linux-kernel - [PATCH 8/8] tty: n_gsm: Enable reception of frames separated with a single SOF marker

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Message-Id: <1456090717-19913-9-git-send-email-ak77@tnode.com>
Date:	Sun, 21 Feb 2016 22:38:37 +0100
From:	Andrej Krpic <ak77@...de.com>
To:	linux-kernel@...r.kernel.org
Cc:	jslaby@...e.com, gregkh@...uxfoundation.org,
	Andrej Krpic <ak77@...de.com>
Subject: [PATCH 8/8] tty: n_gsm: Enable reception of frames separated with a single SOF marker

Frames may be separated with a single SOF (5.2.6.1 of 27.010 mux spec).
While transmission of a single SOF between frames is implemented in
gsm_data_kick, the reception isn't.

As a side effect, it is now possible to receive and ignore a stream of
consecutive SOFs (5.2.5 of 27.010 mux spec).

Signed-off-by: Andrej Krpic <ak77@...de.com>
---
 drivers/tty/n_gsm.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index b0d9edd..12b149d 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1895,9 +1895,14 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
 		}
 		break;
 	case GSM_ADDRESS:	/* Address EA */
-		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		/* Ignore (not first) GSM0_SOF as it decodes into
+		 * reserved DLCI 62 (5.6 of the 27.010 mux spec).
+		 */
+		if (c == GSM0_SOF)
+			break;
 		if (gsm_read_ea(&gsm->address, c))
 			gsm->state = GSM_CONTROL;
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
 		break;
 	case GSM_CONTROL:	/* Control Byte */
 		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
@@ -1944,13 +1949,10 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
 	case GSM_FCS:		/* FCS follows the packet */
 		gsm->received_fcs = c;
 		gsm_queue(gsm);
-		gsm->state = GSM_SSOF;
-		break;
-	case GSM_SSOF:
-		if (c == GSM0_SOF) {
-			gsm->state = GSM_SEARCH;
-			break;
-		}
+		/* Frames can be separated with a single GSM0_SOF.
+		 * Deal with consecutive GSM0_SOFs in GSM_ADDRESS.
+		 */
+		gsm->state = GSM_SEARCH;
 		break;
 	}
 }
-- 
2.7.0