drm-buffer: Don't always generate fb_id on construction

It might not be needed by the user of the buffer, so don't always
require it up front. Instead make sure that any user that needs it first
calls "meta_drm_buffer_ensure_fb_id()" to create the ID.

Only the plain gbm implementation creates the ID lazilly, the other
still does it on construction due to the objects used to create them
only existing during construction.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1854>

src/backends/native/meta-drm-buffer-dumb.c

@@ -251,7 +251,7 @@ init_dumb_buffer (MetaDrmBufferDumb  *buffer_dumb,
     .handles = { create_arg.handle },
     .strides = { create_arg.pitch },
   };
-  if (!meta_drm_buffer_ensure_fb_id (buffer, &fb_args, error))
+  if (!meta_drm_buffer_do_ensure_fb_id (buffer, &fb_args, error))
     goto err_add_fb;
 
   map_arg = (struct drm_mode_map_dumb) {

src/backends/native/meta-drm-buffer-gbm.c

@@ -133,11 +133,12 @@ meta_drm_buffer_gbm_get_modifier (MetaDrmBuffer *buffer)
 }
 
 static gboolean
-init_fb_id (MetaDrmBufferGbm  *buffer_gbm,
-            struct gbm_bo     *bo,
-            GError           **error)
+meta_drm_buffer_gbm_ensure_fb_id (MetaDrmBuffer  *buffer,
+                                  GError        **error)
 {
+  MetaDrmBufferGbm *buffer_gbm = META_DRM_BUFFER_GBM (buffer);
   MetaDrmFbArgs fb_args = { 0, };
+  struct gbm_bo *bo = buffer_gbm->bo;
 
   if (gbm_bo_get_handle_for_plane (bo, 0).s32 == -1)
     {
@@ -164,8 +165,8 @@ init_fb_id (MetaDrmBufferGbm  *buffer_gbm,
   fb_args.height = gbm_bo_get_height (bo);
   fb_args.format = gbm_bo_get_format (bo);
 
-  if (!meta_drm_buffer_ensure_fb_id (META_DRM_BUFFER (buffer_gbm),
-                                     &fb_args, error))
+  if (!meta_drm_buffer_do_ensure_fb_id (META_DRM_BUFFER (buffer_gbm),
+                                        &fb_args, error))
     return FALSE;
 
   return TRUE;
@@ -185,7 +186,7 @@ lock_front_buffer (MetaDrmBufferGbm  *buffer_gbm,
       return FALSE;
     }
 
-  return init_fb_id (buffer_gbm, buffer_gbm->bo, error);
+  return meta_drm_buffer_gbm_ensure_fb_id (META_DRM_BUFFER (buffer_gbm), error);
 }
 
 MetaDrmBufferGbm *
@@ -223,13 +224,6 @@ meta_drm_buffer_gbm_new_take (MetaDeviceFile      *device_file,
                              "device-file", device_file,
                              "flags", flags,
                              NULL);
-
-  if (!init_fb_id (buffer_gbm, bo, error))
-    {
-      g_object_unref (buffer_gbm);
-      return NULL;
-    }
-
   buffer_gbm->bo = bo;
 
   return buffer_gbm;
@@ -510,6 +504,7 @@ meta_drm_buffer_gbm_class_init (MetaDrmBufferGbmClass *klass)
   object_class->finalize = meta_drm_buffer_gbm_finalize;
 
   buffer_class->export_fd = meta_drm_buffer_gbm_export_fd;
+  buffer_class->ensure_fb_id = meta_drm_buffer_gbm_ensure_fb_id;
   buffer_class->get_width = meta_drm_buffer_gbm_get_width;
   buffer_class->get_height = meta_drm_buffer_gbm_get_height;
   buffer_class->get_stride = meta_drm_buffer_gbm_get_stride;

src/backends/native/meta-drm-buffer-import.c

@@ -181,9 +181,9 @@ import_gbm_buffer (MetaDrmBufferImport  *buffer_import,
 
   fb_args.handles[0] = gbm_bo_get_handle (imported_bo).u32;
 
-  ret = meta_drm_buffer_ensure_fb_id (META_DRM_BUFFER (buffer_import),
-                                      &fb_args,
-                                      error);
+  ret = meta_drm_buffer_do_ensure_fb_id (META_DRM_BUFFER (buffer_import),
+                                         &fb_args,
+                                         error);
 
   gbm_bo_destroy (imported_bo);
 

src/backends/native/meta-drm-buffer-private.h

@@ -42,6 +42,9 @@ struct _MetaDrmBufferClass
   int (* export_fd) (MetaDrmBuffer  *buffer,
                      GError        **error);
 
