arch/riscv/lib/delay.c

   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);