doc: clarify enable_sdap as RRC SDAP header policy

Log, param help, and rrc-usage.md described enable_sdap as toggling the
SDAP layer, while it only sets sdap-HeaderUL/DL in RRC SDAP-Config.
Signed-off-by: Guido Casati <guido.casati@openairinterface.org>

doc/RRC/rrc-usage.md

@@ -90,9 +90,9 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
       `0xffffff` is a reserved value and means "no SD"
     Note that: SST=1, no SD is "eMBB"; SST=2, no SD is "URLLC"; SST=3, no SD
     is "mMTC"
-- `enable_sdap` (default: true): enable the use of the SDAP layer. If
-  deactivated, a transparent SDAP header is prepended to packets, but no
-  further processing is being done.
+- `enable_sdap` (default: true): set `sdap-HeaderUL` and `sdap-HeaderDL` to
+  present in the RRC `SDAP-Config` IE for SA PDU sessions. If false, both
+  headers are absent (per DRB). SDAP entities are still created, SDAP layer always enabled.
 - `cu_sibs` (default: `[]`) list of SIBs to give to the DU for transmission.
   Currently supported:
   - SIB2: serving-cell reselection parameters (configured in `sib2_config`)

openair2/GNB_APP/gnb_config.c

@@ -2435,7 +2435,7 @@ gNB_RRC_INST *RCconfig_NRRRC()
 
         nrrrc_config.num_plmn = set_plmn_config(nrrrc_config.plmn, k);
         nrrrc_config.enable_sdap = *GNBParamList.paramarray[i][GNB_ENABLE_SDAP_IDX].iptr;
-        LOG_I(GNB_APP, "SDAP layer is %s\n", nrrrc_config.enable_sdap ? "enabled" : "disabled");
+        LOG_I(GNB_APP, "SDAP UL/DL headers in RRC are %s\n", nrrrc_config.enable_sdap ? "present" : "absent");
         nrrrc_config.um_on_default_drb = *(GNBParamList.paramarray[i][GNB_UMONDEFAULTDRB_IDX].uptr);
 
       }//

openair2/GNB_APP/gnb_paramdef.h

@@ -102,7 +102,7 @@ typedef enum {
 #define GNB_CONFIG_STRING_1ST_ACTIVE_BWP                "first_active_bwp"
 #define GNB_CONFIG_STRING_LIMIT_RSRP_REPORT             "max_num_RSRP_reported"
 
-#define GNB_CONFIG_HLP_STRING_ENABLE_SDAP               "enable the SDAP layer\n"
+#define GNB_CONFIG_HLP_STRING_ENABLE_SDAP               "set sdap-HeaderUL/DL present in RRC SDAP-Config (false = both absent)\n"
 #define GNB_CONFIG_HLP_FORCE256QAMOFF                   "suppress activation of 256 QAM despite UE support"
 #define GNB_CONFIG_HLP_MAXMIMOLAYERS                    "limit on maxMIMO-layers for DL"
 #define GNB_CONFIG_HLP_DISABLE_HARQ                     "disable feedback for all HARQ processes (REL17 feature)"