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Commit 8759ef32 authored by Oskar Schirmer's avatar Oskar Schirmer Committed by Linus Torvalds
Browse files

lib: isolate rational fractions helper function 



Provide a helper function to determine optimum numerator
denominator value pairs taking into account restricted
register size. Useful especially with PLL and other clock
configurations.

Signed-off-by: default avatarOskar Schirmer <os@emlix.com>
Signed-off-by: default avatarAlan Cox <alan@linux.intel.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9f322ad0

include/linux/rational.h

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/*

 * rational fractions

 *

 * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <os@emlix.com>

 *

 * helper functions when coping with rational numbers,

 * e.g. when calculating optimum numerator/denominator pairs for

 * pll configuration taking into account restricted register size

 */



#ifndef _LINUX_RATIONAL_H

#define _LINUX_RATIONAL_H



void rational_best_approximation(

	unsigned long given_numerator, unsigned long given_denominator,

	unsigned long max_numerator, unsigned long max_denominator,

	unsigned long *best_numerator, unsigned long *best_denominator);



#endif /* _LINUX_RATIONAL_H */

lib/Kconfig

+3 −0
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@@ -10,6 +10,9 @@ menu "Library routines" 
config BITREVERSE

	tristate



config RATIONAL

	boolean



config GENERIC_FIND_FIRST_BIT

	bool

lib/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -50,6 +50,7 @@ ifneq ($(CONFIG_HAVE_DEC_LOCK),y) 
endif



obj-$(CONFIG_BITREVERSE) += bitrev.o

obj-$(CONFIG_RATIONAL)	+= rational.o

obj-$(CONFIG_CRC_CCITT)	+= crc-ccitt.o

obj-$(CONFIG_CRC16)	+= crc16.o

obj-$(CONFIG_CRC_T10DIF)+= crc-t10dif.o

lib/rational.c

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+62 −0
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/*

 * rational fractions

 *

 * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <os@emlix.com>

 *

 * helper functions when coping with rational numbers

 */



#include <linux/rational.h>



/*

 * calculate best rational approximation for a given fraction

 * taking into account restricted register size, e.g. to find

 * appropriate values for a pll with 5 bit denominator and

 * 8 bit numerator register fields, trying to set up with a

 * frequency ratio of 3.1415, one would say:

 *

 * rational_best_approximation(31415, 10000,

 *		(1 << 8) - 1, (1 << 5) - 1, &n, &d);

 *

 * you may look at given_numerator as a fixed point number,

 * with the fractional part size described in given_denominator.

 *

 * for theoretical background, see:

 * http://en.wikipedia.org/wiki/Continued_fraction

 */



void rational_best_approximation(

	unsigned long given_numerator, unsigned long given_denominator,

	unsigned long max_numerator, unsigned long max_denominator,

	unsigned long *best_numerator, unsigned long *best_denominator)

{

	unsigned long n, d, n0, d0, n1, d1;

	n = given_numerator;

	d = given_denominator;

	n0 = d1 = 0;

	n1 = d0 = 1;

	for (;;) {

		unsigned long t, a;

		if ((n1 > max_numerator) || (d1 > max_denominator)) {

			n1 = n0;

			d1 = d0;

			break;

		}

		if (d == 0)

			break;

		t = d;

		a = n / d;

		d = n % d;

		n = t;

		t = n0 + a * n1;

		n0 = n1;

		n1 = t;

		t = d0 + a * d1;

		d0 = d1;

		d1 = t;

	}

	*best_numerator = n1;

	*best_denominator = d1;

}



EXPORT_SYMBOL(rational_best_approximation);