eglfs: Detect llvmpipe and show a warning

Do ourselves and our users a favor by pointing out why Qt (Quick)
apps perform horribly on commonly used distros on the RPi. Using
a software rasterizer on such boards is not going to cut it.

Task-number: QTBUG-50533
Change-Id: I087f502ddb9c6bdde84343e6abd85c87cdc474f0
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>

src/platformsupport/eglconvenience/qeglplatformcontext.cpp

@@ -275,6 +275,12 @@ void QEGLPlatformContext::destroyTemporaryOffscreenSurface(EGLSurface surface)
     eglDestroySurface(m_eglDisplay, surface);
 }
 
+void QEGLPlatformContext::runGLChecks()
+{
+    // Nothing to do here, subclasses may override in order to perform OpenGL
+    // queries needing a context.
+}
+
 void QEGLPlatformContext::updateFormatFromGL()
 {
 #ifndef QT_NO_OPENGL
@@ -346,6 +352,7 @@ void QEGLPlatformContext::updateFormatFromGL()
                 }
             }
         }
+        runGLChecks();
         eglMakeCurrent(prevDisplay, prevSurfaceDraw, prevSurfaceRead, prevContext);
     } else {
         qWarning("QEGLPlatformContext: Failed to make temporary surface current, format not updated (%x)", eglGetError());

src/platformsupport/eglconvenience/qeglplatformcontext_p.h

@@ -84,6 +84,7 @@ protected:
     virtual EGLSurface eglSurfaceForPlatformSurface(QPlatformSurface *surface) = 0;
     virtual EGLSurface createTemporaryOffscreenSurface();
     virtual void destroyTemporaryOffscreenSurface(EGLSurface surface);
+    virtual void runGLChecks();
 
 private:
     void init(const QSurfaceFormat &format, QPlatformOpenGLContext *share);

src/plugins/platforms/eglfs/qeglfscontext.cpp

@@ -85,6 +85,16 @@ void QEglFSContext::destroyTemporaryOffscreenSurface(EGLSurface surface)
     }
 }
 
+void QEglFSContext::runGLChecks()
+{
+    // Note that even though there is an EGL context current here,
+    // QOpenGLContext and QOpenGLFunctions are not yet usable at this stage.
+    const char *renderer = reinterpret_cast<const char *>(glGetString(GL_RENDERER));
+    // Be nice and warn about a common source of confusion.
+    if (renderer && strstr(renderer, "llvmpipe"))
+        qWarning("Running on a software rasterizer (LLVMpipe), expect limited performance.");
+}
+
 void QEglFSContext::swapBuffers(QPlatformSurface *surface)
 {
     // draw the cursor

src/plugins/platforms/eglfs/qeglfscontext.h

@@ -48,6 +48,7 @@ public:
     EGLSurface eglSurfaceForPlatformSurface(QPlatformSurface *surface) Q_DECL_OVERRIDE;
     EGLSurface createTemporaryOffscreenSurface() Q_DECL_OVERRIDE;
     void destroyTemporaryOffscreenSurface(EGLSurface surface) Q_DECL_OVERRIDE;
+    void runGLChecks() Q_DECL_OVERRIDE;
     void swapBuffers(QPlatformSurface *surface) Q_DECL_OVERRIDE;
 
 private: