1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Generic polynomial calculation using integer coefficients.
4 *
5 * Copyright (C) 2020 BAIKAL ELECTRONICS, JSC
6 *
7 * Authors:
8 * Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
9 * Serge Semin <Sergey.Semin@baikalelectronics.ru>
10 *
11 */
12
13 #include <linux/kernel.h>
14 #include <linux/module.h>
15 #include <linux/polynomial.h>
16
17 /*
18 * Originally this was part of drivers/hwmon/bt1-pvt.c.
19 * There the following conversion is used and should serve as an example here:
20 *
21 * The original translation formulae of the temperature (in degrees of Celsius)
22 * to PVT data and vice-versa are following:
23 *
24 * N = 1.8322e-8*(T^4) + 2.343e-5*(T^3) + 8.7018e-3*(T^2) + 3.9269*(T^1) +
25 * 1.7204e2
26 * T = -1.6743e-11*(N^4) + 8.1542e-8*(N^3) + -1.8201e-4*(N^2) +
27 * 3.1020e-1*(N^1) - 4.838e1
28 *
29 * where T = [-48.380, 147.438]C and N = [0, 1023].
30 *
31 * They must be accordingly altered to be suitable for the integer arithmetics.
32 * The technique is called 'factor redistribution', which just makes sure the
33 * multiplications and divisions are made so to have a result of the operations
34 * within the integer numbers limit. In addition we need to translate the
35 * formulae to accept millidegrees of Celsius. Here what they look like after
36 * the alterations:
37 *
38 * N = (18322e-20*(T^4) + 2343e-13*(T^3) + 87018e-9*(T^2) + 39269e-3*T +
39 * 17204e2) / 1e4
40 * T = -16743e-12*(D^4) + 81542e-9*(D^3) - 182010e-6*(D^2) + 310200e-3*D -
41 * 48380
42 * where T = [-48380, 147438] mC and N = [0, 1023].
43 *
44 * static const struct polynomial poly_temp_to_N = {
45 * .total_divider = 10000,
46 * .terms = {
47 * {4, 18322, 10000, 10000},
48 * {3, 2343, 10000, 10},
49 * {2, 87018, 10000, 10},
50 * {1, 39269, 1000, 1},
51 * {0, 1720400, 1, 1}
52 * }
53 * };
54 *
55 * static const struct polynomial poly_N_to_temp = {
56 * .total_divider = 1,
57 * .terms = {
58 * {4, -16743, 1000, 1},
59 * {3, 81542, 1000, 1},
60 * {2, -182010, 1000, 1},
61 * {1, 310200, 1000, 1},
62 * {0, -48380, 1, 1}
63 * }
64 * };
65 */
66
67 /**
68 * polynomial_calc - calculate a polynomial using integer arithmetic
69 *
70 * @poly: pointer to the descriptor of the polynomial
71 * @data: input value of the polynimal
72 *
73 * Calculate the result of a polynomial using only integer arithmetic. For
74 * this to work without too much loss of precision the coefficients has to
75 * be altered. This is called factor redistribution.
76 *
77 * Returns the result of the polynomial calculation.
78 */
79 long polynomial_calc(const struct polynomial *poly, long data)
80 {
81 const struct polynomial_term *term = poly->terms;
82 long total_divider = poly->total_divider ?: 1;
83 long tmp, ret = 0;
84 int deg;
85
86 /*
87 * Here is the polynomial calculation function, which performs the
88 * redistributed terms calculations. It's pretty straightforward.
89 * We walk over each degree term up to the free one, and perform
90 * the redistributed multiplication of the term coefficient, its
91 * divider (as for the rationale fraction representation), data
92 * power and the rational fraction divider leftover. Then all of
93 * this is collected in a total sum variable, which value is
94 * normalized by the total divider before being returned.
95 */
96 do {
97 tmp = term->coef;
98 for (deg = 0; deg < term->deg; ++deg)
99 tmp = mult_frac(tmp, data, term->divider);
100 ret += tmp / term->divider_leftover;
101 } while ((term++)->deg);
102
103 return ret / total_divider;
104 }
105 EXPORT_SYMBOL_GPL(polynomial_calc);
106
107 MODULE_DESCRIPTION("Generic polynomial calculations");
108 MODULE_LICENSE("GPL");
109
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