drm-backend: address case in which writeback takes longer than atomic commit

In commit "drm-backend: add writeback connector screenshooter to
DRM-backend" we were failing the writeback screenshot when the DRM/KMS
driver would take longer than the atomic commit to finish. In this patch
we address such case.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

libweston/backend-drm/drm-internal.h

@@ -567,6 +567,12 @@ enum writeback_screenshot_state {
 	 * commit is handled by DRM it will give us a sync fd that gets
 	 * signalled when the writeback is done. */
 	DRM_OUTPUT_WB_SCREENSHOT_CHECK_FENCE,
+	/* The atomic commit completed and we received the sync fd from the
+	 * kernel. We've polled to check if the writeback was over, but it
+	 * wasn't. Now we must stop the repaint loop and wait until the
+	 * writeback is complete, because we can't commit with KMS objects
+	 * (CRTC, planes, etc) that are in used by the writeback job. */
+	DRM_OUTPUT_WB_SCREENSHOT_WAITING_SIGNAL,
 };
 
 struct drm_writeback_state {
@@ -578,6 +584,7 @@ struct drm_writeback_state {
 
 	struct drm_fb *fb;
 	int32_t out_fence_fd;
+	struct wl_event_source *wb_source;
 
 	/* Reference to fb's being used by the writeback job. These are all the
 	 * framebuffers in every drm_plane_state of the output state that we've
@@ -699,7 +706,7 @@ void
 drm_writeback_reference_planes(struct drm_writeback_state *state,
 			       struct wl_list *plane_state_list);
 bool
-drm_writeback_has_finished(struct drm_writeback_state *state);
+drm_writeback_should_wait_completion(struct drm_writeback_state *state);
 void
 drm_writeback_fail_screenshot(struct drm_writeback_state *state,
 			      const char *err_msg);

libweston/backend-drm/drm.c

@@ -2670,7 +2670,20 @@ drm_writeback_fail_screenshot(struct drm_writeback_state *state,
 	output->wb_state = NULL;
 }
 
-bool
+static int
+drm_writeback_save_callback(int fd, uint32_t mask, void *data)
+{
+	struct drm_writeback_state *state = data;
+
+	wl_event_source_remove(state->wb_source);
+	close(fd);
+
+	drm_writeback_success_screenshot(state);
+
+	return 0;
+}
+
+static bool
 drm_writeback_has_finished(struct drm_writeback_state *state)
 {
 	struct pollfd pollfd;
@@ -2695,6 +2708,40 @@ drm_writeback_has_finished(struct drm_writeback_state *state)
 	return false;
 }
 
+bool
+drm_writeback_should_wait_completion(struct drm_writeback_state *state)
+{
+	struct weston_compositor *ec = state->output->base.compositor;
+	struct wl_event_loop *event_loop;
+
+	if (state->state == DRM_OUTPUT_WB_SCREENSHOT_WAITING_SIGNAL)
+		return true;
+
+	if (state->state == DRM_OUTPUT_WB_SCREENSHOT_CHECK_FENCE) {
+		if (drm_writeback_has_finished(state))
+			return false;
+
+		/* The writeback has not finished yet. So add callback that gets
+		 * called when the sync fd of the writeback job gets signalled.
+		 * We need to wait for that to resume the repaint loop. */
+		event_loop = wl_display_get_event_loop(ec->wl_display);
+		state->wb_source =
+			wl_event_loop_add_fd(event_loop, state->out_fence_fd,
+					     WL_EVENT_READABLE,
+					     drm_writeback_save_callback, state);
+		if (!state->wb_source) {
+			drm_writeback_fail_screenshot(state, "drm: out of memory");
+			return false;
+		}
+
+		state->state = DRM_OUTPUT_WB_SCREENSHOT_WAITING_SIGNAL;
+
+		return true;
+	}
+
+	return false;
+}
+
 void
 drm_writeback_reference_planes(struct drm_writeback_state *state,
 			       struct wl_list *plane_state_list)

libweston/backend-drm/kms.c

@@ -1750,6 +1750,7 @@ on_drm_input(int fd, uint32_t mask, void *data)
 	struct drm_device *device = data;
 	struct drm_writeback_state *state;
 	struct drm_crtc *crtc;
+	bool wait_wb_completion = false;
 	drmEventContext evctx;
 
 	/* If we have a pending writeback job for this output, we can't continue
@@ -1758,9 +1759,11 @@ on_drm_input(int fd, uint32_t mask, void *data)
 	 * uses the KMS objects (CRTC, planes, etc) in use by the writeback. */
 	wl_list_for_each(crtc, &device->crtc_list, link) {
 		state = crtc->output ? crtc->output->wb_state : NULL;
-		if (state && !drm_writeback_has_finished(state))
-			drm_writeback_fail_screenshot(state, "drm: out fence not signalled yet");
+		if (state && drm_writeback_should_wait_completion(state))
+			wait_wb_completion = true;
 	}
+	if (wait_wb_completion)
+		return 1;
 
 	memset(&evctx, 0, sizeof evctx);
 	evctx.version = 3;