delay.c 2.7 KB

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  1. // SPDX-License-Identifier: GPL-2.0-only
  2. /*
  3. * Copyright (C) 2012 Regents of the University of California
  4. */
  5. #include <linux/delay.h>
  6. #include <linux/param.h>
  7. #include <linux/timex.h>
  8. #include <linux/export.h>
  9. /*
  10. * This is copies from arch/arm/include/asm/delay.h
  11. *
  12. * Loop (or tick) based delay:
  13. *
  14. * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
  15. *
  16. * where:
  17. *
  18. * jiffies_per_sec = HZ
  19. * us_per_sec = 1000000
  20. *
  21. * Therefore the constant part is HZ / 1000000 which is a small
  22. * fractional number. To make this usable with integer math, we
  23. * scale up this constant by 2^31, perform the actual multiplication,
  24. * and scale the result back down by 2^31 with a simple shift:
  25. *
  26. * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
  27. *
  28. * where:
  29. *
  30. * UDELAY_MULT = 2^31 * HZ / 1000000
  31. * = (2^31 / 1000000) * HZ
  32. * = 2147.483648 * HZ
  33. * = 2147 * HZ + 483648 * HZ / 1000000
  34. *
  35. * 31 is the biggest scale shift value that won't overflow 32 bits for
  36. * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
  37. */
  38. #define MAX_UDELAY_US 2000
  39. #define MAX_UDELAY_HZ 1000
  40. #define UDELAY_MULT (2147UL * HZ + 483648UL * HZ / 1000000UL)
  41. #define UDELAY_SHIFT 31
  42. #if HZ > MAX_UDELAY_HZ
  43. #error "HZ > MAX_UDELAY_HZ"
  44. #endif
  45. /*
  46. * RISC-V supports both UDELAY and NDELAY. This is largely the same as above,
  47. * but with different constants. I added 10 bits to the shift to get this, but
  48. * the result is that I need a 64-bit multiply, which is slow on 32-bit
  49. * platforms.
  50. *
  51. * NDELAY_MULT = 2^41 * HZ / 1000000000
  52. * = (2^41 / 1000000000) * HZ
  53. * = 2199.02325555 * HZ
  54. * = 2199 * HZ + 23255550 * HZ / 1000000000
  55. *
  56. * The maximum here is to avoid 64-bit overflow, but it isn't checked as it
  57. * won't happen.
  58. */
  59. #define MAX_NDELAY_NS (1ULL << 42)
  60. #define MAX_NDELAY_HZ MAX_UDELAY_HZ
  61. #define NDELAY_MULT ((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL))
  62. #define NDELAY_SHIFT 41
  63. #if HZ > MAX_NDELAY_HZ
  64. #error "HZ > MAX_NDELAY_HZ"
  65. #endif
  66. void __delay(unsigned long cycles)
  67. {
  68. u64 t0 = get_cycles();
  69. while ((unsigned long)(get_cycles() - t0) < cycles)
  70. cpu_relax();
  71. }
  72. EXPORT_SYMBOL(__delay);
  73. void udelay(unsigned long usecs)
  74. {
  75. u64 ucycles = (u64)usecs * lpj_fine * UDELAY_MULT;
  76. u64 n;
  77. if (unlikely(usecs > MAX_UDELAY_US)) {
  78. n = (u64)usecs * riscv_timebase;
  79. do_div(n, 1000000);
  80. __delay(n);
  81. return;
  82. }
  83. __delay(ucycles >> UDELAY_SHIFT);
  84. }
  85. EXPORT_SYMBOL(udelay);
  86. void ndelay(unsigned long nsecs)
  87. {
  88. /*
  89. * This doesn't bother checking for overflow, as it won't happen (it's
  90. * an hour) of delay.
  91. */
  92. unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT;
  93. __delay(ncycles >> NDELAY_SHIFT);
  94. }
  95. EXPORT_SYMBOL(ndelay);