From 06d5afd4d640eea67f5623e76cd5fc03359b7f3c Mon Sep 17 00:00:00 2001
From: Daniel Starke <daniel.starke@siemens.com>
Date: Thu, 14 Apr 2022 02:42:10 -0700
Subject: [PATCH] tty: n_gsm: fix wrong signal octet encoding in convergence layer type 2
Git-commit: 06d5afd4d640eea67f5623e76cd5fc03359b7f3c
Patch-mainline: v5.18-rc5
References: git-fixes

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.5.2 describes that the signal octet in
convergence layer type 2 can be either one or two bytes. The length is
encoded in the EA bit. This is set 1 for the last byte in the sequence.
gsmtty_modem_update() handles this correctly but gsm_dlci_data_output()
fails to set EA to 1. There is no case in which we encode two signal octets
as there is no case in which we send out a break signal.
Therefore, always set the EA bit to 1 for the signal octet to fix this.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220414094225.4527-5-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/tty/n_gsm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index de97a3810731..3ba2505908e3 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -832,7 +832,7 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 			break;
 		case 2:	/* Unstructed with modem bits.
 		Always one byte as we never send inline break data */
-			*dp++ = gsm_encode_modem(dlci);
+			*dp++ = (gsm_encode_modem(dlci) << 1) | EA;
 			break;
 		}
 		WARN_ON(kfifo_out_locked(&dlci->fifo, dp , len, &dlci->lock) != len);
-- 
2.35.3