edge: Send a second BSN block in an RLC message if possible

Currently only one BSN block is encoded in each RLC data message,
even if MSC7-9 are used, which transport two independant (except for a
max BSN delta of 512) BSN blocks. In that case, the same block is
just put twice into the same message.

The current create_dl_acked_block(fn, ts) method handles
block selection (resend, new BSN, dummy block, ...) and restart
handling in one method and is too complex to extend it accordingly.

Therefore this commit move the block selection/creation handling into
a new method (take_next_bsn) which delivers the next BSN along with a
hint, whether it may be combined with another block. In that case,
the function can be called a second time (this time with a valid
previous BSN, that's the one returned by the first call) to get the
second BSN. The real block generation method
create_dl_acked_block(fn, ts, index, index2) is then called with both
BSNs as indices (the second must be -1, if there is only one BSN).

Note that every BSN returned by take_next_bsn should be passed to
create_dl_acked_block to avoid state inconsistencies.

Sponsored-by: On-Waves ehf

3 files changed, 109 insertions, 54 deletions

diff --git a/src/rlc.h b/src/rlc.h
index 28913bd0..54f28dfd 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -31,6 +31,7 @@
 #define RLC_EGPRS_MIN_WS 64 /* min window size */
 #define RLC_EGPRS_MAX_WS 1024 /* min window size */
 #define RLC_EGPRS_SNS 2048 /* EGPRS, must be power of 2 */
+#define RLC_EGPRS_MAX_BSN_DELTA 512
 #define RLC_MAX_SNS  RLC_EGPRS_SNS
 #define RLC_MAX_WS   RLC_EGPRS_MAX_WS
 #define RLC_MAX_LEN 74 /* MCS-9 data unit */
diff --git a/src/tbf.h b/src/tbf.h
index 2eefbeea..6c030d23 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -406,7 +406,10 @@ protected:
 		unsigned lost_bytes;
 	};
 
-	struct msgb *create_new_bsn(const uint32_t fn, const uint8_t ts);
+	int take_next_bsn(uint32_t fn, int previous_bsn,
+		GprsCodingScheme *next_cs);
+	bool restart_bsn_cycle();
+	int create_new_bsn(const uint32_t fn, GprsCodingScheme cs);
 	struct msgb *create_dl_acked_block(const uint32_t fn, const uint8_t ts,
 					int index, int index2 = -1);
 	int update_window(const uint8_t ssn, const uint8_t *rbb);
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 21b2ba13..e2521f84 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -326,81 +326,136 @@ drop_frame:
 	return msg;
 }
 
-/*
- * Create DL data block
- * The messages are fragmented and forwarded as data blocks.
- */
-struct msgb *gprs_rlcmac_dl_tbf::create_dl_acked_block(uint32_t fn, uint8_t ts)
+bool gprs_rlcmac_dl_tbf::restart_bsn_cycle()
 {
-	LOGP(DRLCMACDL, LOGL_DEBUG, "%s downlink (V(A)==%d .. "
-		"V(S)==%d)\n", tbf_name(this),
-		m_window.v_a(), m_window.v_s());
+	/* If V(S) == V(A) and finished state, we would have received
+	 * acknowledgement of all transmitted block.  In this case we would
+	 * have transmitted the final block, and received ack from MS. But in
+	 * this case we did not receive the final ack indication from MS.  This
+	 * should never happen if MS works correctly.
+	 */
+	if (m_window.window_empty()) {
+		LOGP(DRLCMACDL, LOGL_DEBUG, "- MS acked all blocks\n");
+		return false;
+	}
 
