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);
74
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