compositor-drm: Ignore views on other outputs

When we come to assign_planes, try very hard to ignore views which are
only visible on other outputs, rather than forcibly moving them to the
primary plane, which causes damage all round and unnecessary repaints.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Emre Ucan <eucan@de.adit-jv.com>

libweston/compositor-drm.c

@@ -1562,10 +1562,6 @@ drm_fb_get_from_view(struct drm_output_state *state, struct weston_view *ev)
 	struct linux_dmabuf_buffer *dmabuf;
 	struct drm_fb *fb;
 
-	/* Don't import buffers which span multiple outputs. */
-	if (ev->output_mask != (1u << output->base.id))
-		return NULL;
-
 	if (ev->alpha != 1.0f)
 		return NULL;
 
@@ -3135,10 +3131,6 @@ drm_output_prepare_cursor_view(struct drm_output_state *output_state,
 	if (plane->state_cur->output && plane->state_cur->output != output)
 		return NULL;
 
-	/* Don't import buffers which span multiple outputs. */
-	if (ev->output_mask != (1u << output->base.id))
-		return NULL;
-
 	/* We use GBM to import SHM buffers. */
 	if (b->gbm == NULL)
 		return NULL;
@@ -3296,6 +3288,16 @@ drm_output_propose_state(struct weston_output *output_base,
 	wl_list_for_each(ev, &output_base->compositor->view_list, link) {
 		struct weston_plane *next_plane = NULL;
 
+		/* If this view doesn't touch our output at all, there's no
+		 * reason to do anything with it. */
+		if (!(ev->output_mask & (1u << output->base.id)))
+			continue;
+
+		/* We only assign planes to views which are exclusively present
+		 * on our output. */
+		if (ev->output_mask != (1u << output->base.id))
+			next_plane = primary;
+
 		/* Since we process views from top to bottom, we know that if
 		 * the view intersects the calculated renderer region, it must
 		 * be part of, or occluded by, it, and cannot go on a plane. */
@@ -3345,6 +3347,11 @@ drm_assign_planes(struct weston_output *output_base, void *repaint_data)
 	wl_list_for_each(ev, &output_base->compositor->view_list, link) {
 		struct drm_plane *target_plane = NULL;
 
+		/* If this view doesn't touch our output at all, there's no
+		 * reason to do anything with it. */
+		if (!(ev->output_mask & (1u << output->base.id)))
+			continue;
+
 		/* Test whether this buffer can ever go into a plane:
 		 * non-shm, or small enough to be a cursor.
 		 *