tbf: Use put_frame in append_data if the TBF has no data

Currently if append_data() is used when there is no LLC data in the DL TBF,
it will either call reuse_tbf() which in turn will call put_frame(),
or it will append the LLC message to the queue, even if the queue and
the frame buffer are empty. This only happens with the test case so
far, but this would change when idle DL TBFs are kept open for some
time. It results in empty LLC message being sent to the MS (see log
below).

This commit changes append_data to check for this case and to
eventually use put_frame() instead of appending the LLC data to the
queue.

Addresses:
  TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 ..
  V(S)==0)
  - Sending new block at BSN 0
  -- Chunk with length 0 is less than remaining space (20): add length
    header to to delimit LLC frame
  Complete DL frame for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)len=0
  - Dequeue next LLC for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)
    (len=200)

Sponsored-by: On-Waves ehf

src/tbf_dl.cpp

@@ -71,6 +71,10 @@ int gprs_rlcmac_dl_tbf::append_data(const uint8_t ms_class,
 			"(T3193), so reuse TBF\n", tbf_name(this));
 		tbf_update_ms_class(this, ms_class);
 		reuse_tbf(data, len);
+	} else if (!have_data()) {
+		m_llc.put_frame(data, len);
+		bts->llc_frame_sched();
+		tbf_update_ms_class(this, ms_class);
 	} else {
 		/* the TBF exists, so we must write it in the queue
 		 * we prepend lifetime in front of PDU */

tests/tbf/TbfTest.err

@@ -37,19 +37,16 @@ Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=0 block=0 data=4f 08 20 00 44
 Scheduling data message at RTS for DL TFI=0 (TRX=0, TS=4)
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 .. V(S)==0)
 - Sending new block at BSN 0
--- Chunk with length 0 is less than remaining space (20): add length header to to delimit LLC frame
-Complete DL frame for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)len=0
-- Dequeue next LLC for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) (len=200)
--- Chunk with length 200 larger than space (19) left in block: copy only remaining space, and we are done
-data block: 07 00 00 03 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 
-Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=4 block=1 data=07 00 00 03 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 
+-- Chunk with length 200 larger than space (20) left in block: copy only remaining space, and we are done
+data block: 07 00 01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 
+Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=4 block=1 data=07 00 01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 
 Received RTS for PDCH: TRX=0 TS=4 FN=8 block_nr=2 scheduling free USF for polling at FN=13 of TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)
 Scheduling data message at RTS for DL TFI=0 (TRX=0, TS=4)
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 .. V(S)==1)
 - Sending new block at BSN 1
--- Chunk with length 181 larger than space (20) left in block: copy only remaining space, and we are done
-data block: 07 00 03 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 
-Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=8 block=2 data=07 00 03 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 
+-- Chunk with length 180 larger than space (20) left in block: copy only remaining space, and we are done
+data block: 07 00 03 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 
+Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=8 block=2 data=07 00 03 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink acknowledge
 - Final ACK received.
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) changes state from FLOW to WAIT RELEASE
@@ -104,19 +101,16 @@ Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=0 block=0 data=4f 08 20 00 44
 Scheduling data message at RTS for DL TFI=0 (TRX=0, TS=4)
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 .. V(S)==0)
 - Sending new block at BSN 0
--- Chunk with length 0 is less than remaining space (20): add length header to to delimit LLC frame
-Complete DL frame for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)len=0
-- Dequeue next LLC for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) (len=200)
--- Chunk with length 200 larger than space (19) left in block: copy only remaining space, and we are done
-data block: 07 00 00 03 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 
-Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=4 block=1 data=07 00 00 03 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 
+-- Chunk with length 200 larger than space (20) left in block: copy only remaining space, and we are done
+data block: 07 00 01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 
+Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=4 block=1 data=07 00 01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 
 Received RTS for PDCH: TRX=0 TS=4 FN=8 block_nr=2 scheduling free USF for polling at FN=13 of TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)
 Scheduling data message at RTS for DL TFI=0 (TRX=0, TS=4)
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 .. V(S)==1)
 - Sending new block at BSN 1
--- Chunk with length 181 larger than space (20) left in block: copy only remaining space, and we are done
-data block: 07 00 03 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 
-Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=8 block=2 data=07 00 03 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 
+-- Chunk with length 180 larger than space (20) left in block: copy only remaining space, and we are done
+data block: 07 00 03 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 
+Sending data request: trx=0 ts=4 sapi=5 arfcn=0 fn=8 block=2 data=07 00 03 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink acknowledge
 - Final ACK received.
 TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) changes state from FLOW to WAIT RELEASE