lib/math/reciprocal_div.c

   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
  17 struct 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 }
  33 EXPORT_SYMBOL(reciprocal_value);
  34
  35 struct 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 }
  73 EXPORT_SYMBOL(reciprocal_value_adv);