Properly set TA_VALID bit

Check Timing Advance validity and set corresponding bit for Immediate
Assignment message. Previously !polling was errorneously used (polling
bit has nothing to do with TA validity according to 3GPP TS 44.018 Table
10.5.2.16.1) which lead to TA being always valid as polling is always 0
in other parts of the code.

Change-Id: I5d7ecc7f71402b945cae99332be2ebc0b17b9d44
Related: OS#1526

src/encoding.cpp

@@ -29,13 +29,16 @@ extern "C" {
 #include <osmocom/gprs/protocol/gsm_04_60.h>
 }
 
+#include <osmocom/gsm/protocol/gsm_04_08.h>
+
+#include <stdbool.h>
 #include <errno.h>
 #include <string.h>
 
 static int write_ia_rest_downlink(
 	gprs_rlcmac_dl_tbf *tbf,
 	bitvec * dest, unsigned& wp,
-	uint8_t polling, uint32_t fn,
+	uint8_t polling, bool ta_valid, uint32_t fn,
 	uint8_t alpha, uint8_t gamma, int8_t ta_idx)
 {
 	if (!tbf) {
@@ -58,7 +61,7 @@ static int write_ia_rest_downlink(
 	}
 	bitvec_write_field(dest, wp,gamma,5);   // GAMMA power control parameter
 	bitvec_write_field(dest, wp,polling,1);   // Polling Bit
-	bitvec_write_field(dest, wp,!polling,1);   // TA_VALID ???
+	bitvec_write_field(dest, wp, ta_valid, 1); // N. B: NOT related to TAI!
 	if (ta_idx < 0) {
 		bitvec_write_field(dest, wp,0x0,1);   // switch TIMING_ADVANCE_INDEX = off
 	} else {
@@ -208,7 +211,7 @@ int Encoding::write_immediate_assignment(
 
 	if (downlink)
 		rc = write_ia_rest_downlink(as_dl_tbf(tbf), dest, wp,
-			polling, fn,
+					    polling, gsm48_ta_is_valid(ta), fn,
 			alpha, gamma, ta_idx);
 	else if (as_ul_tbf(tbf) && as_ul_tbf(tbf)->is_egprs_enabled())
 		rc = write_ia_rest_egprs_uplink(as_ul_tbf(tbf), dest, wp,