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 
reciprocal_value(u32 d)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 
reciprocal_value_adv(u32 d,u8 prec)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