include/asm-generic/delay.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef __ASM_GENERIC_DELAY_H
   3 #define __ASM_GENERIC_DELAY_H
   4
   5 #include <linux/math.h>
   6 #include <vdso/time64.h>
   7
   8 /* Undefined functions to get compile-time errors */
   9 extern void __bad_udelay(void);
  10 extern void __bad_ndelay(void);
  11
  12 extern void __udelay(unsigned long usecs);
  13 extern void __ndelay(unsigned long nsecs);
  14 extern void __const_udelay(unsigned long xloops);
  15 extern void __delay(unsigned long loops);
  16
  17 /*
  18  * The microseconds/nanosecond delay multiplicators are used to convert a
  19  * constant microseconds/nanoseconds value to a value which can be used by the
  20  * architectures specific implementation to transform it into loops.
  21  */
  22 #define UDELAY_CONST_MULT       ((unsigned long)DIV_ROUND_UP(1ULL << 32, USEC_PER_SEC))
  23 #define NDELAY_CONST_MULT       ((unsigned long)DIV_ROUND_UP(1ULL << 32, NSEC_PER_SEC))
  24
  25 /*
  26  * The maximum constant udelay/ndelay value picked out of thin air to prevent
  27  * too long constant udelays/ndelays.
  28  */
  29 #define DELAY_CONST_MAX   20000
  30
  31 /**
  32  * udelay - Inserting a delay based on microseconds with busy waiting
  33  * @usec:       requested delay in microseconds
  34  *
  35  * When delaying in an atomic context ndelay(), udelay() and mdelay() are the
  36  * only valid variants of delaying/sleeping to go with.
  37  *
  38  * When inserting delays in non atomic context which are shorter than the time
  39  * which is required to queue e.g. an hrtimer and to enter then the scheduler,
  40  * it is also valuable to use udelay(). But it is not simple to specify a
  41  * generic threshold for this which will fit for all systems. An approximation
  42  * is a threshold for all delays up to 10 microseconds.
  43  *
  44  * When having a delay which is larger than the architecture specific
  45  * %MAX_UDELAY_MS value, please make sure mdelay() is used. Otherwise a overflow
  46  * risk is given.
  47  *
  48  * Please note that ndelay(), udelay() and mdelay() may return early for several
  49  * reasons (https://lists.openwall.net/linux-kernel/2011/01/09/56):
  50  *
  51  * #. computed loops_per_jiffy too low (due to the time taken to execute the
  52  *    timer interrupt.)
  53  * #. cache behaviour affecting the time it takes to execute the loop function.
  54  * #. CPU clock rate changes.
  55  */
  56 static __always_inline void udelay(unsigned long usec)
  57 {
  58         if (__builtin_constant_p(usec)) {
  59                 if (usec >= DELAY_CONST_MAX)
  60                         __bad_udelay();
  61                 else
  62                         __const_udelay(usec * UDELAY_CONST_MULT);
  63         } else {
  64                 __udelay(usec);
  65         }
  66 }
  67
  68 /**
  69  * ndelay - Inserting a delay based on nanoseconds with busy waiting
  70  * @nsec:       requested delay in nanoseconds
  71  *
  72  * See udelay() for basic information about ndelay() and it's variants.
  73  */
  74 static __always_inline void ndelay(unsigned long nsec)
  75 {
  76         if (__builtin_constant_p(nsec)) {
  77                 if (nsec >= DELAY_CONST_MAX)
  78                         __bad_ndelay();
  79                 else
  80                         __const_udelay(nsec * NDELAY_CONST_MULT);
  81         } else {
  82                 __ndelay(nsec);
  83         }
  84 }
  85 #define ndelay(x) ndelay(x)
  86
  87 #endif /* __ASM_GENERIC_DELAY_H */