Drift correction and better accuracy for repeating timers in Symbian

Timers on Symbian were always firing at least 2ms late. This was partly
due to a paranoid extra delay in Qt and the kernel being very cautious.

For one shot timers this is not so bad although a bit too much. But for
repeating timers, this can cause significant reductions in firing rate
particularly for short period timers.

The timer active object now corrects timer lateness by up to 4ms per
event. This is enough to compensate for the kernel lateness without
the possibility of wild corrections.

Task-number: QTBUG-18549
Reviewed-by: iain

2 files changed, 25 insertions, 5 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_symbian.cpp b/src/corelib/kernel/qeventdispatcher_symbian.cpp
index 47dd558..84825af 100644
--- a/src/corelib/kernel/qeventdispatcher_symbian.cpp
+++ b/src/corelib/kernel/qeventdispatcher_symbian.cpp
@@ -210,8 +210,10 @@ void QWakeUpActiveObject::RunL()
 
 QTimerActiveObject::QTimerActiveObject(QEventDispatcherSymbian *dispatcher, SymbianTimerInfo *timerInfo)
     : QActiveObject((timerInfo->interval) ? TIMER_PRIORITY : NULLTIMER_PRIORITY , dispatcher),
-      m_timerInfo(timerInfo)
+      m_timerInfo(timerInfo), m_expectedTimeSinceLastEvent(0)
 {
+    // start the timeout timer to ensure initialisation
+    m_timeoutTimer.start();
 }
 
 QTimerActiveObject::~QTimerActiveObject()
@@ -255,10 +257,23 @@ void QTimerActiveObject::StartTimer()
         m_rTimer.After(iStatus, MAX_SYMBIAN_TIMEOUT_MS * 1000);
         m_timerInfo->msLeft -= MAX_SYMBIAN_TIMEOUT_MS;
     } else {
-        //HighRes gives the 1ms accuracy expected by Qt, the +1 is to ensure that
-        //"Timers will never time out earlier than the specified timeout value"
-        //condition is always met.
-        m_rTimer.HighRes(iStatus, (m_timerInfo->msLeft + 1) * 1000);
+        // this algorithm implements drift correction for repeating timers
+        // calculate how late we are for this event
+        int timeSinceLastEvent = m_timeoutTimer.restart();
+        int overshoot = timeSinceLastEvent - m_expectedTimeSinceLastEvent;
+        if (overshoot > m_timerInfo->msLeft) {
+            // we skipped a whole timeout, restart from here
+            overshoot = 0;
+        }
+        // calculate when the next event should happen
+        int waitTime = m_timerInfo->msLeft - overshoot;
+        m_expectedTimeSinceLastEvent = waitTime;
+        // limit the actual ms wait time to avoid wild corrections
+        // this will cause the real event time to slowly drift back to the expected event time
+        // measurements show that Symbian timers always fire 1 or 2 ms late
+        const int limit = 4;
+        waitTime = qMax(m_timerInfo->msLeft - limit, waitTime);
+        m_rTimer.HighRes(iStatus, waitTime * 1000);
         m_timerInfo->msLeft = 0;
     }
     SetActive();
@@ -305,6 +320,8 @@ void QTimerActiveObject::Start()
         if (!m_rTimer.Handle()) {
             qt_symbian_throwIfError(m_rTimer.CreateLocal());
         }
+        m_timeoutTimer.start();
+        m_expectedTimeSinceLastEvent = 0;
         StartTimer();
     } else {
         iStatus = KRequestPending;
diff --git a/src/corelib/kernel/qeventdispatcher_symbian_p.h b/src/corelib/kernel/qeventdispatcher_symbian_p.h
index b785aea..a31a446 100644
--- a/src/corelib/kernel/qeventdispatcher_symbian_p.h
+++ b/src/corelib/kernel/qeventdispatcher_symbian_p.h
@@ -63,6 +63,7 @@
 #include <qwaitcondition.h>
 #include <qsocketnotifier.h>
 #include <qdatetime.h>
+#include <qelapsedtimer.h>
 
 #include <e32base.h>
 
@@ -143,6 +144,8 @@ private:
 
 private:
     SymbianTimerInfo *m_timerInfo;
+    QElapsedTimer m_timeoutTimer;
+    int m_expectedTimeSinceLastEvent;
     RTimer m_rTimer;
 };