tty: n_gsm: fix wrong tty control line for flow control

stable inclusion
from stable-v5.10.103
commit e4c8cb95d03543c721e7a1265253eda2ce1b8370
bugzilla: https://gitee.com/openeuler/kernel/issues/I56NE7

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e4c8cb95d03543c721e7a1265253eda2ce1b8370

--------------------------------

commit c19d9354 upstream.

tty flow control is handled via gsmtty_throttle() and gsmtty_unthrottle().
Both functions propagate the outgoing hardware flow control state to the
remote side via MSC (modem status command) frames. The local state is taken
from the RTS (ready to send) flag of the tty. However, RTS gets mapped to
DTR (data terminal ready), which is wrong.
This patch corrects this by mapping RTS to RTS.

Fixes: e1eaea46 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-5-daniel.starke@siemens.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYu Liao <liaoyu15@huawei.com>
Reviewed-by: NWei Li <liwei391@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/tty/n_gsm.c

@@ -3178,9 +3178,9 @@ static void gsmtty_throttle(struct tty_struct *tty)
 	if (dlci->state == DLCI_CLOSED)
 		return;
 	if (C_CRTSCTS(tty))
-		dlci->modem_tx &= ~TIOCM_DTR;
+		dlci->modem_tx &= ~TIOCM_RTS;
 	dlci->throttled = true;
-	/* Send an MSC with DTR cleared */
+	/* Send an MSC with RTS cleared */
 	gsmtty_modem_update(dlci, 0);
 }
 
@@ -3190,9 +3190,9 @@ static void gsmtty_unthrottle(struct tty_struct *tty)
 	if (dlci->state == DLCI_CLOSED)
 		return;
 	if (C_CRTSCTS(tty))
-		dlci->modem_tx |= TIOCM_DTR;
+		dlci->modem_tx |= TIOCM_RTS;
 	dlci->throttled = false;
-	/* Send an MSC with DTR set */
+	/* Send an MSC with RTS set */
 	gsmtty_modem_update(dlci, 0);
 }