backends/drm: Improve device seat assignment handling

This fixes the drm backend adding hotplugged gpus that belong to other
seats and makes the udev helper depend on less stuff from the layer
above backends.

src/backends/drm/drm_backend.cpp

@@ -215,7 +215,9 @@ bool DrmBackend::initialize()
     } else {
         const auto devices = m_udev->listGPUs();
         for (const UdevDevice::Ptr &device : devices) {
-            addGpu(device->devNode());
+            if (device->seat() == m_session->seat()) {
+                addGpu(device->devNode());
+            }
         }
     }
 
@@ -244,8 +246,16 @@ void DrmBackend::handleUdevEvent()
         if (!m_active) {
             continue;
         }
-        if (!m_explicitGpus.isEmpty() && !m_explicitGpus.contains(device->devNode())) {
-            continue;
+
+        // Ignore the device seat if the KWIN_DRM_DEVICES envvar is set.
+        if (!m_explicitGpus.isEmpty()) {
+            if (!m_explicitGpus.contains(device->devNode())) {
+                continue;
+            }
+        } else {
+            if (device->seat() != m_session->seat()) {
+                continue;
+            }
         }
 
         if (device->action() == QStringLiteral("add")) {

src/utils/udev.cpp

@@ -7,9 +7,6 @@
     SPDX-License-Identifier: GPL-2.0-or-later
 */
 #include "udev.h"
-#include "main.h"
-#include "platform.h"
-#include "session.h"
 #include "utils/common.h"
 // Qt
 #include <QByteArray>
@@ -102,23 +99,14 @@ std::vector<UdevDevice::Ptr> UdevEnumerate::find()
         return {};
     }
     std::vector<UdevDevice::Ptr> vect;
-    QString defaultSeat = QStringLiteral("seat0");
     udev_list_entry *it = udev_enumerate_get_list_entry(m_enumerate.data());
     while (it) {
         auto current = it;
         it = udev_list_entry_get_next(it);
         auto device = m_udev->deviceFromSyspath(udev_list_entry_get_name(current));
-        if (!device) {
-            continue;
+        if (device) {
+            vect.push_back(std::move(device));
         }
-        QString deviceSeat = device->property("ID_SEAT");
-        if (deviceSeat.isEmpty()) {
-            deviceSeat = defaultSeat;
-        }
-        if (deviceSeat != kwinApp()->session()->seat()) {
-            continue;
-        }
-        vect.push_back(std::move(device));
     }
     return vect;
 }
@@ -230,6 +218,15 @@ bool UdevDevice::isBootVga() const
     return systAttrValue && qstrcmp(systAttrValue, "1") == 0;
 }
 
+QString UdevDevice::seat() const
+{
+    QString deviceSeat = udev_device_get_property_value(m_device, "ID_SEAT");
+    if (deviceSeat.isEmpty()) {
+        deviceSeat = QStringLiteral("seat0");
+    }
+    return deviceSeat;
+}
+
 QString UdevDevice::action() const
 {
     return QString::fromLocal8Bit(udev_device_get_action(m_device));

src/utils/udev.h

@@ -38,6 +38,7 @@ public:
 
     QMap<QByteArray, QByteArray> properties() const;
     bool isBootVga() const;
+    QString seat() const;
 
     operator udev_device *() const
     {