1 /* 2 * Copyright (C) 2012 Regents of the University of California 3 * 4 * This program is free software; you can redistribute it and/or 5 * modify it under the terms of the GNU General Public License 6 * as published by the Free Software Foundation, version 2. 7 * 8 * This program is distributed in the hope that it will be useful, 9 * but WITHOUT ANY WARRANTY; without even the implied warranty of 10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 11 * GNU General Public License for more details. 12 */ 13 14 #include <linux/delay.h> 15 #include <linux/param.h> 16 #include <linux/timex.h> 17 #include <linux/export.h> 18 19 /* 20 * This is copies from arch/arm/include/asm/delay.h 21 * 22 * Loop (or tick) based delay: 23 * 24 * loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec 25 * 26 * where: 27 * 28 * jiffies_per_sec = HZ 29 * us_per_sec = 1000000 30 * 31 * Therefore the constant part is HZ / 1000000 which is a small 32 * fractional number. To make this usable with integer math, we 33 * scale up this constant by 2^31, perform the actual multiplication, 34 * and scale the result back down by 2^31 with a simple shift: 35 * 36 * loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31 37 * 38 * where: 39 * 40 * UDELAY_MULT = 2^31 * HZ / 1000000 41 * = (2^31 / 1000000) * HZ 42 * = 2147.483648 * HZ 43 * = 2147 * HZ + 483648 * HZ / 1000000 44 * 45 * 31 is the biggest scale shift value that won't overflow 32 bits for 46 * delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000. 47 */ 48 #define MAX_UDELAY_US 2000 49 #define MAX_UDELAY_HZ 1000 50 #define UDELAY_MULT (2147UL * HZ + 483648UL * HZ / 1000000UL) 51 #define UDELAY_SHIFT 31 52 53 #if HZ > MAX_UDELAY_HZ 54 #error "HZ > MAX_UDELAY_HZ" 55 #endif 56 57 /* 58 * RISC-V supports both UDELAY and NDELAY. This is largely the same as above, 59 * but with different constants. I added 10 bits to the shift to get this, but 60 * the result is that I need a 64-bit multiply, which is slow on 32-bit 61 * platforms. 62 * 63 * NDELAY_MULT = 2^41 * HZ / 1000000000 64 * = (2^41 / 1000000000) * HZ 65 * = 2199.02325555 * HZ 66 * = 2199 * HZ + 23255550 * HZ / 1000000000 67 * 68 * The maximum here is to avoid 64-bit overflow, but it isn't checked as it 69 * won't happen. 70 */ 71 #define MAX_NDELAY_NS (1ULL << 42) 72 #define MAX_NDELAY_HZ MAX_UDELAY_HZ 73 #define NDELAY_MULT ((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL)) 74 #define NDELAY_SHIFT 41 75 76 #if HZ > MAX_NDELAY_HZ 77 #error "HZ > MAX_NDELAY_HZ" 78 #endif 79 80 void __delay(unsigned long cycles) 81 { 82 u64 t0 = get_cycles(); 83 84 while ((unsigned long)(get_cycles() - t0) < cycles) 85 cpu_relax(); 86 } 87 EXPORT_SYMBOL(__delay); 88 89 void udelay(unsigned long usecs) 90 { 91 unsigned long ucycles = usecs * lpj_fine * UDELAY_MULT; 92 93 if (unlikely(usecs > MAX_UDELAY_US)) { 94 __delay((u64)usecs * riscv_timebase / 1000000ULL); 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