backend-drm: Move drm_fb reference to state-propose

Currently we take a reference on the underlying client buffer every time
we materialise a drm_fb from a view, and release it when the drm_fb is
destroyed. This means that we need to create and destroy a drm_fb every
time we want to use it, which is pathologically unperformant on some
platforms.

To start working towards being able to cache drm_fb, only take the
reference when we apply it to a plane state.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/fb.c

@@ -410,17 +410,6 @@ err_free:
 	free(fb);
 	return NULL;
 }
-
-static void
-drm_fb_set_buffer(struct drm_fb *fb, struct weston_buffer *buffer,
-		  struct weston_buffer_release *buffer_release)
-{
-	assert(fb->buffer_ref.buffer == NULL);
-	assert(fb->type == BUFFER_CLIENT || fb->type == BUFFER_DMABUF);
-	weston_buffer_reference(&fb->buffer_ref, buffer);
-	weston_buffer_release_reference(&fb->buffer_release_ref,
-					buffer_release);
-}
 #endif
 
 void
@@ -528,8 +517,6 @@ drm_fb_get_from_view(struct drm_output_state *state, struct weston_view *ev,
 
 	drm_debug(b, "\t\t\t[view] view %p format: %s\n",
 		  ev, fb->format->drm_format_name);
-	drm_fb_set_buffer(fb, buffer,
-			  ev->surface->buffer_release_ref.buffer_release);
 	return fb;
 }
 #endif

libweston/backend-drm/state-propose.c

@@ -532,6 +532,7 @@ drm_output_try_view_on_plane(struct drm_plane *plane,
 	struct drm_plane *scanout_plane = state->output->scanout_plane;
 	struct drm_plane_state *ps = NULL;
 	const char *p_name = drm_output_get_plane_type_name(plane);
+	struct weston_surface *surface = ev->surface;
 	enum {
 		NO_PLANES,	/* generic err-handle */
 		NO_PLANES_ACCEPTED,
@@ -603,6 +604,19 @@ out:
 			     plane->plane_id, ev, p_name);
 		break;
 	case PLACED_ON_PLANE:
+		/* Take a reference on the buffer so that we don't release it
+		 * back to the client until we're done with it; cursor buffers
+		 * don't require a reference since we copy them. */
+		assert(fb->buffer_ref.buffer == NULL);
+		assert(fb->buffer_release_ref.buffer_release == NULL);
+		if (ps->plane->type == WDRM_PLANE_TYPE_CURSOR) {
+			assert(ps->fb->type == BUFFER_CURSOR);
+		} else if (fb->type == BUFFER_CLIENT || fb->type == BUFFER_DMABUF) {
+			assert(ps->fb == fb);
+			weston_buffer_reference(&fb->buffer_ref, surface->buffer_ref.buffer);
+			weston_buffer_release_reference(&fb->buffer_release_ref,
+							surface->buffer_release_ref.buffer_release);
+		}
 		break;
 	}