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