tty: n_gsm: fix software flow control handling

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 5.4.8.1 states that XON/XOFF characters
shall be used instead of Fcon/Fcoff command in advanced option mode to
handle flow control. Chapter 5.4.8.2 describes how XON/XOFF characters
shall be handled. Basic option mode only used Fcon/Fcoff commands and no
XON/XOFF characters. These are treated as data bytes here.
The current implementation uses the gsm_mux field 'constipated' to handle
flow control from the remote peer and the gsm_dlci field 'constipated' to
handle flow control from each DLCI. The later is unrelated to this patch.
The gsm_mux field is correctly set for Fcon/Fcoff commands in
gsm_control_message(). However, the same is not true for XON/XOFF
characters in gsm1_receive().
Disable software flow control handling in the tty to allow explicit
handling by n_gsm.
Add the missing handling in advanced option mode for gsm_mux in
gsm1_receive() to comply with the standard.

This patch depends on the following commit:
Commit 8838b2a ("tty: n_gsm: fix SW flow control encoding/handling")

Fixes: e1eaea4 ("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/20220422071025.5490-3-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dstarke-siemens

drivers/tty/n_gsm.c

@@ -232,6 +232,7 @@ struct gsm_mux {
 	int initiator;			/* Did we initiate connection */
 	bool dead;			/* Has the mux been shut down */
 	struct gsm_dlci *dlci[NUM_DLCI];
+	int old_c_iflag;		/* termios c_iflag value before attach */
 	bool constipated;		/* Asked by remote to shut up */
 
 	spinlock_t tx_lock;
@@ -2022,6 +2023,16 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
 
 static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
 {
+	/* handle XON/XOFF */
+	if ((c & ISO_IEC_646_MASK) == XON) {
+		gsm->constipated = true;
+		return;
+	} else if ((c & ISO_IEC_646_MASK) == XOFF) {
+		gsm->constipated = false;
+		/* Kick the link in case it is idling */
+		gsm_data_kick(gsm, NULL);
+		return;
+	}
 	if (c == GSM1_SOF) {
 		/* EOF is only valid in frame if we have got to the data state */
 		if (gsm->state == GSM_DATA) {
@@ -2449,6 +2460,9 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	int ret, i;
 
 	gsm->tty = tty_kref_get(tty);
+	/* Turn off tty XON/XOFF handling to handle it explicitly. */
+	gsm->old_c_iflag = tty->termios.c_iflag;
+	tty->termios.c_iflag &= (IXON | IXOFF);
 	ret =  gsm_activate_mux(gsm);
 	if (ret != 0)
 		tty_kref_put(gsm->tty);
@@ -2489,6 +2503,8 @@ static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	WARN_ON(tty != gsm->tty);
 	for (i = 1; i < NUM_DLCI; i++)
 		tty_unregister_device(gsm_tty_driver, base + i);
+	/* Restore tty XON/XOFF handling. */
+	gsm->tty->termios.c_iflag = gsm->old_c_iflag;
 	tty_kref_put(gsm->tty);
 	gsm->tty = NULL;
 }