[PATCH] shift and clocksource

[Carlo Caione]

Added clocks_calc_mult_shift()

Signed-off-by: Carlo Caione <carlo.caione at gmail.com>
---
 common/clock.c  |   55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 include/clock.h |    2 ++
 2 files changed, 57 insertions(+), 0 deletions(-)

diff --git a/common/clock.c b/common/clock.c
index 15df0ab..79c06c8 100644
--- a/common/clock.c
+++ b/common/clock.c
@@ -57,6 +57,61 @@ uint64_t get_time_ns(void)
 EXPORT_SYMBOL(get_time_ns);
 
 /**
+ * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
+ * @mult:       pointer to mult variable
+ * @shift:      pointer to shift variable
+ * @from:       frequency to convert from
+ * @to:         frequency to convert to
+ * @maxsec:     guaranteed runtime conversion range in seconds
+ *
+ * The function evaluates the shift/mult pair for the scaled math
+ * operations of clocksources and clockevents.
+ *
+ * @to and @from are frequency values in HZ. For clock sources @to is
+ * NSEC_PER_SEC == 1GHz and @from is the counter frequency. For clock
+ * event @to is the counter frequency and @from is NSEC_PER_SEC.
+ *
+ * The @maxsec conversion range argument controls the time frame in
+ * seconds which must be covered by the runtime conversion with the
+ * calculated mult and shift factors. This guarantees that no 64bit
+ * overflow happens when the input value of the conversion is
+ * multiplied with the calculated mult factor. Larger ranges may
+ * reduce the conversion accuracy by chosing smaller mult and shift
+ * factors.
+ */
+
+void clocks_calc_mult_shift(uint32_t *mult, uint32_t *shift, uint32_t from, uint32_t to, uint32_t maxsec)
+{
+        uint64_t tmp;
+        uint32_t sft, sftacc = 32;
+
+        /*
+         * Calculate the shift factor which is limiting the conversion
+         * range:
+         */
+        tmp = ((uint64_t)maxsec * from) >> 32;
+        while (tmp) {
+                tmp >>=1;
+                sftacc--;
+        }
+
+        /*
+         * Find the conversion shift/mult pair which has the best
+         * accuracy and fits the maxsec conversion range:
+         */
+        for (sft = 32; sft > 0; sft--) {
+                tmp = (uint64_t) to << sft;
+                tmp += from / 2;
+                do_div(tmp, from);
+                if ((tmp >> sftacc) == 0)
+                        break;
+        }
+        *mult = tmp;
+        *shift = sft;
+}
+
+
+/**
  * clocksource_hz2mult - calculates mult from hz and shift
  * @hz:                 Clocksource frequency in Hz
  * @shift_constant:     Clocksource shift factor
diff --git a/include/clock.h b/include/clock.h
index b9acdb9..af5b939 100644
--- a/include/clock.h
+++ b/include/clock.h
@@ -25,6 +25,8 @@ int init_clock(struct clocksource *);
 
 uint64_t get_time_ns(void);
 
+void clocks_calc_mult_shift(uint32_t *mult, uint32_t *shift, uint32_t from, uint32_t to, uint32_t maxsec);
+
 uint32_t clocksource_hz2mult(uint32_t hz, uint32_t shift_constant);
 
 int is_timeout(uint64_t start_ns, uint64_t time_offset_ns);
-- 
1.7.5.2