-do_resend:
-	/* check if there is a block with negative acknowledgement */
-	int resend_bsn = m_window.resend_needed();
-	if (resend_bsn >= 0) {
-		LOGP(DRLCMACDL, LOGL_DEBUG, "- Resending BSN %d\n", resend_bsn);
-		/* re-send block with negative aknowlegement */
-		m_window.m_v_b.mark_unacked(resend_bsn);
-		bts->rlc_resent();
-		return create_dl_acked_block(fn, ts, resend_bsn);
+	/* cycle through all unacked blocks */
+	int resend = m_window.mark_for_resend();
+
+	/* At this point there should be at least one unacked block
+	 * to be resent. If not, this is an software error. */
+	if (resend == 0) {
+		LOGP(DRLCMACDL, LOGL_ERROR, "Software error: "
+			"There are no unacknowledged blocks, but V(A) "
+			" != V(S). PLEASE FIX!\n");
+		return false;
 	}
 
-	/* if the window has stalled, or transfer is complete,
-	 * send an unacknowledged block */
-	if (state_is(GPRS_RLCMAC_FINISHED)) {
+	return true;
+}
+
+int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
+	int previous_bsn, GprsCodingScheme *next_cs)
+{
+	int bsn;
+	GprsCodingScheme cs2;
+	GprsCodingScheme force_cs;
+
+	bsn = m_window.resend_needed();
+
+	if (previous_bsn >= 0) {
+		force_cs = m_rlc.block(previous_bsn)->cs;
+		if (!force_cs.isEgprs())
+			return -1;
+	}
+
+	if (bsn >= 0) {
+		if (previous_bsn == bsn)
+			return -1;
+
+		if (previous_bsn >= 0 &&
+			m_window.mod_sns(bsn - previous_bsn) > RLC_EGPRS_MAX_BSN_DELTA)
+			return -1;
+
+		cs2 = m_rlc.block(bsn)->cs;
+		if (force_cs && !cs2.isCombinable(force_cs))
+			return -1;
+		LOGP(DRLCMACDL, LOGL_DEBUG, "- Resending BSN %d\n", bsn);
+		/* re-send block with negative aknowlegement */
+		m_window.m_v_b.mark_unacked(bsn);
+		bts->rlc_resent();
+	} else if (state_is(GPRS_RLCMAC_FINISHED)) {
 		LOGP(DRLCMACDL, LOGL_DEBUG, "- Restarting at BSN %d, "
 			"because all blocks have been transmitted.\n",
 			m_window.v_a());
 		bts->rlc_restarted();
+		if (restart_bsn_cycle())
+			return take_next_bsn(fn, previous_bsn, next_cs);
 	} else if (dl_window_stalled()) {
 		LOGP(DRLCMACDL, LOGL_NOTICE, "- Restarting at BSN %d, "
-			"because all window is stalled.\n",
+			"because the window is stalled.\n",
 			m_window.v_a());
 		bts->rlc_stalled();
+		if (restart_bsn_cycle())
+			return take_next_bsn(fn, previous_bsn, next_cs);
 	} else if (have_data()) {
 		/* New blocks may be send */
-		return create_new_bsn(fn, ts);
+		cs2 = force_cs ? force_cs : current_cs();
+		LOGP(DRLCMACDL, LOGL_DEBUG,
+			"- Sending new block at BSN %d, CS=%s\n",
+			m_window.v_s(), cs2.name());
+
+		bsn = create_new_bsn(fn, cs2);
 	} else if (!m_window.window_empty()) {
 		LOGP(DRLCMACDL, LOGL_DEBUG, "- Restarting at BSN %d, "
 			"because all blocks have been transmitted (FLOW).\n",
 			m_window.v_a());
 		bts->rlc_restarted();
+		if (restart_bsn_cycle())
+			return take_next_bsn(fn, previous_bsn, next_cs);
 	} else {
 		/* Nothing left to send, create dummy LLC commands */
-		return create_new_bsn(fn, ts);
+		LOGP(DRLCMACDL, LOGL_DEBUG,
+			"- Sending new dummy block at BSN %d, CS=%s\n",
+			m_window.v_s(), current_cs().name());
+		bsn = create_new_bsn(fn, current_cs());
+		/* Don't send a second block */
 	}
 
