LXR linux/lib/reciprocal

   1// SPDX-License-Identifier: GPL-2.0
   2#include <linux/bug.h>
   3#include <linux/kernel.h>
   4#include <asm/div64.h>
   5#include <linux/reciprocal_div.h>
   6#include <linux/export.h>
   7
   8/*
   9 * For a description of the algorithm please have a look at
  10 * include/linux/reciprocal_div.h
  11 */
  12
  13struct reciprocal_value reciprocal_value(u32 d)
  14{
  15        struct reciprocal_value R;
  16        u64 m;
  17        int l;
  18
  19        l = fls(d - 1);
  20        m = ((1ULL << 32) * ((1ULL << l) - d));
  21        do_div(m, d);
  22        ++m;
  23        R.m = (u32)m;
  24        R.sh1 = min(l, 1);
  25        R.sh2 = max(l - 1, 0);
  26
  27        return R;
  28}
  29EXPORT_SYMBOL(reciprocal_value);
  30
  31struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec)
  32{
  33        struct reciprocal_value_adv R;
  34        u32 l, post_shift;
  35        u64 mhigh, mlow;
  36
  37        /* ceil(log2(d)) */
  38        l = fls(d - 1);
  39        /* NOTE: mlow/mhigh could overflow u64 when l == 32. This case needs to
  40         * be handled before calling "reciprocal_value_adv", please see the
  41         * comment at include/linux/reciprocal_div.h.
  42         */
  43        WARN(l == 32,
  44             "ceil(log2(0x%08x)) == 32, %s doesn't support such divisor",
  45             d, __func__);
  46        post_shift = l;
  47        mlow = 1ULL << (32 + l);
  48        do_div(mlow, d);
  49        mhigh = (1ULL << (32 + l)) + (1ULL << (32 + l - prec));
  50        do_div(mhigh, d);
  51
  52        for (; post_shift > 0; post_shift--) {
  53                u64 lo = mlow >> 1, hi = mhigh >> 1;
  54
  55                if (lo >= hi)
  56                        break;
  57
  58                mlow = lo;
  59                mhigh = hi;
  60        }
  61
  62        R.m = (u32)mhigh;
  63        R.sh = post_shift;
  64        R.exp = l;
  65        R.is_wide_m = mhigh > U32_MAX;
  66
  67        return R;
  68}
  69EXPORT_SYMBOL(reciprocal_value_adv);
  70