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