From 20542f9546637bd649c928226249be0ffc91841b Mon Sep 17 00:00:00 2001
From: Jani Hautakangas <jani.hautakangas@nokia.com>
Date: Wed, 19 Oct 2011 09:39:22 +0300
Subject: Workaround to VideoCore III scissor bug.
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Some versions of VideoCore III drivers seem
to pollute and use stencil buffer when using
glScissors. Workaround is to clear stencil buffer
before disabling scissoring.

Task-number: QT-5308
Reviewed-by: Samuel Rødal
---
 src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp |  2 ++
 src/opengl/qgl.cpp                                     |  2 ++
 src/opengl/qgl_egl.cpp                                 | 11 ++++++++++-
 src/opengl/qgl_p.h                                     |  2 ++
 4 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
index 047d589..999a074 100644
--- a/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
+++ b/src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp
@@ -2059,6 +2059,8 @@ void QGL2PaintEngineExPrivate::updateClipScissorTest()
     currentScissorBounds = bounds;
 
     if (bounds == QRect(0, 0, width, height)) {
+        if (ctx->d_func()->workaround_brokenScissor)
+            clearClip(0);
         glDisable(GL_SCISSOR_TEST);
     } else {
         glEnable(GL_SCISSOR_TEST);
diff --git a/src/opengl/qgl.cpp b/src/opengl/qgl.cpp
index 3b3da43..af160d8 100644
--- a/src/opengl/qgl.cpp
+++ b/src/opengl/qgl.cpp
@@ -1716,6 +1716,8 @@ void QGLContextPrivate::init(QPaintDevice *dev, const QGLFormat &format)
     workaround_brokenTextureFromPixmap = false;
     workaround_brokenTextureFromPixmap_init = false;
 
+    workaround_brokenScissor = false;
+
     for (int i = 0; i < QT_GL_VERTEX_ARRAY_TRACKED_COUNT; ++i)
         vertexAttributeArraysEnabledState[i] = false;
 }
diff --git a/src/opengl/qgl_egl.cpp b/src/opengl/qgl_egl.cpp
index a589118..07134cf 100644
--- a/src/opengl/qgl_egl.cpp
+++ b/src/opengl/qgl_egl.cpp
@@ -192,7 +192,9 @@ void QGLContext::makeCurrent()
         if (!d->workaroundsCached) {
             d->workaroundsCached = true;
             const char *renderer = reinterpret_cast<const char *>(glGetString(GL_RENDERER));
-            if (renderer && (strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
+            if (!renderer)
+                return;
+            if ((strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
                 // PowerVR MBX/SGX chips needs to clear all buffers when starting to render
                 // a new frame, otherwise there will be a performance penalty to pay for
                 // each frame.
@@ -229,6 +231,13 @@ void QGLContext::makeCurrent()
                         d->workaround_brokenFBOReadBack = true;
                     }
                 }
+            } else if (strstr(renderer, "VideoCore III")) {
+                // Some versions of VideoCore III drivers seem to pollute and use
+                // stencil buffer when using glScissors even if stencil test is disabled.
+                // Workaround is to clear stencil buffer before disabling scissoring.
+
+                // qDebug() << "Found VideoCore III driver, enabling brokenDisableScissorTest"; 
+                d->workaround_brokenScissor = true;
             }
         }
     }
diff --git a/src/opengl/qgl_p.h b/src/opengl/qgl_p.h
index c56b2db..d76f0b0 100644
--- a/src/opengl/qgl_p.h
+++ b/src/opengl/qgl_p.h
@@ -415,6 +415,8 @@ public:
     uint workaround_brokenTextureFromPixmap : 1;
     uint workaround_brokenTextureFromPixmap_init : 1;
 
+    uint workaround_brokenScissor : 1;
+
     QPaintDevice *paintDevice;
     QColor transpColor;
     QGLContext *q_ptr;
-- 
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