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
72 void __delay(unsigned long cycles)
73 {
74 u64 t0 = get_cycles();
75
76 while ((unsigned long)(get_cycles() - t0) < cycles)
77 cpu_relax();
78 }
79 EXPORT_SYMBOL(__delay);
80
81 void 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 }
96 EXPORT_SYMBOL(udelay);
97
98 void 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 }
107 EXPORT_SYMBOL(ndelay);