tty: serial_core: Set port active bit in uart_port_activate

A bug was introduced by commit b3b576461864 ("tty: serial_core: convert
uart_open to use tty_port_open"). It caused a constant warning printed
into the system log regarding the tty and port counter mismatch:

[ 21.644197] ttyS ttySx: tty_port_close_start: tty->count = 1 port count = 2

in case if session hangup was detected so the warning is printed starting
from the second open-close iteration.

Particularly the problem was discovered in situation when there is a
serial tty device without hardware back-end being setup. It is considered
by the tty-serial subsystems as a hardware problem with session hang up.
In this case uart_startup() will return a positive value with TTY_IO_ERROR
flag set in corresponding tty_struct instance. The same value will get
passed to be returned from the activate() callback and then being returned
from tty_port_open(). But since in this case tty_port_block_til_ready()
isn't called the TTY_PORT_ACTIVE flag isn't set (while the method had been
called before tty_port_open conversion was introduced and the rest of the
subsystem code expected the bit being set in this case), which prevents the
uart_hangup() method to perform any cleanups including the tty port
counter setting to zero. So the next attempt to open/close the tty device
will discover the counters mismatch.

In order to fix the problem we need to manually set the TTY_PORT_ACTIVE
flag in case if uart_startup() returned a positive value. In this case
the hang up procedure will perform a full set of cleanup actions including
the port ref-counter resetting.

Fixes: b3b576461864 "tty: serial_core: convert uart_open to use tty_port_open"
Signed-off-by: Serge Semin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/serial_core.c

···
 {
 	struct uart_state *state = container_of(port, struct uart_state, port);
 	struct uart_port *uport;
+	int ret;
 
 	uport = uart_port_check(state);
 	if (!uport || uport->flags & UPF_DEAD)
···
 	/*
 	 * Start up the serial port.
 	 */
-	return uart_startup(tty, state, 0);
+	ret = uart_startup(tty, state, 0);
+	if (ret > 0)
+		tty_port_set_active(port, 1);
+
+	return ret;
 }
 
 static const char *uart_type(struct uart_port *port)