109 lines · c
1// SPDX-License-Identifier: GPL-2.0-only2/*3 * Generic polynomial calculation using integer coefficients.4 *5 * Copyright (C) 2020 BAIKAL ELECTRONICS, JSC6 *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 * 1.7204e226 * T = -1.6743e-11*(N^4) + 8.1542e-8*(N^3) + -1.8201e-4*(N^2) +27 * 3.1020e-1*(N^1) - 4.838e128 *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 the33 * multiplications and divisions are made so to have a result of the operations34 * within the integer numbers limit. In addition we need to translate the35 * formulae to accept millidegrees of Celsius. Here what they look like after36 * the alterations:37 *38 * N = (18322e-20*(T^4) + 2343e-13*(T^3) + 87018e-9*(T^2) + 39269e-3*T +39 * 17204e2) / 1e440 * T = -16743e-12*(D^4) + 81542e-9*(D^3) - 182010e-6*(D^2) + 310200e-3*D -41 * 4838042 * 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 arithmetic69 *70 * @poly: pointer to the descriptor of the polynomial71 * @data: input value of the polynimal72 *73 * Calculate the result of a polynomial using only integer arithmetic. For74 * this to work without too much loss of precision the coefficients has to75 * be altered. This is called factor redistribution.76 *77 * Returns the result of the polynomial calculation.78 */79long 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 the88 * redistributed terms calculations. It's pretty straightforward.89 * We walk over each degree term up to the free one, and perform90 * the redistributed multiplication of the term coefficient, its91 * divider (as for the rationale fraction representation), data92 * power and the rational fraction divider leftover. Then all of93 * this is collected in a total sum variable, which value is94 * 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}105EXPORT_SYMBOL_GPL(polynomial_calc);106 107MODULE_DESCRIPTION("Generic polynomial calculations");108MODULE_LICENSE("GPL");109