xref: /linux/arch/riscv/lib/delay.c (revision 03ab8e6297acd1bc0eedaa050e2a1635c576fd11)
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/math.h>
8 #include <linux/param.h>
9 #include <linux/timex.h>
10 #include <linux/types.h>
11 #include <linux/export.h>
12 
13 #include <asm/processor.h>
14 
15 /*
16  * This is copies from arch/arm/include/asm/delay.h
17  *
18  * Loop (or tick) based delay:
19  *
20  * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec
21  *
22  * where:
23  *
24  * jiffies_per_sec = HZ
25  * us_per_sec = 1000000
26  *
27  * Therefore the constant part is HZ / 1000000 which is a small
28  * fractional number. To make this usable with integer math, we
29  * scale up this constant by 2^31, perform the actual multiplication,
30  * and scale the result back down by 2^31 with a simple shift:
31  *
32  * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31
33  *
34  * where:
35  *
36  * UDELAY_MULT = 2^31 * HZ / 1000000
37  *             = (2^31 / 1000000) * HZ
38  *             = 2147.483648 * HZ
39  *             = 2147 * HZ + 483648 * HZ / 1000000
40  *
41  * 31 is the biggest scale shift value that won't overflow 32 bits for
42  * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000.
43  */
44 #define MAX_UDELAY_US	2000
45 #define MAX_UDELAY_HZ	1000
46 #define UDELAY_MULT	(2147UL * HZ + 483648UL * HZ / 1000000UL)
47 #define UDELAY_SHIFT	31
48 
49 #if HZ > MAX_UDELAY_HZ
50 #error "HZ > MAX_UDELAY_HZ"
51 #endif
52 
53 /*
54  * RISC-V supports both UDELAY and NDELAY.  This is largely the same as above,
55  * but with different constants.  I added 10 bits to the shift to get this, but
56  * the result is that I need a 64-bit multiply, which is slow on 32-bit
57  * platforms.
58  *
59  * NDELAY_MULT = 2^41 * HZ / 1000000000
60  *             = (2^41 / 1000000000) * HZ
61  *             = 2199.02325555 * HZ
62  *             = 2199 * HZ + 23255550 * HZ / 1000000000
63  *
64  * The maximum here is to avoid 64-bit overflow, but it isn't checked as it
65  * won't happen.
66  */
67 #define MAX_NDELAY_NS   (1ULL << 42)
68 #define MAX_NDELAY_HZ	MAX_UDELAY_HZ
69 #define NDELAY_MULT	((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL))
70 #define NDELAY_SHIFT	41
71 
72 #if HZ > MAX_NDELAY_HZ
73 #error "HZ > MAX_NDELAY_HZ"
74 #endif
75 
__delay(unsigned long cycles)76 void __delay(unsigned long cycles)
77 {
78 	u64 t0 = get_cycles();
79 
80 	while ((unsigned long)(get_cycles() - t0) < cycles)
81 		cpu_relax();
82 }
83 EXPORT_SYMBOL(__delay);
84 
udelay(unsigned long usecs)85 void udelay(unsigned long usecs)
86 {
87 	u64 ucycles = (u64)usecs * lpj_fine * UDELAY_MULT;
88 	u64 n;
89 
90 	if (unlikely(usecs > MAX_UDELAY_US)) {
91 		n = (u64)usecs * riscv_timebase;
92 		do_div(n, 1000000);
93 
94 		__delay(n);
95 		return;
96 	}
97 
98 	__delay(ucycles >> UDELAY_SHIFT);
99 }
100 EXPORT_SYMBOL(udelay);
101 
ndelay(unsigned long nsecs)102 void ndelay(unsigned long nsecs)
103 {
104 	/*
105 	 * This doesn't bother checking for overflow, as it won't happen (it's
106 	 * an hour) of delay.
107 	 */
108 	unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT;
109 	__delay(ncycles >> NDELAY_SHIFT);
110 }
111 EXPORT_SYMBOL(ndelay);
112