Avoid rounding problems in ptimer_get_count

Signed-off-by: Paul Brook <paul@codesourcery.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6961 c046a42c-6fe2-441c-8c8c-71466251a162

1 files changed, 31 insertions, 1 deletions

diff --git a/hw/ptimer.c b/hw/ptimer.c
index 9d3862724d..b36f9a6f07 100644
--- a/hw/ptimer.c
+++ b/hw/ptimer.c
@@ -7,7 +7,7 @@
  */
 #include "hw.h"
 #include "qemu-timer.h"
-
+#include "host-utils.h"
 
 struct ptimer_state
 {
@@ -78,9 +78,39 @@ uint64_t ptimer_get_count(ptimer_state *s)
         } else {
             uint64_t rem;
             uint64_t div;
+            uint32_t frac;
+            int clz1, clz2;
+            int shift;
+
+            /* We need to divide time by period, where time is stored in
+               rem (64-bit integer) and period is stored in period/period_frac
+               (64.32 fixed point).
+              
+               Doing full precision division is hard, so scale values and
+               do a 64-bit division.  The result should be rounded down,
+               so that the rounding error never causes the timer to go
+               backwards.
+            */
 
             rem = s->next_event - now;
             div = s->period;
+
+            clz1 = clz64(rem);
+            clz2 = clz64(div);
+            shift = clz1 < clz2 ? clz1 : clz2;
+
+            rem <<= shift;
+            div <<= shift;
+            if (shift >= 32) {
+                div |= ((uint64_t)s->period_frac << (shift - 32));
+            } else {
+                if (shift != 0)
+                    div |= (s->period_frac >> (32 - shift));
+                /* Look at remaining bits of period_frac and round div up if 
+                   necessary.  */
+                if ((uint32_t)(s->period_frac << shift))
+                    div += 1;
+            }
             counter = rem / div;
         }
     } else {