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Some temperature and voltage sensors use a polynomial to convert between raw data points and actual temperature or voltage. The polynomial is usually the result of a curve fitting of the diode characteristic. The BT1 PVT hwmon driver already uses such a polynonmial calculation which is rather generic. Move it to lib/ so other drivers can reuse it. Signed-off-by: Michael Walle <michael@walle.cc> Reviewed-by: Guenter Roeck <linux@roeck-us.net> Link: https://lore.kernel.org/r/20220401214032.3738095-2-michael@walle.cc Signed-off-by: Guenter Roeck <linux@roeck-us.net>
109 lines
3.6 KiB
C
109 lines
3.6 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Generic polynomial calculation using integer coefficients.
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*
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* Copyright (C) 2020 BAIKAL ELECTRONICS, JSC
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*
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* Authors:
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* Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
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* Serge Semin <Sergey.Semin@baikalelectronics.ru>
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*
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*/
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/polynomial.h>
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/*
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* Originally this was part of drivers/hwmon/bt1-pvt.c.
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* There the following conversion is used and should serve as an example here:
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*
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* The original translation formulae of the temperature (in degrees of Celsius)
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* to PVT data and vice-versa are following:
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*
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* N = 1.8322e-8*(T^4) + 2.343e-5*(T^3) + 8.7018e-3*(T^2) + 3.9269*(T^1) +
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* 1.7204e2
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* T = -1.6743e-11*(N^4) + 8.1542e-8*(N^3) + -1.8201e-4*(N^2) +
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* 3.1020e-1*(N^1) - 4.838e1
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*
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* where T = [-48.380, 147.438]C and N = [0, 1023].
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*
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* They must be accordingly altered to be suitable for the integer arithmetics.
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* The technique is called 'factor redistribution', which just makes sure the
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* multiplications and divisions are made so to have a result of the operations
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* within the integer numbers limit. In addition we need to translate the
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* formulae to accept millidegrees of Celsius. Here what they look like after
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* the alterations:
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*
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* N = (18322e-20*(T^4) + 2343e-13*(T^3) + 87018e-9*(T^2) + 39269e-3*T +
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* 17204e2) / 1e4
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* T = -16743e-12*(D^4) + 81542e-9*(D^3) - 182010e-6*(D^2) + 310200e-3*D -
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* 48380
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* where T = [-48380, 147438] mC and N = [0, 1023].
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*
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* static const struct polynomial poly_temp_to_N = {
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* .total_divider = 10000,
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* .terms = {
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* {4, 18322, 10000, 10000},
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* {3, 2343, 10000, 10},
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* {2, 87018, 10000, 10},
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* {1, 39269, 1000, 1},
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* {0, 1720400, 1, 1}
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* }
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* };
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*
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* static const struct polynomial poly_N_to_temp = {
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* .total_divider = 1,
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* .terms = {
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* {4, -16743, 1000, 1},
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* {3, 81542, 1000, 1},
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* {2, -182010, 1000, 1},
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* {1, 310200, 1000, 1},
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* {0, -48380, 1, 1}
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* }
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* };
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*/
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/**
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* polynomial_calc - calculate a polynomial using integer arithmetic
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*
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* @poly: pointer to the descriptor of the polynomial
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* @data: input value of the polynimal
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*
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* Calculate the result of a polynomial using only integer arithmetic. For
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* this to work without too much loss of precision the coefficients has to
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* be altered. This is called factor redistribution.
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*
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* Returns the result of the polynomial calculation.
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*/
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long polynomial_calc(const struct polynomial *poly, long data)
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{
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const struct polynomial_term *term = poly->terms;
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long total_divider = poly->total_divider ?: 1;
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long tmp, ret = 0;
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int deg;
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/*
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* Here is the polynomial calculation function, which performs the
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* redistributed terms calculations. It's pretty straightforward.
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* We walk over each degree term up to the free one, and perform
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* the redistributed multiplication of the term coefficient, its
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* divider (as for the rationale fraction representation), data
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* power and the rational fraction divider leftover. Then all of
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* this is collected in a total sum variable, which value is
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* normalized by the total divider before being returned.
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*/
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do {
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tmp = term->coef;
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for (deg = 0; deg < term->deg; ++deg)
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tmp = mult_frac(tmp, data, term->divider);
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ret += tmp / term->divider_leftover;
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} while ((term++)->deg);
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return ret / total_divider;
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}
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EXPORT_SYMBOL_GPL(polynomial_calc);
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MODULE_DESCRIPTION("Generic polynomial calculations");
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MODULE_LICENSE("GPL");
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