backends/drm: don't wake displays up when outputs get temporarily removed

When some displays go to sleep, that can be wrongly detected as a temporary
hotunplug by the driver. In order to not wrongly wake up the system with
such a display, detect that scenario and set the 'new' output to dpms off
again.

BUG: 452553
BUG: 379474

src/backends/drm/drm_backend.cpp

@@ -212,6 +212,21 @@ DrmGpu *DrmBackend::addGpu(const QString &fileName)
 
 void DrmBackend::addOutput(DrmAbstractOutput *o)
 {
+    const bool allOff = std::all_of(m_outputs.begin(), m_outputs.end(), [](Output *output) {
+        return output->dpmsMode() != Output::DpmsMode::On;
+    });
+    if (allOff && m_recentlyUnpluggedDpmsOffOutputs.contains(o->uuid())) {
+        if (DrmOutput *drmOutput = qobject_cast<DrmOutput *>(o)) {
+            // When the system is in dpms power saving mode, KWin turns on all outputs if the user plugs a new output in
+            // as that's an intentional action and they expect to see the output light up.
+            // Some outputs however temporarily disconnect in some situations, most often shortly after they go into standby.
+            // To not turn on outputs in that case, restore the previous dpms state
+            drmOutput->updateDpmsMode(Output::DpmsMode::Off);
+            drmOutput->pipeline()->setActive(false);
+            drmOutput->renderLoop()->inhibit();
+            m_recentlyUnpluggedDpmsOffOutputs.removeOne(drmOutput->uuid());
+        }
+    }
     m_outputs.append(o);
     Q_EMIT outputAdded(o);
     o->updateEnabled(true);
@@ -219,6 +234,13 @@ void DrmBackend::addOutput(DrmAbstractOutput *o)
 
 void DrmBackend::removeOutput(DrmAbstractOutput *o)
 {
+    if (o->dpmsMode() == Output::DpmsMode::Off) {
+        const QUuid id = o->uuid();
+        m_recentlyUnpluggedDpmsOffOutputs.push_back(id);
+        QTimer::singleShot(1000, this, [this, id]() {
+            m_recentlyUnpluggedDpmsOffOutputs.removeOne(id);
+        });
+    }
     o->updateEnabled(false);
     m_outputs.removeOne(o);
     Q_EMIT outputRemoved(o);

src/backends/drm/drm_backend.h

@@ -89,7 +89,7 @@ private:
     std::unique_ptr<QSocketNotifier> m_socketNotifier;
     Session *m_session;
     QVector<DrmAbstractOutput *> m_outputs;
-    DrmVirtualOutput *m_placeHolderOutput = nullptr;
+    QVector<QUuid> m_recentlyUnpluggedDpmsOffOutputs;
 
     const QStringList m_explicitGpus;
     std::vector<std::unique_ptr<DrmGpu>> m_gpus;

src/workspace.cpp

@@ -1292,6 +1292,9 @@ void Workspace::updateOutputs(const QVector<Output *> &outputOrder)
         m_tileManagers[output] = std::make_unique<TileManager>(output);
         connect(output, &Output::aboutToTurnOff, this, &Workspace::createDpmsFilter);
         connect(output, &Output::dpmsModeChanged, this, &Workspace::maybeDestroyDpmsFilter);
+        if (output->dpmsMode() != Output::DpmsMode::On) {
+            createDpmsFilter();
+        }
         Q_EMIT outputAdded(output);
     }
     maybeDestroyDpmsFilter();