LXR linux/arch/arm/include/asm/delay.h

   1/* SPDX-License-Identifier: GPL-2.0 */
   2/*
   3 * Copyright (C) 1995-2004 Russell King
   4 *
   5 * Delay routines, using a pre-computed "loops_per_second" value.
   6 */
   7#ifndef __ASM_ARM_DELAY_H
   8#define __ASM_ARM_DELAY_H
   9
  10#include <asm/memory.h>
  11#include <asm/param.h>  /* HZ */
  12
  13/*
  14 * Loop (or tick) based delay:
  15 *
  16 * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
  17 *
  18 * where:
  19 *
  20 * jiffies_per_sec = HZ
  21 * us_per_sec = 1000000
  22 *
  23 * Therefore the constant part is HZ / 1000000 which is a small
  24 * fractional number. To make this usable with integer math, we
  25 * scale up this constant by 2^31, perform the actual multiplication,
  26 * and scale the result back down by 2^31 with a simple shift:
  27 *
  28 * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
  29 *
  30 * where:
  31 *
  32 * UDELAY_MULT = 2^31 * HZ / 1000000
  33 *             = (2^31 / 1000000) * HZ
  34 *             = 2147.483648 * HZ
  35 *             = 2147 * HZ + 483648 * HZ / 1000000
  36 *
  37 * 31 is the biggest scale shift value that won't overflow 32 bits for
  38 * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
  39 */
  40#define MAX_UDELAY_MS   2
  41#define UDELAY_MULT     UL(2147 * HZ + 483648 * HZ / 1000000)
  42#define UDELAY_SHIFT    31
  43
  44#ifndef __ASSEMBLY__
  45
  46struct delay_timer {
  47        unsigned long (*read_current_timer)(void);
  48        unsigned long freq;
  49};
  50
  51extern struct arm_delay_ops {
  52        void (*delay)(unsigned long);
  53        void (*const_udelay)(unsigned long);
  54        void (*udelay)(unsigned long);
  55        unsigned long ticks_per_jiffy;
  56} arm_delay_ops;
  57
  58#define __delay(n)              arm_delay_ops.delay(n)
  59
  60/*
  61 * This function intentionally does not exist; if you see references to
  62 * it, it means that you're calling udelay() with an out of range value.
  63 *
  64 * With currently imposed limits, this means that we support a max delay
  65 * of 2000us. Further limits: HZ<=1000
  66 */
  67extern void __bad_udelay(void);
  68
  69/*
  70 * division by multiplication: you don't have to worry about
  71 * loss of precision.
  72 *
  73 * Use only for very small delays ( < 2 msec).  Should probably use a
  74 * lookup table, really, as the multiplications take much too long with
  75 * short delays.  This is a "reasonable" implementation, though (and the
  76 * first constant multiplications gets optimized away if the delay is
  77 * a constant)
  78 */
  79#define __udelay(n)             arm_delay_ops.udelay(n)
  80#define __const_udelay(n)       arm_delay_ops.const_udelay(n)
  81
  82#define udelay(n)                                                       \
  83        (__builtin_constant_p(n) ?                                      \
  84          ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :              \
  85                        __const_udelay((n) * UDELAY_MULT)) :            \
  86          __udelay(n))
  87
  88/* Loop-based definitions for assembly code. */
  89extern void __loop_delay(unsigned long loops);
  90extern void __loop_udelay(unsigned long usecs);
  91extern void __loop_const_udelay(unsigned long);
  92
  93/* Delay-loop timer registration. */
  94#define ARCH_HAS_READ_CURRENT_TIMER
  95extern void register_current_timer_delay(const struct delay_timer *timer);
  96
  97#endif /* __ASSEMBLY__ */
  98
  99#endif /* defined(_ARM_DELAY_H) */
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