prevent yield/swap cpu overhead on nvidia

CCBUG: 322060
REVIEW: 111663

kwin/eglonxbackend.cpp

@@ -277,6 +277,18 @@ void EglOnXBackend::present()
             eglWaitGL();
             if (char result = m_swapProfiler.end()) {
                 gs_tripleBufferUndetected = gs_tripleBufferNeedsDetection = false;
+                if (result == 'd' && GLPlatform::instance()->driver() == Driver_NVidia) {
+                    // TODO this is a workaround, we should get __GL_YIELD set before libGL checks it
+                    if (qstrcmp(qgetenv("__GL_YIELD"), "USLEEP")) {
+                        options->setGlPreferBufferSwap(0);
+                        eglSwapInterval(dpy, 0);
+                        kWarning(1212) << "\nIt seems you are using the nvidia driver without triple buffering\n"
+                                          "You must export __GL_YIELD=\"USLEEP\" to prevent large CPU overhead on synced swaps\n"
+                                          "Preferably, enable the TripleBuffer Option in the xorg.conf Device\n"
+                                          "For this reason, the tearing prevention has been disabled.\n"
+                                          "See https://bugs.kde.org/show_bug.cgi?id=322060\n";
+                    }
+                }
                 setBlocksForRetrace(result == 'd');
             }
         }

kwin/glxbackend.cpp

@@ -432,6 +432,18 @@ void GlxBackend::present()
                 glXWaitGL();
                 if (char result = m_swapProfiler.end()) {
                     gs_tripleBufferUndetected = gs_tripleBufferNeedsDetection = false;
+                    if (result == 'd' && GLPlatform::instance()->driver() == Driver_NVidia) {
+                        // TODO this is a workaround, we should get __GL_YIELD set before libGL checks it
+                        if (qstrcmp(qgetenv("__GL_YIELD"), "USLEEP")) {
+                            options->setGlPreferBufferSwap(0);
+                            setSwapInterval(0);
+                            kWarning(1212) << "\nIt seems you are using the nvidia driver without triple buffering\n"
+                                              "You must export __GL_YIELD=\"USLEEP\" to prevent large CPU overhead on synced swaps\n"
+                                              "Preferably, enable the TripleBuffer Option in the xorg.conf Device\n"
+                                              "For this reason, the tearing prevention has been disabled.\n"
+                                              "See https://bugs.kde.org/show_bug.cgi?id=322060\n";
+                        }
+                    }
                     setBlocksForRetrace(result == 'd');
                 }
             }