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: Oskar Schirmer <os@emlix.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 62 insertions, 0 deletions

diff --git a/lib/rational.c b/lib/rational.c
new file mode 100644
index 000000000000..b3c099b5478e
--- /dev/null
+++ b/lib/rational.c
@@ -0,0 +1,62 @@
+/*
+ * 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);