This is a note to let you know that I've just added the patch titled serial: core: Keep mctrl register state and cached copy in sync to the 5.10-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: serial-core-keep-mctrl-register-state-and-cached-cop.patch and it can be found in the queue-5.10 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 148db6a691bf78d59ab2018e28749a0dac00a495 Author: Lukas Wunner <lukas@xxxxxxxxx> Date: Sun Jan 2 18:52:44 2022 +0100 serial: core: Keep mctrl register state and cached copy in sync [ 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> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 046bedca7b8f5..55108db5b64bf 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -2414,7 +2414,8 @@ 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); /*