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 */
9extern void __bad_udelay(void);
10extern void __bad_ndelay(void);
11
12extern void __udelay(unsigned long usecs);
13extern void __ndelay(unsigned long nsecs);
14extern void __const_udelay(unsigned long xloops);
15extern 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 */
56static __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(xloops: usec * UDELAY_CONST_MULT);
63 } else {
64 __udelay(usecs: 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 */
74static __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(xloops: nsec * NDELAY_CONST_MULT);
81 } else {
82 __ndelay(nsecs: nsec);
83 }
84}
85#define ndelay(x) ndelay(x)
86
87#endif /* __ASM_GENERIC_DELAY_H */
88

source code of linux/include/asm-generic/delay.h