Implement downgrade to DL MCS1-4 when USF for GPRS_only MS

In previous status, if USF for GPRS-only MS was selected, then EGPRS
TBFs were skipped and either a GPRS TBF was selected or a Dummy Block
was sent. That means the behavior was unfair towards EGPRS TBFs, because
sometimes they were skipped in favor of GPRS ones.
This patch imporves the situation in the above mentioned USF scenario, by
first, under specific conditions, allowing selection of an EGPRS TBF and
then forcing it to transmit in EGPRS-GMSK (MCS1-4) so that the
USF-targeted MS can still decode the USF, while at the same time
providing more fairness by allowing the EGPRS TBF to transmit data.

The specific conditions mentioned above are, mainly, related to the fact
that once a DL data block has been sent, and hence a BSN was assigned to
it, it cannot be retransmitted later using another MCS, since lower
MCS1-4 wouldn't be able to contain higher MCS RLC payload.

The set of conditions could be expanded in the future by also selecting
the EGPRS TBF if retransmition is required and the block to be
retransmitted was originally transmitted as MCS1-4.

Related: OS#4544
Change-Id: I9af23e175435fe9ae7b0e4119ad52fcd4707b9ca

1 files changed, 25 insertions, 11 deletions

diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 4b184b83..ad98510c 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -442,7 +442,7 @@ bool gprs_rlcmac_dl_tbf::restart_bsn_cycle()
 }
 
 int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
-	int previous_bsn, bool *may_combine)
+	int previous_bsn, enum mcs_kind req_mcs_kind, bool *may_combine)
 {
 	int bsn;
 	int data_len2, force_data_len = -1;
@@ -456,6 +456,19 @@ int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
 		if (!mcs_is_edge(force_cs))
 			return -1;
 		force_data_len = m_rlc.block(previous_bsn)->len;
+	} else if (bsn < 0 && is_egprs_enabled() && req_mcs_kind == EGPRS_GMSK) {
+		/* New data to be sent for EGPRS TBF but we are required to downgrade to
+		 * MCS1-4, because USF for GPRS-only MS will be sent */
+		force_cs = m_ms->current_cs_dl();
+		if (force_cs > MCS4) {
+			force_cs = bts->cs_dl_is_supported(MCS4) ? MCS4 :
+				   bts->cs_dl_is_supported(MCS3) ? MCS3 :
+				   bts->cs_dl_is_supported(MCS2) ? MCS2 :
+				   MCS1;
+			LOGPTBFDL(this, LOGL_DEBUG,
+				  "Force downgrading DL %s -> %s due to USF for GPRS-only MS\n",
+				  mcs_name(m_ms->current_cs_dl()), mcs_name(force_cs));
+		}
 	}
 
 	if (bsn >= 0) {
@@ -510,7 +523,7 @@ int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
 			  m_window.v_a());
 		bts->do_rate_ctr_inc(CTR_RLC_RESTARTED);
 		if (restart_bsn_cycle())
-			return take_next_bsn(fn, previous_bsn, may_combine);
+			return take_next_bsn(fn, previous_bsn, req_mcs_kind, may_combine);
 	} else if (dl_window_stalled()) {
 		/* There are no more packages to send, but the window is stalled.
 		 * Restart the bsn_cycle to resend all unacked messages */
@@ -519,14 +532,15 @@ int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
 			  m_window.v_a());
 		bts->do_rate_ctr_inc(CTR_RLC_STALLED);
 		if (restart_bsn_cycle())
-			return take_next_bsn(fn, previous_bsn, may_combine);
+			return take_next_bsn(fn, previous_bsn, req_mcs_kind, may_combine);
 	} else if (have_data()) {
 		/* The window has space left, generate new bsn */
 		enum CodingScheme new_cs;
 		new_cs = force_cs ? force_cs : current_cs();
 		LOGPTBFDL(this, LOGL_DEBUG,
-			  "Sending new block at BSN %d, CS=%s\n",
-			  m_window.v_s(), mcs_name(new_cs));
+			  "Sending new block at BSN %d, CS=%s%s\n",
+			  m_window.v_s(), mcs_name(new_cs),
+			  force_cs ? " (forced)" : "");
 
 		bsn = create_new_bsn(fn, new_cs);
 	} else if (bts->bts_data()->dl_tbf_preemptive_retransmission && !m_window.window_empty()) {
@@ -537,7 +551,7 @@ int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
 			  m_window.v_a());
 		bts->do_rate_ctr_inc(CTR_RLC_RESTARTED);
 		if (restart_bsn_cycle())
-			return take_next_bsn(fn, previous_bsn, may_combine);
+			return take_next_bsn(fn, previous_bsn, req_mcs_kind, may_combine);
 	} else {
 		/* Nothing left to send, create dummy LLC commands */
 		LOGPTBFDL(this, LOGL_DEBUG,
@@ -565,20 +579,20 @@ int gprs_rlcmac_dl_tbf::take_next_bsn(uint32_t fn,
  * 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)
+struct msgb *gprs_rlcmac_dl_tbf::create_dl_acked_block(uint32_t fn, uint8_t ts, enum mcs_kind req_mcs_kind)
 {
 	int bsn, bsn2 = -1;
 	bool may_combine;
 
-	LOGPTBFDL(this, LOGL_DEBUG, "downlink (V(A)==%d .. V(S)==%d)\n",
-		  m_window.v_a(), m_window.v_s());
+	LOGPTBFDL(this, LOGL_DEBUG, "downlink (V(A)==%d .. V(S)==%d) mcs_mode_restrict=%s\n",
+		  m_window.v_a(), m_window.v_s(), mode_name(req_mcs_kind));
 
-	bsn = take_next_bsn(fn, -1, &may_combine);
+	bsn = take_next_bsn(fn, -1, req_mcs_kind, &may_combine);
 	if (bsn < 0)
 		return NULL;
 
 	if (may_combine)
-		bsn2 = take_next_bsn(fn, bsn, &may_combine);
+		bsn2 = take_next_bsn(fn, bsn, req_mcs_kind, &may_combine);
 
 	return create_dl_acked_block(fn, ts, bsn, bsn2);
 }