gsm/lapdm: Fix TA and power level handling in the ACCH header

Timing advance and power level indicated by MS (measurement reports)
and BTS (SI 5/6) are now stored for use at ACCH data link connection.

This is part of a set of commit to fix LAPDm to handle datalink
connection on ACCH (SAPI 3)

This is required to transfer SMS on SACCH of TCH/f or SDCCH/8 (4).

Written-by: Andreas Eversberg <jolly@eversberg.eu>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

2 files changed, 19 insertions, 8 deletions

diff --git a/include/osmocom/gsm/lapdm.h b/include/osmocom/gsm/lapdm.h
index adffbebb..52e8fc52 100644
--- a/include/osmocom/gsm/lapdm.h
+++ b/include/osmocom/gsm/lapdm.h
@@ -72,8 +72,8 @@ struct lapdm_msg_ctx {
 	int lapdm_fmt;
 	uint8_t chan_nr;
 	uint8_t link_id;
-	uint8_t ta_ind;
-	uint8_t tx_power_ind;
+	uint8_t ta_ind;		/* TA indicated by network */
+	uint8_t tx_power_ind;	/* MS power indicated by network */
 };
 
 /*! \brief LAPDm datalink like TS 04.06 / Section 3.5.2 */
@@ -113,6 +113,9 @@ struct lapdm_entity {
 
 	/*! \brief pointer to \ref lapdm_channel of which we're part */
 	struct lapdm_channel *lapdm_ch;
+
+	uint8_t ta;		/* TA used and indicated to network */
+	uint8_t tx_power;	/* MS power used and indicated to network */
 };
 
 /*! \brief the two lapdm_entities that form a GSM logical channel (ACCH + DCCH) */
diff --git a/src/gsm/lapdm.c b/src/gsm/lapdm.c
index 7e4b0380..aac203e2 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -479,6 +479,15 @@ static int lapdm_send_ph_data_req(struct lapd_msg_ctx *lctx, struct msgb *msg)
 	if (lctx->more)
 		msg->l2h[2] |= LAPDm_MORE;
 
+	/* add ACCH header with last indicated tx-power and TA */
+	if ((mctx->link_id & 0x40)) {
+		struct lapdm_entity *le = mdl->entity;
+
+		msg->l2h = msgb_push(msg, 2);
+		msg->l2h[0] = le->tx_power;
+		msg->l2h[1] = le->ta;
+	}
+
 	return tx_ph_data_enqueue(mctx->dl, msg, mctx->chan_nr, mctx->link_id,
 			23);
 }
@@ -799,17 +808,16 @@ static int rslms_rx_rll_udata_req(struct msgb *msg, struct lapdm_datalink *dl)
 	uint8_t sapi = link_id & 7;
 	struct tlv_parsed tv;
 	int length;
-	uint8_t ta = 0, tx_power = 0;
 
 	/* check if the layer3 message length exceeds N201 */
 
 	rsl_tlv_parse(&tv, rllh->data, msgb_l2len(msg)-sizeof(*rllh));
 
 	if (TLVP_PRESENT(&tv, RSL_IE_TIMING_ADVANCE)) {
-		ta = *TLVP_VAL(&tv, RSL_IE_TIMING_ADVANCE);
+		le->ta = *TLVP_VAL(&tv, RSL_IE_TIMING_ADVANCE);
 	}
 	if (TLVP_PRESENT(&tv, RSL_IE_MS_POWER)) {
-		tx_power = *TLVP_VAL(&tv, RSL_IE_MS_POWER);
+		le->tx_power = *TLVP_VAL(&tv, RSL_IE_MS_POWER);
 	}
 	if (!TLVP_PRESENT(&tv, RSL_IE_L3_INFO)) {
 		LOGP(DLLAPD, LOGL_ERROR, "unit data request without message "
@@ -828,7 +836,7 @@ static int rslms_rx_rll_udata_req(struct msgb *msg, struct lapdm_datalink *dl)
 	}
 
 	LOGP(DLLAPD, LOGL_INFO, "sending unit data (tx_power=%d, ta=%d)\n",
-		tx_power, ta);
+		le->tx_power, le->ta);
 
 	/* Remove RLL header from msgb and set length to L3-info */
 	msgb_pull_l2h(msg);
@@ -837,8 +845,8 @@ static int rslms_rx_rll_udata_req(struct msgb *msg, struct lapdm_datalink *dl)
 
 	/* Push L1 + LAPDm header on msgb */
 	msg->l2h = msgb_push(msg, 2 + 3);
-	msg->l2h[0] = tx_power;
-	msg->l2h[1] = ta;
+	msg->l2h[0] = le->tx_power;
+	msg->l2h[1] = le->ta;
 	msg->l2h[2] = LAPDm_ADDR(LAPDm_LPD_NORMAL, sapi, dl->dl.cr.loc2rem.cmd);
 	msg->l2h[3] = LAPDm_CTRL_U(LAPDm_U_UI, 0);
 	msg->l2h[4] = LAPDm_LEN(length);