From: Lukas Wunner lukas@wunner.de
[ 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@wunner.de Link: https://lore.kernel.org/r/bceeaba030b028ed810272d55d5fc6f3656ddddb.164112975... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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);
/*