[PATCH 1/5] mc146818rtc: update periodic timer only if it is needed

[guangrong.xiao@gmail.com]

From: Xiao Guangrong <xiaoguangrong@tencent.com>

Currently, the timer is updated whenever RegA or RegB is written
even if the periodic timer related configuration is not changed

This patch optimizes it slightly to make the update happen only
if its period or enable-status is changed, also later patches are
depend on this optimization

Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
---
 hw/timer/mc146818rtc.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
index 4165450..749e206 100644
--- a/hw/timer/mc146818rtc.c
+++ b/hw/timer/mc146818rtc.c
@@ -387,10 +387,25 @@ static void rtc_update_timer(void *opaque)
     check_update_timer(s);
 }
 
+static bool rtc_periodic_timer_updated_by_regA(RTCState *s, uint64_t data)
+{
+    uint8_t orig = s->cmos_data[RTC_REG_A];
+
+    return (orig & 0x0f) != (data & 0x0f);
+}
+
+static bool rtc_periodic_timer_updated_by_regB(RTCState *s, uint64_t data)
+{
+    uint8_t orig = s->cmos_data[RTC_REG_B];
+
+    return (orig & REG_B_PIE) != (data & REG_B_PIE);
+}
+
 static void cmos_ioport_write(void *opaque, hwaddr addr,
                               uint64_t data, unsigned size)
 {
     RTCState *s = opaque;
+    bool update_periodic_timer;
 
     if ((addr & 1) == 0) {
         s->cmos_index = data & 0x7f;
@@ -423,6 +438,8 @@ static void cmos_ioport_write(void *opaque, hwaddr addr,
             }
             break;
         case RTC_REG_A:
+            update_periodic_timer = rtc_periodic_timer_updated_by_regA(s, data);
+
             if ((data & 0x60) == 0x60) {
                 if (rtc_running(s)) {
                     rtc_update_time(s);
@@ -445,10 +462,16 @@ static void cmos_ioport_write(void *opaque, hwaddr addr,
             /* UIP bit is read only */
             s->cmos_data[RTC_REG_A] = (data & ~REG_A_UIP) |
                 (s->cmos_data[RTC_REG_A] & REG_A_UIP);
-            periodic_timer_update(s, qemu_clock_get_ns(rtc_clock));
+
+            if (update_periodic_timer) {
+                periodic_timer_update(s, qemu_clock_get_ns(rtc_clock));
+            }
+
             check_update_timer(s);
             break;
         case RTC_REG_B:
+            update_periodic_timer = rtc_periodic_timer_updated_by_regB(s, data);
+
             if (data & REG_B_SET) {
                 /* update cmos to when the rtc was stopping */
                 if (rtc_running(s)) {
@@ -475,7 +498,11 @@ static void cmos_ioport_write(void *opaque, hwaddr addr,
                 qemu_irq_lower(s->irq);
             }
             s->cmos_data[RTC_REG_B] = data;
-            periodic_timer_update(s, qemu_clock_get_ns(rtc_clock));
+
+            if (update_periodic_timer) {
+                periodic_timer_update(s, qemu_clock_get_ns(rtc_clock));
+            }
+
             check_update_timer(s);
             break;
         case RTC_REG_C:
-- 
2.9.3