return -EINVAL;
        }
 
+       /* Backwards compatibility for firmwares without
+        * ATH10K_FW_IE_HTT_OP_VERSION.
+        */
+       if (ar->htt.op_version == ATH10K_FW_HTT_OP_VERSION_UNSET) {
+               switch (ar->wmi.op_version) {
+               case ATH10K_FW_WMI_OP_VERSION_MAIN:
+                       ar->htt.op_version = ATH10K_FW_HTT_OP_VERSION_MAIN;
+                       break;
+               case ATH10K_FW_WMI_OP_VERSION_10_1:
+               case ATH10K_FW_WMI_OP_VERSION_10_2:
+               case ATH10K_FW_WMI_OP_VERSION_10_2_4:
+                       ar->htt.op_version = ATH10K_FW_HTT_OP_VERSION_10_1;
+                       break;
+               case ATH10K_FW_WMI_OP_VERSION_TLV:
+                       ar->htt.op_version = ATH10K_FW_HTT_OP_VERSION_TLV;
+                       break;
+               case ATH10K_FW_WMI_OP_VERSION_UNSET:
+               case ATH10K_FW_WMI_OP_VERSION_MAX:
+                       WARN_ON(1);
+                       return -EINVAL;
+               }
+       }
+
        return 0;
 }
 
 
                ar->htt.t2h_msg_types_max = HTT_TLV_T2H_NUM_MSGS;
                break;
        case ATH10K_FW_HTT_OP_VERSION_MAIN:
-       case ATH10K_FW_HTT_OP_VERSION_UNSET:
                ar->htt.t2h_msg_types = htt_main_t2h_msg_types;
                ar->htt.t2h_msg_types_max = HTT_MAIN_T2H_NUM_MSGS;
                break;
        case ATH10K_FW_HTT_OP_VERSION_MAX:
+       case ATH10K_FW_HTT_OP_VERSION_UNSET:
                WARN_ON(1);
                return -EINVAL;
        }