multigpu: fix multi gpu checking (#13277)

* multigpu: fix multi gpu checking

drmFD() from allocators is not always equal, because we reopen them
inside AQ for refcounting, meaning they get duplicated and become their
own fds, so checking if fd1 == fd2 ends up wrong.

introduce sameGpu in MiscFunctions that checks the actual drmDevice
meaning we can now even check if a rendernode is the same gpu as a
display node if we want.

* multigpu: move sameGpu to DRM namespace

move sameGpu out of MiscFunctions to DRM namespace.

src/helpers/Drm.cpp

@@ -0,0 +1,20 @@
+#include <xf86drm.h>
+#include "Drm.hpp"
+
+bool DRM::sameGpu(int fd1, int fd2) {
+    drmDevice* devA = nullptr;
+    drmDevice* devB = nullptr;
+
+    if (drmGetDevice2(fd1, 0, &devA) != 0)
+        return false;
+    if (drmGetDevice2(fd2, 0, &devB) != 0) {
+        drmFreeDevice(&devA);
+        return false;
+    }
+
+    bool same = drmDevicesEqual(devA, devB);
+
+    drmFreeDevice(&devA);
+    drmFreeDevice(&devB);
+    return same;
+}

src/helpers/Drm.hpp

@@ -0,0 +1,5 @@
+#pragma once
+
+namespace DRM {
+    bool sameGpu(int fd1, int fd2);
+}

src/helpers/Monitor.cpp

@@ -32,6 +32,7 @@
 #include "time/Time.hpp"
 #include "../desktop/view/LayerSurface.hpp"
 #include "../desktop/state/FocusState.hpp"
+#include "Drm.hpp"
 #include <aquamarine/output/Output.hpp>
 #include "debug/log/Logger.hpp"
 #include "debug/HyprNotificationOverlay.hpp"
@@ -1895,7 +1896,7 @@ bool CMonitor::attemptDirectScanout() {
     m_output->state->addDamage(PSURFACE->m_current.accumulateBufferDamage());
 
     // multigpu needs a fence to trigger fence syncing blits and also committing with the recreated dgpu fence
-    if (m_output->getBackend()->preferredAllocator()->drmFD() != g_pCompositor->m_drm.fd && g_pHyprOpenGL->explicitSyncSupported()) {
+    if (!DRM::sameGpu(m_output->getBackend()->preferredAllocator()->drmFD(), g_pCompositor->m_drm.fd) && g_pHyprOpenGL->explicitSyncSupported()) {
         auto sync = CEGLSync::create();
 
         if (sync->fd().isValid()) {

src/managers/PointerManager.cpp

@@ -17,6 +17,7 @@
 #include "../desktop/state/FocusState.hpp"
 #include "SeatManager.hpp"
 #include "../helpers/time/Time.hpp"
+#include "../helpers/Drm.hpp"
 #include <cstring>
 #include <gbm.h>
 #include <cairo/cairo.h>
@@ -440,7 +441,7 @@ SP<Aquamarine::IBuffer> CPointerManager::renderHWCursorBuffer(SP<CPointerManager
         options.length   = 2;
         options.scanout  = true;
         options.cursor   = true;
-        options.multigpu = state->monitor->m_output->getBackend()->preferredAllocator()->drmFD() != g_pCompositor->m_drm.fd;
+        options.multigpu = !DRM::sameGpu(state->monitor->m_output->getBackend()->preferredAllocator()->drmFD(), g_pCompositor->m_drm.fd);
         // We do not set the format (unless shm). If it's unset (DRM_FORMAT_INVALID) then the swapchain will pick for us,
         // but if it's set, we don't wanna change it.
         if (shouldUseCpuBuffer)