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 | |