Use Qt::TimerType on Windows when starting timers

Make all Qt::PreciseTimers and timers with intervals < 20ms use
Multimedia timers for maximum accuracy. Qt::CoarseTimers and
Qt::VeryCoarseTimers use normal Windows timers, with
Qt::VeryCoarseTimers having their interval rounded to the nearest
full second.

Note that the Windows timer implementation does not attempt to align
timers and reduce CPU wakeups like the UNIX implementation does. This
might be done in the future, though. However, this change does the
best we can do now, keeping most timers working as-is, while allowing
explicit use of Multimedia timers via Qt::PreciseTimer.

Change-Id: I1898272b101c572a2a9b9454fef27a651eb178f5
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index 58ca8d312e..dbc399809f 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -2875,4 +2875,10 @@
     \value PreciseTimer Precise timers try to keep millisecond accuracy
     \value CoarseTimer Coarse timers try to keep accuracy within 5% of the desired interval
     \value VeryCoarseTimer Very coarse timers only keep full second accuracy
+
+    On Windows, Qt will use Windows's Multimedia timer facility (if available)
+    for Qt::PreciseTimer and normal Windows timers for Qt::CoarseTimer and
+    Qt::VeryCoarseTimer. The interval for Qt::VeryCoarseTimer is rounded to the
+    nearest full second (e.g. an interval of 23500ms will be rounded to
+    24000ms, and 20300ms to 20000ms).
 */