Improve the implementation of the _NET_WM_SYNC_REQUEST protocol

This change improves the synced resizes of xcb windows and adds support
for synced resizes of glx windows.

The QXcbWindow keeps a better track on whether the window manager
expects a sync and can be in one of three states:
* no sync required
* sync required, but configure notify event not yet received
* sync required and configured

By tracking this in the QXcbWindow itself the backing store can make
use of this information and doesn't need an own heuristic to decide
whether a sync is needed.

Also this allows to add support for synced resizes of windows with an
OpenGLSurface. This is accomplished by checking the sync state after
swapping buffers. As the OpenGL context may be bound to a background
thread the sync is done using a QueuedConnection to ensure that the
sync happens in the thread which created the xcb window.
So far this is only added for GLX.

This significantly improves the resize experience of QQuickWindow and
also the initial mapping with a composited window manager in case the
compositor uses the sync protocol to determine whether the window is
ready to get painted on screen.

Change-Id: Ied0261873043d785dec652d2821fc3638292fa36
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

1 files changed, 23 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbwindow.cpp b/src/plugins/platforms/xcb/qxcbwindow.cpp
index 3645b6469a..a46fe437d8 100644
--- a/src/plugins/platforms/xcb/qxcbwindow.cpp
+++ b/src/plugins/platforms/xcb/qxcbwindow.cpp
@@ -200,6 +200,7 @@ QXcbWindow::QXcbWindow(QWindow *window)
     , m_eglSurface(0)
 #endif
     , m_lastWindowStateEvent(-1)
+    , m_syncState(NoSyncNeeded)
 {
     m_screen = static_cast<QXcbScreen *>(window->screen()->handle());
 
@@ -367,7 +368,12 @@ void QXcbWindow::create()
     properties[propertyCount++] = atom(QXcbAtom::WM_TAKE_FOCUS);
     properties[propertyCount++] = atom(QXcbAtom::_NET_WM_PING);
 
-    m_usingSyncProtocol = m_screen->syncRequestSupported() && window()->surfaceType() != QSurface::OpenGLSurface;
+    m_usingSyncProtocol = m_screen->syncRequestSupported();
+#if !defined(XCB_USE_GLX)
+    // synced resize only implemented on GLX
+    if (window()->supportsOpenGL())
+        m_usingSyncProtocol = false;
+#endif
 
     if (m_usingSyncProtocol)
         properties[propertyCount++] = atom(QXcbAtom::_NET_WM_SYNC_REQUEST);
@@ -1596,6 +1602,8 @@ void QXcbWindow::handleClientMessageEvent(const xcb_client_message_event_t *even
             connection()->setTime(event->data.data32[1]);
             m_syncValue.lo = event->data.data32[2];
             m_syncValue.hi = event->data.data32[3];
+            if (m_usingSyncProtocol)
+                m_syncState = SyncReceived;
 #ifndef QT_NO_WHATSTHIS
         } else if (event->data.data32[0] == atom(QXcbAtom::_NET_WM_CONTEXT_HELP)) {
             QWindowSystemInterface::handleEnterWhatsThisEvent();
@@ -1669,6 +1677,9 @@ void QXcbWindow::handleConfigureNotifyEvent(const xcb_configure_notify_event_t *
         QWindowSystemInterface::handleExposeEvent(window(), QRect(QPoint(), geometry().size()));
     }
 
+    if (m_usingSyncProtocol && m_syncState == SyncReceived)
+        m_syncState = SyncAndConfigureReceived;
+
     m_dirtyFrameMargins = true;
 }
 
@@ -1945,12 +1956,17 @@ void QXcbWindow::handleFocusOutEvent(const xcb_focus_out_event_t *)
 
 void QXcbWindow::updateSyncRequestCounter()
 {
+    if (m_syncState != SyncAndConfigureReceived) {
+        // window manager does not expect a sync event yet.
+        return;
+    }
     if (m_usingSyncProtocol && (m_syncValue.lo != 0 || m_syncValue.hi != 0)) {
         Q_XCB_CALL(xcb_sync_set_counter(xcb_connection(), m_syncCounter, m_syncValue));
-        connection()->sync();
+        xcb_flush(xcb_connection());
 
         m_syncValue.lo = 0;
         m_syncValue.hi = 0;
+        m_syncState = NoSyncNeeded;
     }
 }
 
@@ -2173,4 +2189,9 @@ void QXcbWindow::setAlertState(bool enabled)
     }
 }
 
+bool QXcbWindow::needsSync() const
+{
+    return m_syncState == SyncAndConfigureReceived;
+}
+
 QT_END_NAMESPACE