From 93a770b7e16772530196674ffc79bb13fa927dc6 Mon Sep 17 00:00:00 2001

From: Lukas Wunner <lukas@wunner.de>

Date: Sun, 2 Jan 2022 18:52:44 +0100

Subject: [PATCH] serial: core: Keep mctrl register state and cached copy in sync

Git-commit: 93a770b7e16772530196674ffc79bb13fa927dc6

Patch-mainline: v5.17-rc1

References: git-fixes

struct uart_port contains a cached copy of the Modem Control signals.

It is used to skip register writes in uart_update_mctrl() if the new

signal state equals the old signal state.  It also avoids a register

read to obtain the current state of output signals.

When a uart_port is registered, uart_configure_port() changes signal

state but neglects to keep the cached copy in sync.  That may cause

a subsequent register write to be incorrectly skipped.  Fix it before

it trips somebody up.

This behavior has been present ever since the serial core was introduced

in 2002:

https://git.kernel.org/history/history/c/33c0d1b0c3eb

So far it was never an issue because the cached copy is initialized to 0

by kzalloc() and when uart_configure_port() is executed, at most DTR has

been set by uart_set_options() or sunsu_console_setup().  Therefore,

a stable designation seems unnecessary.

Signed-off-by: Lukas Wunner <lukas@wunner.de>

Link: https://lore.kernel.org/r/bceeaba030b028ed810272d55d5fc6f3656ddddb.1641129752.git.lukas@wunner.de

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Acked-by: Takashi Iwai <tiwai@suse.de>

---

 drivers/tty/serial/serial_core.c | 3 ++-

 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c

index 259f28ec6c8b..dc40c4155356 100644

--- a/drivers/tty/serial/serial_core.c

+++ b/drivers/tty/serial/serial_core.c

@@ -2389,7 +2389,8 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,

 		 * We probably don't need a spinlock around this, but

 		 */

 		spin_lock_irqsave(&port->lock, flags);

-		port->ops->set_mctrl(port, port->mctrl & TIOCM_DTR);

+		port->mctrl &= TIOCM_DTR;

+		port->ops->set_mctrl(port, port->mctrl);

 		spin_unlock_irqrestore(&port->lock, flags);

 		/*

-- 

2.31.1