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 unsigned long ucycles = usecs * lpj_fine * UDELAY_MULT; 84 85 if (unlikely(usecs > MAX_UDELAY_US)) { 86 __delay((u64)usecs * riscv_timebase / 1000000ULL); 87 return; 88 } 89 90 __delay(ucycles >> UDELAY_SHIFT); 91 } 92 EXPORT_SYMBOL(udelay); 93 94 void ndelay(unsigned long nsecs) 95 { 96 /* 97 * This doesn't bother checking for overflow, as it won't happen (it's 98 * an hour) of delay. 99 */ 100 unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT; 101 __delay(ncycles >> NDELAY_SHIFT); 102 } 103 EXPORT_SYMBOL(ndelay); 104