From d7ae21ce85fdc8e249dc172caf2df10f31045731 Mon Sep 17 00:00:00 2001
From: Gabor Kiss-Vamosi <kisvegabor@gmail.com>
Date: Sat, 29 Dec 2018 09:18:58 +0100
Subject: [PATCH] lv_vdb: add comments

---
 lv_core/lv_vdb.c | 14 ++++++++------
 lv_core/lv_vdb.h |  6 ++++++
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/lv_core/lv_vdb.c b/lv_core/lv_vdb.c
index 30ebc5a56..c362a48c0 100644
--- a/lv_core/lv_vdb.c
+++ b/lv_core/lv_vdb.c
@@ -108,12 +108,6 @@ void lv_vdb_flush(void)
     lv_disp_flush(vdb_act->area.x1, vdb_act->area.y1, vdb_act->area.x2, vdb_act->area.y2, vdb_act->buf);
 
 
-#if LV_VDB_TRUE_DOUBLE_BUFFERED
-    while(vdb_flushing);
-    memcpy(vdb[(vdb_active + 1) & 0x1].buf, vdb[vdb_active].buf, LV_VDB_SIZE_IN_BYTES);
-#endif
-
-
 #if LV_VDB_DOUBLE
     /*Make the other VDB active. The content of the current will be kept until the next flush*/
     vdb_active++;
@@ -124,6 +118,14 @@ void lv_vdb_flush(void)
         memset(vdb[vdb_active].buf, 0x00, LV_VDB_SIZE_IN_BYTES);
 #  endif  /*LV_COLOR_SCREEN_TRANSP*/
 
+    /* With true double buffering the flushing should be only the address change of the current frame buffer
+     * Wait until the address change is ready and copy the active content to the other frame buffer (new active VDB)
+     * The changes will be written to the new VDB.*/
+#if LV_VDB_TRUE_DOUBLE_BUFFERED
+        while(vdb_flushing);
+        memcpy(vdb[vdb_active].buf, vdb[(vdb_active + 1) & 0x1].buf, LV_VDB_SIZE_IN_BYTES);
+#endif
+
 #endif  /*#if LV_VDB_DOUBLE*/
 
 }
diff --git a/lv_core/lv_vdb.h b/lv_core/lv_vdb.h
index cbb4329a4..278fc4e8e 100644
--- a/lv_core/lv_vdb.h
+++ b/lv_core/lv_vdb.h
@@ -35,6 +35,12 @@ extern "C" {
 #define LV_VDB_PX_BPP LV_COLOR_SIZE
 #endif
 
+
+#if LV_VDB_TRUE_DOUBLE_BUFFERED && (LV_VDB_SIZE != LV_HOR_RES * LV_VER_RES || LV_VDB_DOUBLE == 0)
+#error "With LV_VDB_TRUE_DOUBLE_BUFFERED: (LV_VDB_SIZE = LV_HOR_RES * LV_VER_RES and LV_VDB_DOUBLE = 1 is required"
+#endif
+
+
 /* The size of VDB in bytes.
  * (LV_VDB_SIZE * LV_VDB_PX_BPP) >> 3): just divide by 8 to convert bits to bytes
  * (((LV_VDB_SIZE * LV_VDB_PX_BPP) & 0x7) ? 1 : 0): add an extra byte to round up.