```   1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Copyright (C) 2012 Regents of the University of California
4 */
5
6#include <linux/delay.h>
7#include <linux/param.h>
8#include <linux/timex.h>
9#include <linux/export.h>
10
11/*
12 * This is copies from arch/arm/include/asm/delay.h
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_US   2000
41#define MAX_UDELAY_HZ   1000
42#define UDELAY_MULT     (2147UL * HZ + 483648UL * HZ / 1000000UL)
43#define UDELAY_SHIFT    31
44
45#if HZ > MAX_UDELAY_HZ
46#error "HZ > MAX_UDELAY_HZ"
47#endif
48
49/*
50 * RISC-V supports both UDELAY and NDELAY.  This is largely the same as above,
51 * but with different constants.  I added 10 bits to the shift to get this, but
52 * the result is that I need a 64-bit multiply, which is slow on 32-bit
53 * platforms.
54 *
55 * NDELAY_MULT = 2^41 * HZ / 1000000000
56 *             = (2^41 / 1000000000) * HZ
57 *             = 2199.02325555 * HZ
58 *             = 2199 * HZ + 23255550 * HZ / 1000000000
59 *
60 * The maximum here is to avoid 64-bit overflow, but it isn't checked as it
61 * won't happen.
62 */
63#define MAX_NDELAY_NS   (1ULL << 42)
64#define MAX_NDELAY_HZ   MAX_UDELAY_HZ
65#define NDELAY_MULT     ((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL))
66#define NDELAY_SHIFT    41
67
68#if HZ > MAX_NDELAY_HZ
69#error "HZ > MAX_NDELAY_HZ"
70#endif
71
72void __delay(unsigned long cycles)
73{
74        u64 t0 = get_cycles();
75
76        while ((unsigned long)(get_cycles() - t0) < cycles)
77                cpu_relax();
78}
79EXPORT_SYMBOL(__delay);
80
81void udelay(unsigned long usecs)
82{
83        u64 ucycles = (u64)usecs * lpj_fine * UDELAY_MULT;
84        u64 n;
85
86        if (unlikely(usecs > MAX_UDELAY_US)) {
87                n = (u64)usecs * riscv_timebase;
88                do_div(n, 1000000);
89
90                __delay(n);
91                return;
92        }
93
94        __delay(ucycles >> UDELAY_SHIFT);
95}
96EXPORT_SYMBOL(udelay);
97
98void ndelay(unsigned long nsecs)
99{
100        /*
101         * This doesn't bother checking for overflow, as it won't happen (it's
102         * an hour) of delay.
103         */
104        unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT;
105        __delay(ncycles >> NDELAY_SHIFT);
106}
107EXPORT_SYMBOL(ndelay);
108```