cnt32_to_63.h 2.9 KB

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  1. /*
  2. * include/asm/cnt32_to_63.h -- extend a 32-bit counter to 63 bits
  3. *
  4. * Author: Nicolas Pitre
  5. * Created: December 3, 2006
  6. * Copyright: MontaVista Software, Inc.
  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 version 2
  10. * as published by the Free Software Foundation.
  11. */
  12. #ifndef __INCLUDE_CNT32_TO_63_H__
  13. #define __INCLUDE_CNT32_TO_63_H__
  14. #include <linux/compiler.h>
  15. #include <asm/types.h>
  16. #include <asm/byteorder.h>
  17. /*
  18. * Prototype: u64 cnt32_to_63(u32 cnt)
  19. * Many hardware clock counters are only 32 bits wide and therefore have
  20. * a relatively short period making wrap-arounds rather frequent. This
  21. * is a problem when implementing sched_clock() for example, where a 64-bit
  22. * non-wrapping monotonic value is expected to be returned.
  23. *
  24. * To overcome that limitation, let's extend a 32-bit counter to 63 bits
  25. * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
  26. * by the hardware while bits 32 to 62 are stored in memory. The top bit in
  27. * memory is used to synchronize with the hardware clock half-period. When
  28. * the top bit of both counters (hardware and in memory) differ then the
  29. * memory is updated with a new value, incrementing it when the hardware
  30. * counter wraps around.
  31. *
  32. * Because a word store in memory is atomic then the incremented value will
  33. * always be in synch with the top bit indicating to any potential concurrent
  34. * reader if the value in memory is up to date or not with regards to the
  35. * needed increment. And any race in updating the value in memory is harmless
  36. * as the same value would simply be stored more than once.
  37. *
  38. * The only restriction for the algorithm to work properly is that this
  39. * code must be executed at least once per each half period of the 32-bit
  40. * counter to properly update the state bit in memory. This is usually not a
  41. * problem in practice, but if it is then a kernel timer could be scheduled
  42. * to manage for this code to be executed often enough.
  43. *
  44. * Note that the top bit (bit 63) in the returned value should be considered
  45. * as garbage. It is not cleared here because callers are likely to use a
  46. * multiplier on the returned value which can get rid of the top bit
  47. * implicitly by making the multiplier even, therefore saving on a runtime
  48. * clear-bit instruction. Otherwise caller must remember to clear the top
  49. * bit explicitly.
  50. */
  51. /* this is used only to give gcc a clue about good code generation */
  52. typedef union {
  53. struct {
  54. #if defined(__LITTLE_ENDIAN)
  55. u32 lo, hi;
  56. #elif defined(__BIG_ENDIAN)
  57. u32 hi, lo;
  58. #endif
  59. };
  60. u64 val;
  61. } cnt32_to_63_t;
  62. #define cnt32_to_63(cnt_lo) \
  63. ({ \
  64. static volatile u32 __m_cnt_hi = 0; \
  65. cnt32_to_63_t __x; \
  66. __x.hi = __m_cnt_hi; \
  67. __x.lo = (cnt_lo); \
  68. if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
  69. __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
  70. __x.val; \
  71. })
  72. #endif