From bd2b897b72209ab17a54c329be9a1cdc6508eb25 Mon Sep 17 00:00:00 2001
From: Andreas Eversberg <jolly@eversberg.eu>
Date: Wed, 15 Nov 2023 14:33:53 +0100
Subject: [PATCH] LAPDm: Add a flag to enable suppression of subsequent REJ
 frame

This behaviour was default in earlier versions of LAPDm/LAPD. Because it
is only required for osmocom-bb, a flag is added to enable it there.

Related: OS#5969
Change-Id: I93994dbbd1fc2c9edb8f3015c6b18ecd0fce0565
---
 include/osmocom/gsm/lapdm.h      | 1 +
 include/osmocom/isdn/lapd_core.h | 1 +
 src/gsm/lapdm.c                  | 2 ++
 src/isdn/lapd_core.c             | 5 ++---
 4 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/osmocom/gsm/lapdm.h b/include/osmocom/gsm/lapdm.h
index 08b808c03..42ebbceca 100644
--- a/include/osmocom/gsm/lapdm.h
+++ b/include/osmocom/gsm/lapdm.h
@@ -50,6 +50,7 @@ typedef int (*lapdm_cb_t)(struct msgb *msg, struct lapdm_entity *le, void *ctx);
 
 #define LAPDM_ENT_F_EMPTY_FRAME		0x0001
 #define LAPDM_ENT_F_POLLING_ONLY	0x0002
+#define LAPDM_ENT_F_DROP_2ND_REJ	0x0004
 
 /*! a LAPDm Entity */
 struct lapdm_entity {
diff --git a/include/osmocom/isdn/lapd_core.h b/include/osmocom/isdn/lapd_core.h
index 776d4f41e..1e010afe0 100644
--- a/include/osmocom/isdn/lapd_core.h
+++ b/include/osmocom/isdn/lapd_core.h
@@ -86,6 +86,7 @@ enum lapd_state {
 
 /*! lapd_flags */
 #define LAPD_F_RTS		0x0001
+#define LAPD_F_DROP_2ND_REJ	0x0002
 
 /*! LAPD T200 state in RTS mode */
 enum lapd_t200_rts {
diff --git a/src/gsm/lapdm.c b/src/gsm/lapdm.c
index 28644c28c..fb892d2d1 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -1637,6 +1637,8 @@ void lapdm_entity_set_flags(struct lapdm_entity *le, unsigned int flags)
 	/* Set flags at LAPD. */
 	if (le->flags & LAPDM_ENT_F_POLLING_ONLY)
 		dl_flags |= LAPD_F_RTS;
+	if (le->flags & LAPDM_ENT_F_DROP_2ND_REJ)
+		dl_flags |= LAPD_F_DROP_2ND_REJ;
 
 	for (i = 0; i < ARRAY_SIZE(le->datalink); i++) {
 		dl = &le->datalink[i];
diff --git a/src/isdn/lapd_core.c b/src/isdn/lapd_core.c
index 34748a49f..d98edae2f 100644
--- a/src/isdn/lapd_core.c
+++ b/src/isdn/lapd_core.c
@@ -1633,9 +1633,8 @@ static int lapd_rx_i(struct msgb *msg, struct lapd_msg_ctx *lctx)
 		      ns, dl->v_recv, lapd_state_name(dl->state));
 		/* discard data */
 		msgb_free(msg);
-		if (dl->seq_err_cond != 1) {
-			/* FIXME: help me understand what exactly todo here
-			*/
+		/* Send reject, but suppress second reject if LAPD_F_DROP_2ND_REJ flag is set. */
+		if (dl->seq_err_cond != 1 || !(dl->lapd_flags & LAPD_F_DROP_2ND_REJ)) {
 			dl->seq_err_cond = 1;
 			lapd_send_rej(lctx, lctx->p_f);
 		} else {