The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From c19d93542a6081577e6da9bf5e887979c72e80c1 Mon Sep 17 00:00:00 2001 From: "daniel.starke@siemens.com" daniel.starke@siemens.com Date: Thu, 17 Feb 2022 23:31:21 -0800 Subject: [PATCH] tty: n_gsm: fix wrong tty control line for flow control

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: 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/20220218073123.2121-5-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c index 79397a40a8f8..b4ddac0124b9 100644 --- a/drivers/tty/n_gsm.c +++ b/drivers/tty/n_gsm.c @@ -3243,9 +3243,9 @@ 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); }

@@ -3255,9 +3255,9 @@ 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); }