+  gboolean (* ensure_fb_id) (MetaDrmBuffer  *buffer,
+                             GError        **error);
+
   int (* get_width) (MetaDrmBuffer *buffer);
   int (* get_height) (MetaDrmBuffer *buffer);
   int (* get_stride) (MetaDrmBuffer *buffer);
@@ -58,8 +61,8 @@ struct _MetaDrmBufferClass
 
 MetaDeviceFile * meta_drm_buffer_get_device_file (MetaDrmBuffer *buffer);
 
-gboolean meta_drm_buffer_ensure_fb_id (MetaDrmBuffer        *buffer,
-                                       const MetaDrmFbArgs  *fb_args,
-                                       GError              **error);
+gboolean meta_drm_buffer_do_ensure_fb_id (MetaDrmBuffer        *buffer,
+                                          const MetaDrmFbArgs  *fb_args,
+                                          GError              **error);
 
 #endif /* META_DRM_BUFFER_PRIVATE_H */

src/backends/native/meta-drm-buffer.c

@@ -67,9 +67,21 @@ meta_drm_buffer_get_device_file (MetaDrmBuffer *buffer)
 }
 
 gboolean
-meta_drm_buffer_ensure_fb_id (MetaDrmBuffer        *buffer,
-                              const MetaDrmFbArgs  *fb_args,
-                              GError              **error)
+meta_drm_buffer_ensure_fb_id (MetaDrmBuffer  *buffer,
+                              GError        **error)
+{
+  MetaDrmBufferPrivate *priv = meta_drm_buffer_get_instance_private (buffer);
+
+  if (priv->fb_id)
+    return TRUE;
+
+  return META_DRM_BUFFER_GET_CLASS (buffer)->ensure_fb_id (buffer, error);
+}
+
+gboolean
+meta_drm_buffer_do_ensure_fb_id (MetaDrmBuffer        *buffer,
+                                 const MetaDrmFbArgs  *fb_args,
+                                 GError              **error)
 {
   MetaDrmBufferPrivate *priv = meta_drm_buffer_get_instance_private (buffer);
   int fd;

src/backends/native/meta-drm-buffer.h

@@ -43,6 +43,9 @@ G_DECLARE_DERIVABLE_TYPE (MetaDrmBuffer,
 int meta_drm_buffer_export_fd (MetaDrmBuffer  *buffer,
                                GError        **error);
 
+gboolean meta_drm_buffer_ensure_fb_id (MetaDrmBuffer  *buffer,
+                                       GError        **error);
+
 uint32_t meta_drm_buffer_get_fb_id (MetaDrmBuffer *buffer);
 
 int meta_drm_buffer_get_width (MetaDrmBuffer *buffer);

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

@@ -421,6 +421,9 @@ process_plane_assignment (MetaKmsImplDevice  *impl_device,
 
   buffer = plane_assignment->buffer;
 
+  if (buffer && !meta_drm_buffer_ensure_fb_id (buffer, error))
+    return FALSE;
+
   meta_topic (META_DEBUG_KMS,
               "[atomic] Assigning %s plane (%u, %s) to %u, "
               "%hdx%hd+%hd+%hd -> %dx%d+%d+%d",

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

@@ -396,6 +396,9 @@ process_mode_set (MetaKmsImplDevice  *impl_device,
         }
 
       buffer = plane_assignment->buffer;
+      if (!meta_drm_buffer_ensure_fb_id (buffer, error))
+        return FALSE;
+
       fb_id = meta_drm_buffer_get_fb_id (buffer);
 
       for (l = mode_set->connectors; l; l = l->next)
@@ -822,6 +825,9 @@ mode_set_fallback (MetaKmsImplDeviceSimple  *impl_device_simple,
       return FALSE;
     }
 
+  if (!meta_drm_buffer_ensure_fb_id (plane_assignment->buffer, error))
+    return FALSE;
+
   fill_connector_ids_array (cached_mode_set->connectors,
                             &connectors,
                             &n_connectors);
@@ -926,6 +932,10 @@ dispatch_page_flip (MetaKmsImplDevice    *impl_device,
       return TRUE;
     }
 
+  if (plane_assignment && plane_assignment->buffer &&
+      !meta_drm_buffer_ensure_fb_id (plane_assignment->buffer, error))
+    return FALSE;
+
   fd = meta_kms_impl_device_get_fd (impl_device);
   if (custom_page_flip)
     {