root/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 
   8 /*
   9  * For a description of the algorithm please have a look at
  10  * include/linux/reciprocal_div.h
  11  */
  12 
  13 struct 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 }
  29 EXPORT_SYMBOL(reciprocal_value);
  30 
  31 struct 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 }
  69 EXPORT_SYMBOL(reciprocal_value_adv);