diff --git a/mpp/codec/inc/mpp_buf_slot.h b/mpp/codec/inc/mpp_buf_slot.h
index 55739583..2cc1df3e 100644
--- a/mpp/codec/inc/mpp_buf_slot.h
+++ b/mpp/codec/inc/mpp_buf_slot.h
@@ -146,6 +146,9 @@ RK_U32  mpp_buf_slot_get_size(MppBufSlots slots);
  *
  * called by mpp
  *
+ * mpp_buf_slot_get_decoding
+ *      - mpp_dec need to get the output slot index to check buffer status
+ *
  * mpp_buf_slot_clr_display
  *      - mark a slot has been send out to display
  *
@@ -160,6 +163,7 @@ MPP_RET mpp_buf_slot_set_ref(MppBufSlots slots, RK_U32 index);
 MPP_RET mpp_buf_slot_clr_ref(MppBufSlots slots, RK_U32 index);
 MPP_RET mpp_buf_slot_set_decoding(MppBufSlots slots, RK_U32 index);
 MPP_RET mpp_buf_slot_clr_decoding(MppBufSlots slots, RK_U32 index);
+MPP_RET mpp_buf_slot_get_decoding(MppBufSlots slots, RK_U32 *index);
 MPP_RET mpp_buf_slot_set_display(MppBufSlots slots, RK_U32 index);
 MPP_RET mpp_buf_slot_clr_display(MppBufSlots slots, RK_U32 index);
 
diff --git a/mpp/codec/mpp_buf_slot.cpp b/mpp/codec/mpp_buf_slot.cpp
index 751c3fe0..92676729 100644
--- a/mpp/codec/mpp_buf_slot.cpp
+++ b/mpp/codec/mpp_buf_slot.cpp
@@ -33,6 +33,7 @@
 typedef struct MppBufSlotEntry_t {
     MppBuffer   buffer;
     RK_U32      status;
+    RK_S32      index;
     RK_S64      pts;
 } MppBufSlotEntry;
 
@@ -50,6 +51,9 @@ typedef struct MppBufSlotsImpl_t {
     RK_U32          new_count;
     RK_U32          new_size;
 
+    // to record current output slot index
+    RK_S32          output;
+
     MppBufSlotEntry *slots;
 } MppBufSlotsImpl;
 
@@ -61,6 +65,7 @@ static void check_entry_unused(MppBufSlotEntry *entry)
     if (entry->status == MPP_SLOT_USED) {
         entry->status = MPP_SLOT_UNUSED;
         mpp_buffer_put(entry->buffer);
+        entry->buffer = NULL;
     }
 }
 
@@ -109,6 +114,9 @@ MPP_RET mpp_buf_slot_setup(MppBufSlots slots, RK_U32 count, RK_U32 size, RK_U32
         impl->slots = mpp_calloc(MppBufSlotEntry, count);
         impl->count = count;
         impl->size  = size;
+        for (RK_U32 i = 0; i < count; i++) {
+            impl->slots[i].index = i;
+        }
     } else {
         // need to check info change or not
         if (!changed) {
@@ -116,6 +124,9 @@ MPP_RET mpp_buf_slot_setup(MppBufSlots slots, RK_U32 count, RK_U32 size, RK_U32
             if (count > impl->count) {
                 mpp_realloc(impl->slots, MppBufSlotEntry, count);
                 memset(&impl->slots[impl->count], 0, sizeof(MppBufSlotEntry) * (count - impl->count));
+                for (RK_U32 i = 0; i < count; i++) {
+                    impl->slots[i].index = i;
+                }
             }
         } else {
             // info changed, even size is the same we still need to wait for new configuration
@@ -159,6 +170,9 @@ MPP_RET mpp_buf_slot_ready(MppBufSlots slots)
         if (impl->new_count > impl->count) {
             memset(&impl->slots[impl->count], 0, sizeof(MppBufSlotEntry) * (impl->new_count - impl->count));
         }
+        for (RK_U32 i = 0; i < impl->new_count; i++) {
+            impl->slots[i].index = i;
+        }
     }
     impl->count = impl->new_count;
     return MPP_OK;
@@ -243,6 +257,7 @@ MPP_RET mpp_buf_slot_set_decoding(MppBufSlots slots, RK_U32 index)
     MppBufSlotEntry *slot = impl->slots;
     mpp_assert(index < impl->count);
     slot[index].status |= MPP_SLOT_USED_AS_DECODING;
+    impl->output = index;
     return MPP_OK;
 }
 
@@ -263,6 +278,18 @@ MPP_RET mpp_buf_slot_clr_decoding(MppBufSlots slots, RK_U32 index)
     return MPP_OK;
 }
 
+MPP_RET mpp_buf_slot_get_decoding(MppBufSlots slots, RK_U32 *index)
+{
+    if (NULL == slots || NULL == index) {
+        mpp_err_f("found NULL input\n");
+        return MPP_ERR_NULL_PTR;
+    }
+
+    MppBufSlotsImpl *impl = (MppBufSlotsImpl *)slots;
+    *index = impl->output;
+    return MPP_OK;
+}
+
 MPP_RET mpp_buf_slot_set_display(MppBufSlots slots, RK_U32 index)
 {
     if (NULL == slots) {
diff --git a/mpp/codec/mpp_dec.cpp b/mpp/codec/mpp_dec.cpp
index 2b7a695d..b10294d4 100644
--- a/mpp/codec/mpp_dec.cpp
+++ b/mpp/codec/mpp_dec.cpp
@@ -163,7 +163,8 @@ void *mpp_dec_parser_thread(void *data)
          *       - need buffer in different side, need to send a info change
          *         frame to hal loop.
          */
-        RK_S32 output = task_local.dec.output;
+        RK_U32 output;
+        mpp_buf_slot_get_decoding(slots, &output);
         if (NULL == mpp_buf_slot_get_buffer(slots, output)) {
             MppBuffer buffer = NULL;
             RK_U32 size = mpp_buf_slot_get_size(slots);
@@ -257,7 +258,7 @@ void *mpp_dec_hal_thread(void *data)
          * 5. add frame to output list
          * 6. clear display flag
          */
-        RK_S32 output = task_dec->output;
+        RK_U32 output = task_dec->output;
 
         mpp_buf_slot_clr_decoding(slots, output);
 
@@ -270,11 +271,11 @@ void *mpp_dec_hal_thread(void *data)
         MppBuffer buffer = mpp_buf_slot_get_buffer(slots, output);
         mpp_frame_set_buffer(frame, buffer);
 
+        mpp_buf_slot_clr_display(slots, output);
+
         frames->add_at_tail(&frame, sizeof(frame));
         mpp->mFramePutCount++;
 
-        mpp_buf_slot_clr_display(slots, output);
-
         /*
          * mark previous buffer is complete
          */
diff --git a/mpp/hal/inc/hal_task.h b/mpp/hal/inc/hal_task.h
index 11110fd3..9bdfe4e5 100644
--- a/mpp/hal/inc/hal_task.h
+++ b/mpp/hal/inc/hal_task.h
@@ -71,6 +71,7 @@ typedef struct HalDecTask_t {
     // current tesk protocol syntax information
     MppSyntax       syntax;
 
+    // for test purpose
     // current tesk output slot index
     RK_S32          output;
     // current task reference slot index, -1 for unused