-	/* If V(S) == V(A) and finished state, we would have received
-	 * acknowledgement of all transmitted block. In this case we
-	 * would have transmitted the final block, and received ack
-	 * from MS. But in this case we did not receive the final ack
-	 * indication from MS. This should never happen if MS works
-	 * correctly. */
-	if (m_window.window_empty()) {
-		LOGP(DRLCMACDL, LOGL_DEBUG, "- MS acked all blocks, "
-			"so we re-transmit final block!\n");
+	if (bsn < 0) {
 		/* we just send final block again */
-		int16_t index = m_window.v_s_mod(-1);
+		LOGP(DRLCMACDL, LOGL_DEBUG,
+			"- Nothing else to send, Re-transmit final block!\n");
+		bsn = m_window.v_s_mod(-1);
 		bts->rlc_resent();
-		return create_dl_acked_block(fn, ts, index);
 	}
 
-	/* cycle through all unacked blocks */
-	int resend = m_window.mark_for_resend();
+	*next_cs = cs2;
 
-	/* At this point there should be at least one unacked block
-	 * to be resent. If not, this is an software error. */
-	if (resend == 0) {
-		LOGP(DRLCMACDL, LOGL_ERROR, "Software error: "
-			"There are no unacknowledged blocks, but V(A) "
-			" != V(S). PLEASE FIX!\n");
-		/* we just send final block again */
-		int16_t index = m_window.v_s_mod(-1);
-		return create_dl_acked_block(fn, ts, index);
-	}
-	goto do_resend;
+	return bsn;
+}
+
+/*
+ * Create DL data block
+ * The messages are fragmented and forwarded as data blocks.
+ */
+struct msgb *gprs_rlcmac_dl_tbf::create_dl_acked_block(uint32_t fn, uint8_t ts)
+{
+	int bsn, bsn2 = -1;
+	GprsCodingScheme cs, next_cs;
+
+	LOGP(DRLCMACDL, LOGL_DEBUG, "%s downlink (V(A)==%d .. "
+		"V(S)==%d)\n", tbf_name(this),
+		m_window.v_a(), m_window.v_s());
+
+	bsn = take_next_bsn(fn, -1, &next_cs);
+	if (bsn < 0)
+		return NULL;
+
+	if (next_cs.numDataBlocks() > 1)
+		bsn2 = take_next_bsn(fn, bsn, &next_cs);
+
+	return create_dl_acked_block(fn, ts, bsn, bsn2);
 }
 
 void gprs_rlcmac_dl_tbf::schedule_next_frame()
@@ -425,12 +480,11 @@ void gprs_rlcmac_dl_tbf::schedule_next_frame()
 	m_last_dl_drained_fn = -1;
 }
 
-struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t ts)
+int gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, GprsCodingScheme cs)
 {
 	uint8_t *data;
 	gprs_rlc_data *rlc_data;
 	const uint16_t bsn = m_window.v_s();
-	GprsCodingScheme cs = current_cs();
 	gprs_rlc_data_block_info *rdbi;
 	int num_chunks = 0;
 	int write_offset = 0;
@@ -439,9 +493,6 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 	if (m_llc.frame_length() == 0)
 		schedule_next_frame();
 
-	LOGP(DRLCMACDL, LOGL_DEBUG, "- Sending new block at BSN %d, CS=%s\n",
-		m_window.v_s(), cs.name());
-
 	OSMO_ASSERT(cs.isValid());
 
 	/* length of usable data block (single data unit w/o header) */
@@ -514,7 +565,7 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 	m_window.m_v_b.mark_unacked(bsn);
 	m_window.increment_send();
 
-	return create_dl_acked_block(fn, ts, bsn);
+	return bsn;
 }
 
 struct msgb *gprs_rlcmac_dl_tbf::create_dl_acked_block(