bits*/
  59. .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
  60. .shift = JIFFIES_SHIFT,
  61. };
  62. static int __init init_jiffies_clocksource(void)
  63. {
  64. return clocksource_register(&clocksource_jiffies);
  65. }
  66. core_initcall(init_jiffies_clocksource);