Add means for forcing non-blocking buffer swap support

In case of a driver that does not properly indicate its capability of
supporting non-blocking eglSwapBuffers() call, the need arises to be
able to manually force the resolution of that check.
New environment variable QT_WAYLAND_FORCE_NONBLOCKING_SWAP_SUPPORT
added. If it is defined to 0 (i.e. false), the non-blocking swap
support is forced to behave as if the driver had been detected not to
be supported. Equally setting it to 1 causes the non-blocking swap
support resolution to remain true (default) even if driver reports it
does not support it.

[ChangeLog][Client] Added support for environment variable
QT_WAYLAND_FORCE_NONBLOCKING_SWAP_SUPPORT, which can be used to force
the resolution of detecting if EGL driver supports non-blocking
eglSwapBuffers call. If you encounter application freezing while
waiting indefinitely for a buffer swap, then try setting variable to 0.

Task-number: QTBUG-84226
Pick-to: 5.15
Pick-to: 5.12
Change-Id: I92dc505d12daf4d78dc18e4f85f3a55ecdf109f3
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

1 files changed, 9 insertions, 3 deletions

diff --git a/src/hardwareintegration/client/wayland-egl/qwaylandglcontext.cpp b/src/hardwareintegration/client/wayland-egl/qwaylandglcontext.cpp
index ec55bdcd0..bb903cf1f 100644
--- a/src/hardwareintegration/client/wayland-egl/qwaylandglcontext.cpp
+++ b/src/hardwareintegration/client/wayland-egl/qwaylandglcontext.cpp
@@ -322,9 +322,15 @@ QWaylandGLContext::QWaylandGLContext(EGLDisplay eglDisplay, QWaylandDisplay *dis
     if (!eglGetConfigAttrib(m_eglDisplay, m_config, a, &a) ||
         !eglGetConfigAttrib(m_eglDisplay, m_config, b, &b) ||
         a > 0) {
-       mSupportNonBlockingSwap = false;
+       m_supportNonBlockingSwap = false;
     }
-    if (!mSupportNonBlockingSwap) {
+    {
+        bool ok;
+        int supportNonBlockingSwap = qEnvironmentVariableIntValue("QT_WAYLAND_FORCE_NONBLOCKING_SWAP_SUPPORT", &ok);
+        if (ok)
+            m_supportNonBlockingSwap = supportNonBlockingSwap != 0;
+    }
+    if (!m_supportNonBlockingSwap) {
         qWarning(lcQpaWayland) << "Non-blocking swap buffers not supported."
                                << "Subsurface rendering can be affected."
                                << "It may also cause the event loop to freeze in some situations";
@@ -478,7 +484,7 @@ void QWaylandGLContext::swapBuffers(QPlatformSurface *surface)
         eglMakeCurrent(currentDisplay, currentSurfaceDraw, currentSurfaceRead, currentContext);
     }
 
-    int swapInterval = mSupportNonBlockingSwap ? 0 : m_format.swapInterval();
+    int swapInterval = m_supportNonBlockingSwap ? 0 : m_format.swapInterval();
     eglSwapInterval(m_eglDisplay, swapInterval);
     if (swapInterval == 0 && m_format.swapInterval() > 0) {
         // Emulating a blocking swap