lvgl: call display_flush at the end of a frame

In frames with multiple writes (officially supported through
`CONFIG_LV_Z_VDB_SIZE`) the display needs to be signalled that the
current frame is over and the content should be displayed.
This allows displays to present the UI without tearing artifacts.

Signed-off-by: Martin Stumpf <martin.stumpf@vected.de>

modules/lvgl/Kconfig

@@ -102,6 +102,7 @@ config LV_Z_FLUSH_THREAD_STACK_SIZE
 	default 1024
 	help
 	  Stack size for LVGL flush thread, which will call display_write
+	  and display_flush
 
 config LV_Z_FLUSH_THREAD_PRIO
 	int "LVGL flush thread priority"

modules/lvgl/lvgl_display.c

@@ -24,12 +24,17 @@ void lvgl_flush_thread_entry(void *arg1, void *arg2, void *arg3)
 	while (1) {
 		k_msgq_get(&flush_queue, &flush, K_FOREVER);
 		data = (struct lvgl_disp_data *)flush.disp_drv->user_data;
+		const bool is_last = lv_disp_flush_is_last(flush.disp_drv);
 
 		display_write(data->display_dev, flush.x, flush.y, &flush.desc,
 			      flush.buf);
 
 		lv_disp_flush_ready(flush.disp_drv);
 		k_sem_give(&flush_complete);
+
+		if (is_last) {
+			display_flush(data->display_dev);
+		}
 	}
 }
 
@@ -129,11 +134,15 @@ void lvgl_flush_display(struct lvgl_display_flush *request)
 	k_yield();
 #else
 	/* Write directly to the display */
-	struct lvgl_disp_data *data =
-		(struct lvgl_disp_data *)request->disp_drv->user_data;
+	struct lvgl_disp_data *data = (struct lvgl_disp_data *)request->disp_drv->user_data;
+	const bool is_last = lv_disp_flush_is_last(request->disp_drv);
 
 	display_write(data->display_dev, request->x, request->y,
 		      &request->desc, request->buf);
 	lv_disp_flush_ready(request->disp_drv);
+
+	if (is_last) {
+		display_flush(data->display_dev);
+	}
 #endif
 }

modules/lvgl/lvgl_display_mono.c

@@ -38,6 +38,10 @@ void lvgl_flush_cb_mono(lv_disp_drv_t *disp_drv, const lv_area_t *area, lv_color
 		display_write(display_dev, area->x1, area->y1, &desc, (void *)color_p);
 	}
 
+	if (is_last) {
+		display_flush(display_dev);
+	}
+
 	if (is_epd && is_last && data->blanking_on) {
 		/*
 		 * The entire screen has now been rendered. Update the