kms: Have the device impl open device files themself

This changes the way the KMS backends load; if we're headless, we always
use the dummy one and fail otherwise; in other cases, we first try the
atomic backend, and if that fails, fall back on the simple one.

The aim for this is to have the impl device open and close the device
when needed, using the device pool directly.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1828>

src/backends/native/meta-kms-impl-device-atomic.c

@@ -21,6 +21,7 @@
 
 #include "backends/native/meta-kms-impl-device-atomic.h"
 
+#include "backends/native/meta-backend-native-private.h"
 #include "backends/native/meta-kms-connector-private.h"
 #include "backends/native/meta-kms-crtc-private.h"
 #include "backends/native/meta-kms-device-private.h"
@@ -44,6 +45,8 @@ struct _MetaKmsImplDeviceAtomic
   GHashTable *page_flip_datas;
 };
 
+static GInitableIface *initable_parent_iface;
+
 static void
 initable_iface_init (GInitableIface *iface);
 
@@ -1039,14 +1042,83 @@ meta_kms_impl_device_atomic_finalize (GObject *object)
   G_OBJECT_CLASS (meta_kms_impl_device_atomic_parent_class)->finalize (object);
 }
 
+static MetaDeviceFile *
+meta_kms_impl_device_atomic_open_device_file (MetaKmsImplDevice  *impl_device,
+                                              const char         *path,
+                                              GError            **error)
+{
+  MetaKmsDevice *device = meta_kms_impl_device_get_device (impl_device);
+  MetaKms *kms = meta_kms_device_get_kms (device);
+  MetaBackend *backend = meta_kms_get_backend (kms);
+  MetaDevicePool *device_pool =
+    meta_backend_native_get_device_pool (META_BACKEND_NATIVE (backend));
+  g_autoptr (MetaDeviceFile) device_file = NULL;
+
+  device_file = meta_device_pool_open (device_pool, path,
+                                       META_DEVICE_FILE_FLAG_TAKE_CONTROL,
+                                       error);
+  if (!device_file)
+    return NULL;
+
+  if (drmSetClientCap (meta_device_file_get_fd (device_file),
+                       DRM_CLIENT_CAP_ATOMIC, 1) != 0)
+    {
+      g_set_error (error, META_KMS_ERROR, META_KMS_ERROR_NOT_SUPPORTED,
+                   "DRM_CLIENT_CAP_ATOMIC not supported");
+      return NULL;
+    }
+
+  return g_steal_pointer (&device_file);
+}
+
+static gboolean
+is_atomic_allowed (const char *driver_name)
+{
+  const char *atomic_driver_deny_list[] = {
+    "qxl",
+    "vmwgfx",
+    "vboxvideo",
+    "nvidia-drm",
+    NULL,
+  };
+
+  return !g_strv_contains (atomic_driver_deny_list, driver_name);
+}
+
 static gboolean
 meta_kms_impl_device_atomic_initable_init (GInitable     *initable,
                                            GCancellable  *cancellable,
                                            GError       **error)
 {
   MetaKmsImplDevice *impl_device = META_KMS_IMPL_DEVICE (initable);
+  const char *atomic_kms_enable_env;
 
-  return meta_kms_impl_device_init_mode_setting (impl_device, error);
+  atomic_kms_enable_env = getenv ("MUTTER_DEBUG_ENABLE_ATOMIC_KMS");
+  if (atomic_kms_enable_env && g_strcmp0 (atomic_kms_enable_env, "1") != 0)
+    {
+      g_set_error (error, META_KMS_ERROR, META_KMS_ERROR_USER_INHIBITED,
+                   "Atomic mode setting disable via env var");
+      return FALSE;
+    }
+
+  if (!initable_parent_iface->init (initable, cancellable, error))
+    return FALSE;
+
+  if (!is_atomic_allowed (meta_kms_impl_device_get_driver_name (impl_device)))
+    {
+      g_set_error (error, META_KMS_ERROR, META_KMS_ERROR_DENY_LISTED,
+                   "Atomic mode setting disable via driver deny list");
+      return FALSE;
+    }
+
+  if (!meta_kms_impl_device_init_mode_setting (impl_device, error))
+    return FALSE;
+
+  g_message ("Added device '%s' (%s) using atomic mode setting.",
+             meta_kms_impl_device_get_path (impl_device),
+             meta_kms_impl_device_get_driver_name (impl_device));
+
+  return TRUE;
 }
 
 static void
@@ -1058,6 +1130,8 @@ meta_kms_impl_device_atomic_init (MetaKmsImplDeviceAtomic *impl_device_atomic)
 static void
 initable_iface_init (GInitableIface *iface)
 {
+  initable_parent_iface = g_type_interface_peek_parent (iface);
+
   iface->init = meta_kms_impl_device_atomic_initable_init;
 }
 
@@ -1070,6 +1144,8 @@ meta_kms_impl_device_atomic_class_init (MetaKmsImplDeviceAtomicClass *klass)
 
   object_class->finalize = meta_kms_impl_device_atomic_finalize;
 
+  impl_device_class->open_device_file =
+    meta_kms_impl_device_atomic_open_device_file;
   impl_device_class->setup_drm_event_context =
     meta_kms_impl_device_atomic_setup_drm_event_context;
   impl_device_class->process_update =