linux/lib/math/reciprocal_div.c
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   1// SPDX-License-Identifier: GPL-2.0
   2#include <linux/bitops.h>
   3#include <linux/bug.h>
   4#include <linux/export.h>
   5#include <linux/limits.h>
   6#include <linux/math.h>
   7#include <linux/minmax.h>
   8#include <linux/types.h>
   9
  10#include <linux/reciprocal_div.h>
  11
  12/*
  13 * For a description of the algorithm please have a look at
  14 * include/linux/reciprocal_div.h
  15 */
  16
  17struct reciprocal_value reciprocal_value(u32 d)
  18{
  19        struct reciprocal_value R;
  20        u64 m;
  21        int l;
  22
  23        l = fls(d - 1);
  24        m = ((1ULL << 32) * ((1ULL << l) - d));
  25        do_div(m, d);
  26        ++m;
  27        R.m = (u32)m;
  28        R.sh1 = min(l, 1);
  29        R.sh2 = max(l - 1, 0);
  30
  31        return R;
  32}
  33EXPORT_SYMBOL(reciprocal_value);
  34
  35struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec)
  36{
  37        struct reciprocal_value_adv R;
  38        u32 l, post_shift;
  39        u64 mhigh, mlow;
  40
  41        /* ceil(log2(d)) */
  42        l = fls(d - 1);
  43        /* NOTE: mlow/mhigh could overflow u64 when l == 32. This case needs to
  44         * be handled before calling "reciprocal_value_adv", please see the
  45         * comment at include/linux/reciprocal_div.h.
  46         */
  47        WARN(l == 32,
  48             "ceil(log2(0x%08x)) == 32, %s doesn't support such divisor",
  49             d, __func__);
  50        post_shift = l;
  51        mlow = 1ULL << (32 + l);
  52        do_div(mlow, d);
  53        mhigh = (1ULL << (32 + l)) + (1ULL << (32 + l - prec));
  54        do_div(mhigh, d);
  55
  56        for (; post_shift > 0; post_shift--) {
  57                u64 lo = mlow >> 1, hi = mhigh >> 1;
  58
  59                if (lo >= hi)
  60                        break;
  61
  62                mlow = lo;
  63                mhigh = hi;
  64        }
  65
  66        R.m = (u32)mhigh;
  67        R.sh = post_shift;
  68        R.exp = l;
  69        R.is_wide_m = mhigh > U32_MAX;
  70
  71        return R;
  72}
  73EXPORT_SYMBOL(reciprocal_value_adv);
  74