From: Michael Walle <michael@walle.cc>
Date: Fri, 1 Apr 2022 23:40:29 +0200
Subject: lib: add generic polynomial calculation
Patch-mainline: v5.19-rc1
Git-commit: cd705ea857fdd859a9df09e8adda4cb4c906e8a2
References: jsc#PED-829
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>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
include/linux/polynomial.h | 35 ++++++++++++++
lib/Kconfig | 3 +
lib/Makefile | 2
lib/polynomial.c | 108 +++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 148 insertions(+)
create mode 100644 include/linux/polynomial.h
create mode 100644 lib/polynomial.c
--- /dev/null
+++ b/include/linux/polynomial.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 BAIKAL ELECTRONICS, JSC
+ */
+
+#ifndef _POLYNOMIAL_H
+#define _POLYNOMIAL_H
+
+/*
+ * struct polynomial_term - one term descriptor of a polynomial
+ * @deg: degree of the term.
+ * @coef: multiplication factor of the term.
+ * @divider: distributed divider per each degree.
+ * @divider_leftover: divider leftover, which couldn't be redistributed.
+ */
+struct polynomial_term {
+ unsigned int deg;
+ long coef;
+ long divider;
+ long divider_leftover;
+};
+
+/*
+ * struct polynomial - a polynomial descriptor
+ * @total_divider: total data divider.
+ * @terms: polynomial terms, last term must have degree of 0
+ */
+struct polynomial {
+ long total_divider;
+ struct polynomial_term terms[];
+};
+
+long polynomial_calc(const struct polynomial *poly, long data);
+
+#endif
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -719,3 +719,6 @@ config PLDMFW
config ASN1_ENCODER
tristate
+
+config POLYNOMIAL
+ tristate
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -261,6 +261,8 @@ obj-$(CONFIG_MEMREGION) += memregion.o
obj-$(CONFIG_STMP_DEVICE) += stmp_device.o
obj-$(CONFIG_IRQ_POLL) += irq_poll.o
+obj-$(CONFIG_POLYNOMIAL) += polynomial.o
+
# stackdepot.c should not be instrumented or call instrumented functions.
# Prevent the compiler from calling builtins like memcmp() or bcmp() from this
# file.
--- /dev/null
+++ b/lib/polynomial.c
@@ -0,0 +1,108 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Generic polynomial calculation using integer coefficients.
+ *
+ * Copyright (C) 2020 BAIKAL ELECTRONICS, JSC
+ *
+ * Authors:
+ * Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
+ * Serge Semin <Sergey.Semin@baikalelectronics.ru>
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/polynomial.h>
+
+/*
+ * Originally this was part of drivers/hwmon/bt1-pvt.c.
+ * There the following conversion is used and should serve as an example here:
+ *
+ * The original translation formulae of the temperature (in degrees of Celsius)
+ * to PVT data and vice-versa are following:
+ *
+ * N = 1.8322e-8*(T^4) + 2.343e-5*(T^3) + 8.7018e-3*(T^2) + 3.9269*(T^1) +
+ * 1.7204e2
+ * T = -1.6743e-11*(N^4) + 8.1542e-8*(N^3) + -1.8201e-4*(N^2) +
+ * 3.1020e-1*(N^1) - 4.838e1
+ *
+ * where T = [-48.380, 147.438]C and N = [0, 1023].
+ *
+ * They must be accordingly altered to be suitable for the integer arithmetics.
+ * The technique is called 'factor redistribution', which just makes sure the
+ * multiplications and divisions are made so to have a result of the operations
+ * within the integer numbers limit. In addition we need to translate the
+ * formulae to accept millidegrees of Celsius. Here what they look like after
+ * the alterations:
+ *
+ * N = (18322e-20*(T^4) + 2343e-13*(T^3) + 87018e-9*(T^2) + 39269e-3*T +
+ * 17204e2) / 1e4
+ * T = -16743e-12*(D^4) + 81542e-9*(D^3) - 182010e-6*(D^2) + 310200e-3*D -
+ * 48380
+ * where T = [-48380, 147438] mC and N = [0, 1023].
+ *
+ * static const struct polynomial poly_temp_to_N = {
+ * .total_divider = 10000,
+ * .terms = {
+ * {4, 18322, 10000, 10000},
+ * {3, 2343, 10000, 10},
+ * {2, 87018, 10000, 10},
+ * {1, 39269, 1000, 1},
+ * {0, 1720400, 1, 1}
+ * }
+ * };
+ *
+ * static const struct polynomial poly_N_to_temp = {
+ * .total_divider = 1,
+ * .terms = {
+ * {4, -16743, 1000, 1},
+ * {3, 81542, 1000, 1},
+ * {2, -182010, 1000, 1},
+ * {1, 310200, 1000, 1},
+ * {0, -48380, 1, 1}
+ * }
+ * };
+ */
+
+/**
+ * polynomial_calc - calculate a polynomial using integer arithmetic
+ *
+ * @poly: pointer to the descriptor of the polynomial
+ * @data: input value of the polynimal
+ *
+ * Calculate the result of a polynomial using only integer arithmetic. For
+ * this to work without too much loss of precision the coefficients has to
+ * be altered. This is called factor redistribution.
+ *
+ * Returns the result of the polynomial calculation.
+ */
+long polynomial_calc(const struct polynomial *poly, long data)
+{
+ const struct polynomial_term *term = poly->terms;
+ long total_divider = poly->total_divider ?: 1;
+ long tmp, ret = 0;
+ int deg;
+
+ /*
+ * Here is the polynomial calculation function, which performs the
+ * redistributed terms calculations. It's pretty straightforward.
+ * We walk over each degree term up to the free one, and perform
+ * the redistributed multiplication of the term coefficient, its
+ * divider (as for the rationale fraction representation), data
+ * power and the rational fraction divider leftover. Then all of
+ * this is collected in a total sum variable, which value is
+ * normalized by the total divider before being returned.
+ */
+ do {
+ tmp = term->coef;
+ for (deg = 0; deg < term->deg; ++deg)
+ tmp = mult_frac(tmp, data, term->divider);
+ ret += tmp / term->divider_leftover;
+ } while ((term++)->deg);
+
+ return ret / total_divider;
+}
+EXPORT_SYMBOL_GPL(polynomial_calc);
+
+MODULE_DESCRIPTION("Generic polynomial calculations");
+MODULE_LICENSE("GPL");