serial: 8250: Preserve DLD[7:4] for PORT_XR17V35X

[ Upstream commit 0ab84da2e076948c49d36197ee7d254125c53eab ]

The upper four bits of the XR17V35x fractional divisor register (DLD)
control general chip function (RS-485 direction pin polarity, multidrop
mode, XON/XOFF parity check, and fast IR mode). Don't allow these bits
to be clobbered when setting the baudrate.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f0cc04f..8dcfd49 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2586,8 +2586,11 @@ static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
        serial_dl_write(up, quot);
 
        /* XR17V35x UARTs have an extra fractional divisor register (DLD) */
-       if (up->port.type == PORT_XR17V35X)
+       if (up->port.type == PORT_XR17V35X) {
+               /* Preserve bits not related to baudrate; DLD[7:4]. */
+               quot_frac |= serial_port_in(port, 0x2) & 0xf0;
                serial_port_out(port, 0x2, quot_frac);
+       }
 }
 
 static unsigned int serial8250_get_baud_rate(struct uart_port *port,