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[Qemu-devel] [6961] Avoid rounding problems in ptimer_get_count
From: |
Paul Brook |
Subject: |
[Qemu-devel] [6961] Avoid rounding problems in ptimer_get_count |
Date: |
Tue, 31 Mar 2009 14:34:24 +0000 |
Revision: 6961
http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=6961
Author: pbrook
Date: 2009-03-31 14:34:24 +0000 (Tue, 31 Mar 2009)
Log Message:
-----------
Avoid rounding problems in ptimer_get_count
Signed-off-by: Paul Brook <address@hidden>
Modified Paths:
--------------
trunk/hw/ptimer.c
Modified: trunk/hw/ptimer.c
===================================================================
--- trunk/hw/ptimer.c 2009-03-31 05:55:16 UTC (rev 6960)
+++ trunk/hw/ptimer.c 2009-03-31 14:34:24 UTC (rev 6961)
@@ -7,8 +7,8 @@
*/
#include "hw.h"
#include "qemu-timer.h"
+#include "host-utils.h"
-
struct ptimer_state
{
int enabled; /* 0 = disabled, 1 = periodic, 2 = oneshot. */
@@ -78,9 +78,39 @@
} 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 {
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