cos / third_party / kernel / 01aeb332006e1f1d9cc32ab519c1e843a23172cf / . / arch / riscv / lib / delay.c

/* | |

* Copyright (C) 2012 Regents of the University of California | |

* | |

* This program is free software; you can redistribute it and/or | |

* modify it under the terms of the GNU General Public License | |

* as published by the Free Software Foundation, version 2. | |

* | |

* This program is distributed in the hope that it will be useful, | |

* but WITHOUT ANY WARRANTY; without even the implied warranty of | |

* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |

* GNU General Public License for more details. | |

*/ | |

#include <linux/delay.h> | |

#include <linux/param.h> | |

#include <linux/timex.h> | |

#include <linux/export.h> | |

/* | |

* This is copies from arch/arm/include/asm/delay.h | |

* | |

* Loop (or tick) based delay: | |

* | |

* loops = loops_per_jiffy * jiffies_per_sec * delay_us / us_per_sec | |

* | |

* where: | |

* | |

* jiffies_per_sec = HZ | |

* us_per_sec = 1000000 | |

* | |

* Therefore the constant part is HZ / 1000000 which is a small | |

* fractional number. To make this usable with integer math, we | |

* scale up this constant by 2^31, perform the actual multiplication, | |

* and scale the result back down by 2^31 with a simple shift: | |

* | |

* loops = (loops_per_jiffy * delay_us * UDELAY_MULT) >> 31 | |

* | |

* where: | |

* | |

* UDELAY_MULT = 2^31 * HZ / 1000000 | |

* = (2^31 / 1000000) * HZ | |

* = 2147.483648 * HZ | |

* = 2147 * HZ + 483648 * HZ / 1000000 | |

* | |

* 31 is the biggest scale shift value that won't overflow 32 bits for | |

* delay_us * UDELAY_MULT assuming HZ <= 1000 and delay_us <= 2000. | |

*/ | |

#define MAX_UDELAY_US 2000 | |

#define MAX_UDELAY_HZ 1000 | |

#define UDELAY_MULT (2147UL * HZ + 483648UL * HZ / 1000000UL) | |

#define UDELAY_SHIFT 31 | |

#if HZ > MAX_UDELAY_HZ | |

#error "HZ > MAX_UDELAY_HZ" | |

#endif | |

/* | |

* RISC-V supports both UDELAY and NDELAY. This is largely the same as above, | |

* but with different constants. I added 10 bits to the shift to get this, but | |

* the result is that I need a 64-bit multiply, which is slow on 32-bit | |

* platforms. | |

* | |

* NDELAY_MULT = 2^41 * HZ / 1000000000 | |

* = (2^41 / 1000000000) * HZ | |

* = 2199.02325555 * HZ | |

* = 2199 * HZ + 23255550 * HZ / 1000000000 | |

* | |

* The maximum here is to avoid 64-bit overflow, but it isn't checked as it | |

* won't happen. | |

*/ | |

#define MAX_NDELAY_NS (1ULL << 42) | |

#define MAX_NDELAY_HZ MAX_UDELAY_HZ | |

#define NDELAY_MULT ((unsigned long long)(2199ULL * HZ + 23255550ULL * HZ / 1000000000ULL)) | |

#define NDELAY_SHIFT 41 | |

#if HZ > MAX_NDELAY_HZ | |

#error "HZ > MAX_NDELAY_HZ" | |

#endif | |

void __delay(unsigned long cycles) | |

{ | |

u64 t0 = get_cycles(); | |

while ((unsigned long)(get_cycles() - t0) < cycles) | |

cpu_relax(); | |

} | |

EXPORT_SYMBOL(__delay); | |

void udelay(unsigned long usecs) | |

{ | |

u64 ucycles = (u64)usecs * lpj_fine * UDELAY_MULT; | |

if (unlikely(usecs > MAX_UDELAY_US)) { | |

__delay((u64)usecs * riscv_timebase / 1000000ULL); | |

return; | |

} | |

__delay(ucycles >> UDELAY_SHIFT); | |

} | |

EXPORT_SYMBOL(udelay); | |

void ndelay(unsigned long nsecs) | |

{ | |

/* | |

* This doesn't bother checking for overflow, as it won't happen (it's | |

* an hour) of delay. | |

*/ | |

unsigned long long ncycles = nsecs * lpj_fine * NDELAY_MULT; | |

__delay(ncycles >> NDELAY_SHIFT); | |

} | |

EXPORT_SYMBOL(ndelay); |