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 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 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); 101 102 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