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 */ polynomial_calc(const struct polynomial * poly,long data)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