[1/8] serial: 8250: Don't touch RTS modem control while in rs485 mode

Message ID b1ce34ca9bc4d7bdc6e9852fcf30b1f4e37c8a80.1582895077.git.lukas@wunner.de
State New
Headers show
Series
  • rs485 support for Raspberry Pi aux uart
Related show

Commit Message

Lukas Wunner Feb. 28, 2020, 1:31 p.m.
serial8250_do_set_mctrl() currently allows modifying the RTS modem
control line even when RTS is used as an rs485 Transmit Enable signal.
It is thus possible for user space to interfere with rs485 communication
by invoking a TIOCMSET ioctl().

Ignore such change requests and retain the current RTS polarity when in
rs485 mode.  Note that serial8250_set_mctrl() is always called with
port->lock held, so there's no risk that RTS is changed concurrently.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Matwey V. Kornilov <matwey@sai.msu.ru>
---
 drivers/tty/serial/8250/8250_port.c | 7 +++++++
 1 file changed, 7 insertions(+)

Patch

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f398f162a1fd..a8f4cedde4dc 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1924,6 +1924,13 @@  void serial8250_do_set_mctrl(struct uart_port *port, unsigned int mctrl)
 	struct uart_8250_port *up = up_to_u8250p(port);
 	unsigned char mcr;
 
+	if (port->rs485.flags & SER_RS485_ENABLED) {
+		if (serial8250_in_MCR(up) & UART_MCR_RTS)
+			mctrl |= TIOCM_RTS;
+		else
+			mctrl &= ~TIOCM_RTS;
+	}
+
 	mcr = serial8250_TIOCM_to_MCR(mctrl);
 
 	mcr = (mcr & up->mcr_mask) | up->mcr_force | up->mcr;