arch/riscv/lib/delay.c

   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);