drm: handle CRTC starvation recovery when outputs are disabled

When more displays are connected than CRTCs available, connectors that
arrive after all CRTCs are claimed become starved.  The compositor can
free a CRTC by disabling an output, but nothing reclaimed that CRTC for
the starved connector.

Restructure recheckCRTCs() into two passes: first, disabled outputs
release their CRTCs and enabled connectors get priority assignment;
second, any remaining free CRTCs are given to disabled connectors as
backup slots for quick re-enable.

When applyCommit() detects an enabledState transition, schedule
recheckOutputs() via addIdleEvent so starved connectors pick up the
freed CRTC on the next event loop iteration, without reentrancy or
blocking the event loop.

Author: j4kuuu

src/backend/drm/DRM.cpp

@@ -699,6 +699,12 @@ void Aquamarine::CDRMBackend::recheckCRTCs() {
             continue;
         }
 
+        // disabled outputs release their CRTCs so active outputs get priority
+        if (c->crtc && c->status == DRM_MODE_CONNECTED && c->output && !c->output->enabledState) {
+            backend->log(AQ_LOG_DEBUG, std::format("drm: {} is disabled, releasing crtc {}", c->szName, c->crtc->id));
+            c->crtc.reset();
+        }
+
         if (c->crtc && c->status == DRM_MODE_CONNECTED) {
             backend->log(AQ_LOG_DEBUG, std::format("drm: Skipping connector {}, has crtc {} and is connected", c->szName, c->crtc->id));
             continue;
@@ -727,14 +733,18 @@ void Aquamarine::CDRMBackend::recheckCRTCs() {
 
         bool assigned = false;
 
-        // try to use a connected connector
+        // try to use a connected, enabled connector
         for (auto const& c : recheck) {
             if (!(c->possibleCrtcs & (1 << i)))
                 continue;
 
             if (c->status != DRM_MODE_CONNECTED)
                 continue;
 
+            // Pass 1 only assigns to enabled connectors
+            if (c->output && !c->output->enabledState)
+                continue;
+
             // deactivate old output
             if (c->output && c->output->state && c->output->state->state().enabled) {
                 c->output->state->setEnabled(false);
@@ -754,11 +764,38 @@ void Aquamarine::CDRMBackend::recheckCRTCs() {
             backend->log(AQ_LOG_DEBUG, std::format("drm: slot {} crtc {} unassigned", i, crtcs.at(i)->id));
     }
 
+    // Pass 2: assign remaining CRTCs to disabled connectors as backup slots
+    for (size_t i = 0; i < crtcs.size(); ++i) {
+        bool taken = false;
+        for (auto const& c : connectors) {
+            if (c->crtc == crtcs.at(i)) {
+                taken = true;
+                break;
+            }
+        }
+        if (taken)
+            continue;
+
+        for (auto const& c : recheck) {
+            if (!(c->possibleCrtcs & (1 << i)))
+                continue;
+            if (c->status != DRM_MODE_CONNECTED)
+                continue;
+
+            backend->log(AQ_LOG_DEBUG, std::format("drm: backup slot {} crtc {} assigned to disabled {}", i, crtcs.at(i)->id, c->szName));
+            c->crtc = crtcs.at(i);
+            std::erase(recheck, c);
+            break;
+        }
+    }
+
     for (auto const& c : connectors) {
         if (c->status == DRM_MODE_CONNECTED)
             continue;
 
-        backend->log(AQ_LOG_DEBUG, std::format("drm: Connector {} is not connected{}", c->szName, c->crtc ? std::format(", removing old crtc {}", c->crtc->id) : ""));
+        if (c->crtc)
+            backend->log(AQ_LOG_DEBUG, std::format("drm: {} is not connected, clearing stale crtc {}", c->szName, c->crtc->id));
+        c->crtc.reset();
     }
 
     // tell the user to re-assign a valid mode etc, if needed
@@ -884,6 +921,11 @@ void Aquamarine::CDRMBackend::recheckOutputs() {
     // now that crtcs are assigned, connect outputs
     for (const auto& conn : connectors) {
         if (conn->status == DRM_MODE_CONNECTED && !conn->output && !conn->tilingRedundant) {
+            if (!conn->crtc) {
+                backend->log(AQ_LOG_DEBUG, std::format("drm: {} has no CRTC, deferring connection", conn->szName));
+                continue;
+            }
+
             backend->log(AQ_LOG_DEBUG, std::format("drm: Connector {} connected", conn->szName));
 
             auto drmConn = drmModeGetConnector(gpu->fd, conn->id);
@@ -1726,13 +1768,27 @@ void Aquamarine::SDRMConnector::applyCommit(const SDRMConnectorCommitData& data)
     if (output->state->state().committed & COutputState::AQ_OUTPUT_STATE_MODE)
         refresh = calculateRefresh(data.modeInfo);
 
-    output->enabledState = output->state->state().enabled;
+    const bool wasEnabled = output->enabledState;
+    output->enabledState  = output->state->state().enabled;
 
     if (!output->enabledState)
         releaseFBReferences();
 
     if (!backend->updateSecondaryRendererState())
         backend->backend->log(AQ_LOG_ERROR, std::format("drm: Failed to update renderer state for {} on applyCommit", szName));
+
+    if (wasEnabled != output->enabledState) {
+        auto bk = backend.lock();
+        if (bk) {
+            bk->backend->log(AQ_LOG_DEBUG, std::format("drm: Connector {} enabledState changed {} -> {}", szName, wasEnabled, output->enabledState));
+            auto weak = bk->self;
+            bk->backend->addIdleEvent(makeShared<std::function<void(void)>>([weak] {
+                auto b = weak.lock();
+                if (b)
+                    b->recheckOutputs();
+            }));
+        }
+    }
 }
 
 void Aquamarine::SDRMConnector::rollbackCommit(const SDRMConnectorCommitData& data) {