[PATCH 4.19 176/239] serial: core: Keep mctrl register state and cached copy in sync

[Greg Kroah-Hartman]

From: Lukas Wunner <lukas@xxxxxxxxx>

[ Upstream commit 93a770b7e16772530196674ffc79bb13fa927dc6 ]

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@xxxxxxxxx>
Link: https://lore.kernel.org/r/bceeaba030b028ed810272d55d5fc6f3656ddddb.1641129752.git.lukas@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 63aefe7e91be1..ab4d0f6058c04 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2347,7 +2347,8 @@ 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);
 
 		/*
-- 
2.34.1