drm/fb-helper: use gem_bo.resv, not dma_buf.resv in prepare_fb

With

commit 5f6ed987
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jun 14 22:35:57 2019 +0200

    drm/prime: automatically set gem_obj->resv on import

we consistently set drm_gem_bo.resv for imported buffers. Which means
we don't need to check the dma-buf in the prepare_fb helper, but can
generalize them so they're also useful for display+render drivers
which use gem_bo.resv to track their own rendering for their own
scanout buffers.

Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625204208.5614-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem_framebuffer_helper.c

@@ -271,11 +271,11 @@ EXPORT_SYMBOL_GPL(drm_gem_fb_create_with_dirty);
  * @plane: Plane
  * @state: Plane state the fence will be attached to
  *
- * This function prepares a GEM backed framebuffer for scanout by checking if
- * the plane framebuffer has a DMA-BUF attached. If it does, it extracts the
- * exclusive fence and attaches it to the plane state for the atomic helper to
- * wait on. This function can be used as the &drm_plane_helper_funcs.prepare_fb
- * callback.
+ * This function extracts the exclusive fence from &drm_gem_object.resv and
+ * attaches it to plane state for the atomic helper to wait on. This is
+ * necessary to correctly implement implicit synchronization for any buffers
+ * shared as a struct &dma_buf. This function can be used as the
+ * &drm_plane_helper_funcs.prepare_fb callback.
  *
  * There is no need for &drm_plane_helper_funcs.cleanup_fb hook for simple
  * gem based framebuffer drivers which have their buffers always pinned in
@@ -287,17 +287,15 @@ EXPORT_SYMBOL_GPL(drm_gem_fb_create_with_dirty);
 int drm_gem_fb_prepare_fb(struct drm_plane *plane,
 			  struct drm_plane_state *state)
 {
-	struct dma_buf *dma_buf;
+	struct drm_gem_object *obj;
 	struct dma_fence *fence;
 
 	if (!state->fb)
 		return 0;
 
-	dma_buf = drm_gem_fb_get_obj(state->fb, 0)->dma_buf;
-	if (dma_buf) {
-		fence = reservation_object_get_excl_rcu(dma_buf->resv);
+	obj = drm_gem_fb_get_obj(state->fb, 0);
+	fence = reservation_object_get_excl_rcu(obj->resv);
 	drm_atomic_set_fence_for_plane(state, fence);
-	}
 
 	return 0;
 }
@@ -309,10 +307,11 @@ EXPORT_SYMBOL_GPL(drm_gem_fb_prepare_fb);
  * @pipe: Simple display pipe
  * @plane_state: Plane state
  *
- * This function uses drm_gem_fb_prepare_fb() to check if the plane FB has a
- * &dma_buf attached, extracts the exclusive fence and attaches it to plane
- * state for the atomic helper to wait on. Drivers can use this as their
- * &drm_simple_display_pipe_funcs.prepare_fb callback.
+ * This function uses drm_gem_fb_prepare_fb() to extract the exclusive fence
+ * from &drm_gem_object.resv and attaches it to plane state for the atomic
+ * helper to wait on. This is necessary to correctly implement implicit
+ * synchronization for any buffers shared as a struct &dma_buf. Drivers can use
+ * this as their &drm_simple_display_pipe_funcs.prepare_fb callback.
  *
  * See drm_atomic_set_fence_for_plane() for a discussion of implicit and
  * explicit fencing in atomic modeset updates.