backends/drm: allow up to two composited frames to be pending at the same time

This should improve responsiveness on setups where rendering each frame takes longer
than the refresh cycle of the display.

BUG: 452119
CCBUG: 454098

src/backends/drm/drm_output.cpp

@@ -36,6 +36,7 @@ namespace KWin
 {
 
 static const bool s_allowColorspaceIntel = qEnvironmentVariableIntValue("KWIN_DRM_ALLOW_INTEL_COLORSPACE") == 1;
+static const bool s_disableTripleBuffering = qEnvironmentVariableIntValue("KWIN_DRM_DISABLE_TRIPLE_BUFFERING") == 1;
 
 DrmOutput::DrmOutput(const std::shared_ptr<DrmConnector> &conn)
     : DrmAbstractOutput(conn->gpu())
@@ -44,6 +45,9 @@ DrmOutput::DrmOutput(const std::shared_ptr<DrmConnector> &conn)
 {
     m_pipeline->setOutput(this);
     m_renderLoop->setRefreshRate(m_pipeline->mode()->refreshRate());
+    if (m_gpu->atomicModeSetting() && !s_disableTripleBuffering) {
+        m_renderLoop->setMaxPendingFrameCount(2);
+    }
 
     Capabilities capabilities = Capability::Dpms | Capability::IccProfile;
     State initialState;