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
  46 struct delay_timer {
  47         unsigned long (*read_current_timer)(void);
  48         unsigned long freq;
  49 };
  50
  51 extern 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  */
  67 extern 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. */
  89 extern void __loop_delay(unsigned long loops);
  90 extern void __loop_udelay(unsigned long usecs);
  91 extern void __loop_const_udelay(unsigned long);
  92
  93 /* Delay-loop timer registration. */
  94 #define ARCH_HAS_READ_CURRENT_TIMER
  95 extern void register_current_timer_delay(const struct delay_timer *timer);
  96
  97 #endif /* __ASSEMBLY__ */
  98
  99 #endif /* defined(_ARM_DELAY_H) */
 100