Utilize EGL_KHR_surfaceless_context in QOffscreenSurface when available

Use it also in the context initialization code which also relied on pbuffers
until now.

This is safe to do since QOffscreenSurface is deliberately kept minimal: we only
support it for rendering to FBOs or uploading textures, it is quite useless for
any rendering type of operation. Therefore simply not creating a surface when
the extension is available is viable and should not break anything.

While we should have done this a long time ago, the real inspiration now is
to avoid crashes with some bad EGL implementations.

Mesa in particular tends to crash in its interal configuration handling when
trying to create a pbuffer surface with more exotic attributes, for example with
multisampling enabled. Just getting rid of those properties is not an option
since the resulting pbuffer would not be compatible with the existing context.
Therefore our only option is to avoid the pbuffer altogether.

There's a so-far-ignored patch for Mesa http://patchwork.freedesktop.org/patch/38100/
which tries to solve some of this but it turns out it is not sufficient to avoid problems
with the multisampled case. So to avoid more headache, we just stop using pbuffers
there.

[ChangeLog][QtGui] QOffscreenSurface is now relying on EGL_KHR_surfaceless_context when
available, and avoids creating a pbuffer surface when the extension is present.

Change-Id: Id18742768b8e66c8d92ce65a9bf64b0296e14db7
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

1 files changed, 18 insertions, 6 deletions

diff --git a/src/platformsupport/eglconvenience/qeglplatformcontext.cpp b/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
index d7c4fd6764..984abe123f 100644
--- a/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
+++ b/src/platformsupport/eglconvenience/qeglplatformcontext.cpp
@@ -278,7 +278,13 @@ void QEGLPlatformContext::updateFormatFromGL()
     EGLSurface prevSurfaceDraw = eglGetCurrentSurface(EGL_DRAW);
     EGLSurface prevSurfaceRead = eglGetCurrentSurface(EGL_READ);
 
-    EGLSurface tempSurface = createTemporaryOffscreenSurface();
+    // Rely on the surfaceless extension, if available. This is beneficial since we can
+    // avoid creating an extra pbuffer surface which is apparently troublesome with some
+    // drivers (Mesa) when certain attributes are present (multisampling).
+    EGLSurface tempSurface = EGL_NO_SURFACE;
+    if (!q_hasEglExtension(m_eglDisplay, "EGL_KHR_surfaceless_context"))
+        tempSurface = createTemporaryOffscreenSurface();
+
     if (eglMakeCurrent(m_eglDisplay, tempSurface, tempSurface, m_eglContext)) {
         if (m_format.renderableType() == QSurfaceFormat::OpenGL
             || m_format.renderableType() == QSurfaceFormat::OpenGLES) {
@@ -316,8 +322,11 @@ void QEGLPlatformContext::updateFormatFromGL()
             }
         }
         eglMakeCurrent(prevDisplay, prevSurfaceDraw, prevSurfaceRead, prevContext);
+    } else {
+        qWarning("QEGLPlatformContext: Failed to make temporary surface current, format not updated");
     }
-    destroyTemporaryOffscreenSurface(tempSurface);
+    if (tempSurface != EGL_NO_SURFACE)
+        destroyTemporaryOffscreenSurface(tempSurface);
 #endif // QT_NO_OPENGL
 }
 
@@ -354,7 +363,8 @@ bool QEGLPlatformContext::makeCurrent(QPlatformSurface *surface)
             : surface->format().swapInterval();
         if (requestedSwapInterval >= 0 && m_swapInterval != requestedSwapInterval) {
             m_swapInterval = requestedSwapInterval;
-            eglSwapInterval(eglDisplay(), m_swapInterval);
+            if (eglSurface != EGL_NO_SURFACE) // skip if using surfaceless context
+                eglSwapInterval(eglDisplay(), m_swapInterval);
         }
     } else {
         qWarning("QEGLPlatformContext: eglMakeCurrent failed: %x", eglGetError());
@@ -383,9 +393,11 @@ void QEGLPlatformContext::swapBuffers(QPlatformSurface *surface)
 {
     eglBindAPI(m_api);
     EGLSurface eglSurface = eglSurfaceForPlatformSurface(surface);
-    bool ok = eglSwapBuffers(m_eglDisplay, eglSurface);
-    if (!ok)
-        qWarning("QEGLPlatformContext: eglSwapBuffers failed: %x", eglGetError());
+    if (eglSurface != EGL_NO_SURFACE) { // skip if using surfaceless context
+        bool ok = eglSwapBuffers(m_eglDisplay, eglSurface);
+        if (!ok)
+            qWarning("QEGLPlatformContext: eglSwapBuffers failed: %x", eglGetError());
+    }
 }
 
 void (*QEGLPlatformContext::getProcAddress(const QByteArray &procName)) ()