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    Subject[PATCH 5.4 221/320] serial: core: Keep mctrl register state and cached copy in sync
    From: Lukas Wunner <>

    [ 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:

    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 <>
    Signed-off-by: Greg Kroah-Hartman <>
    Signed-off-by: Sasha Levin <>
    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 aad640b9e3f4b..c8a047ba76ebe 100644
    --- a/drivers/tty/serial/serial_core.c
    +++ b/drivers/tty/serial/serial_core.c
    @@ -2395,7 +2395,8 @@ 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);


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