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Commit 8f6cecff authored by Lukas Wunner's avatar Lukas Wunner Committed by Greg Kroah-Hartman
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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: default avatarLukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/bceeaba030b028ed810272d55d5fc6f3656ddddb.1641129752.git.lukas@wunner.de


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 6f7bd9f7

drivers/tty/serial/serial_core.c

+2 −1
Original line number Diff line number Diff line
@@ -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);



		/*