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 */
 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 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}
105EXPORT_SYMBOL_GPL(polynomial_calc);
106
107MODULE_DESCRIPTION("Generic polynomial calculations");
108MODULE_LICENSE("GPL");