linux-kernel - Re: [PATCH 6/8] lib: isolate rational fractions helper function

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Date:	Tue, 9 Jun 2009 20:13:16 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Oskar Schirmer <os@...ix.com>
Cc:	Sascha Hauer <kernel@...gutronix.de>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] lib: isolate rational fractions helper function

On Tue, Jun 09, 2009 at 03:46:49PM +0200, Oskar Schirmer wrote:
> 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: Oskar Schirmer <os@...ix.com>
> ---
>  include/linux/rational.h |   19 ++++++++++++++
>  lib/Kconfig              |    3 ++
>  lib/Makefile             |    1 +
>  lib/rational.c           |   60 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 83 insertions(+), 0 deletions(-)
>  create mode 100644 include/linux/rational.h
>  create mode 100644 lib/rational.c
> 
> diff --git a/include/linux/rational.h b/include/linux/rational.h
> new file mode 100644
> index 0000000..4f532fc
> --- /dev/null
> +++ b/include/linux/rational.h
> @@ -0,0 +1,19 @@
> +/*
> + * rational fractions
> + *
> + * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <os@...ix.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 */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 8ade0a7..9960be0 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -10,6 +10,9 @@ menu "Library routines"
>  config BITREVERSE
>  	tristate
>  
> +config RATIONAL
> +	boolean
> +
>  config GENERIC_FIND_FIRST_BIT
>  	bool
>  
> diff --git a/lib/Makefile b/lib/Makefile
> index 33a40e4..1f6edef 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -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
> diff --git a/lib/rational.c b/lib/rational.c
> new file mode 100644
> index 0000000..76f5a0c
> --- /dev/null
> +++ b/lib/rational.c
> @@ -0,0 +1,60 @@
> +/*
> + * rational fractions
> + *
> + * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <os@...ix.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(_GPL)?

Sascha

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