lib/math/reciprocal_div.c

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