kms/impl-device: Add meta_kms_impl_device_get_signaled_sync_file helper

It returns a file descriptor which references a signaled sync_file.

v2:
* Change function name and add Doxygen comment to hopefully make its
  purpose a bit clearer (Ivan Molodetskikh)
v3: (Jonas Ådahl)
* Create sync_file from scratch via a syncobj, no buffer needed anymore
* Initialize priv->sync_file = 1 and use g_clear_fd in finalize

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3389>

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

@@ -21,6 +21,8 @@
 
 #include <errno.h>
 #include <glib/gstdio.h>
+#include <linux/dma-buf.h>
+#include <sys/ioctl.h>
 #include <sys/timerfd.h>
 #include <xf86drm.h>
 
@@ -99,6 +101,9 @@ typedef struct _MetaKmsImplDevicePrivate
   GHashTable *crtc_frames;
 
   gboolean deadline_timer_inhibited;
+
+  gboolean sync_file_retrieved;
+  int sync_file;
 } MetaKmsImplDevicePrivate;
 
 static void
@@ -1026,6 +1031,60 @@ meta_kms_impl_device_get_fd (MetaKmsImplDevice *impl_device)
   return meta_device_file_get_fd (priv->device_file);
 }
 
+/**
+ * meta_kms_impl_device_get_signaled_sync_file:
+ * @impl_device: a #MetaKmsImplDevice object
+ *
+ * Returns a file descriptor which references a sync_file. The file descriptor
+ * must not be closed by the caller.
+ *
+ * Always returns the same file descriptor for the same impl_device. The
+ * referenced sync_file will always be considered signaled.
+ *
+ * Returns a negative value if a sync_file fd couldn't be retrieved.
+ */
+int
+meta_kms_impl_device_get_signaled_sync_file (MetaKmsImplDevice *impl_device)
+{
+  MetaKmsImplDevicePrivate *priv =
+    meta_kms_impl_device_get_instance_private (impl_device);
+
+  meta_assert_in_kms_impl (meta_kms_impl_get_kms (priv->impl));
+
+  if (!priv->sync_file_retrieved)
+    {
+      uint32_t syncobj_handle;
+      int drm_fd, ret;
+
+      priv->sync_file = -1;
+      priv->sync_file_retrieved = TRUE;
+
+      drm_fd = meta_kms_impl_device_get_fd (impl_device);
+      ret = drmSyncobjCreate (drm_fd,
+                              DRM_SYNCOBJ_CREATE_SIGNALED,
+                              &syncobj_handle);
+      if (ret < 0)
+        {
+          meta_topic (META_DEBUG_KMS,
+                      "drmSyncobjCreate failed: %s",
+                      g_strerror (errno));
+          return -1;
+        }
+
+      ret = drmSyncobjExportSyncFile (drm_fd, syncobj_handle, &priv->sync_file);
+      if (ret < 0)
+        {
+          meta_topic (META_DEBUG_KMS,
+                      "drmSyncobjExportSyncFile failed: %s",
+                      g_strerror (errno));
+        }
+
+      drmSyncobjDestroy (drm_fd, syncobj_handle);
+    }
+
+  return priv->sync_file;
+}
+
 static void
 disarm_crtc_frame_deadline_timer (CrtcFrame *crtc_frame)
 {
@@ -1825,6 +1884,8 @@ meta_kms_impl_device_finalize (GObject *object)
   g_free (priv->driver_description);
   g_free (priv->path);
 
+  g_clear_fd (&priv->sync_file, NULL);
+
   G_OBJECT_CLASS (meta_kms_impl_device_parent_class)->finalize (object);
 }
 
@@ -1947,6 +2008,8 @@ meta_kms_impl_device_initable_init (GInitable     *initable,
     g_hash_table_new_full (NULL, NULL,
                            NULL, (GDestroyNotify) crtc_frame_free);
 
+  priv->sync_file = -1;
+
   return TRUE;
 }
 

src/backends/native/meta-kms-impl-device.h

@@ -145,6 +145,8 @@ void meta_kms_impl_device_hold_fd (MetaKmsImplDevice *impl_device);
 
 void meta_kms_impl_device_unhold_fd (MetaKmsImplDevice *impl_device);
 
+int meta_kms_impl_device_get_signaled_sync_file (MetaKmsImplDevice *impl_device);
+
 MetaKmsResourceChanges meta_kms_impl_device_update_states (MetaKmsImplDevice *impl_device,
                                                            uint32_t           crtc_id,
                                                            uint32_t           connector_id);