Skip to content

Commit

Permalink
lib: add generic polynomial calculation
Browse files Browse the repository at this point in the history
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 <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Guenter Roeck <[email protected]>
  • Loading branch information
mwalle authored and groeck committed May 22, 2022
1 parent 9054416 commit cd705ea
Show file tree
Hide file tree
Showing 4 changed files with 148 additions and 0 deletions.
35 changes: 35 additions & 0 deletions include/linux/polynomial.h
Original file line number Diff line number Diff line change
@@ -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
3 changes: 3 additions & 0 deletions lib/Kconfig
Original file line number Diff line number Diff line change
Expand Up @@ -737,3 +737,6 @@ config PLDMFW

config ASN1_ENCODER
tristate

config POLYNOMIAL
tristate
2 changes: 2 additions & 0 deletions lib/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -263,6 +263,8 @@ 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.
Expand Down
108 changes: 108 additions & 0 deletions lib/polynomial.c
Original file line number Diff line number Diff line change
@@ -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 <[email protected]>
* Serge Semin <[email protected]>
*
*/

#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");

0 comments on commit cd705ea

Please sign in to comment.