serial: 8250: lock port in startup() callbacks

serial: 8250: lock port in startup() callbacks

uart_ops startup() callback is called without interrupts
disabled and without port->lock locked, relatively late during the
boot process (from the call path of console_on_rootfs()). If the
device is a console, it was already previously registered and could
be actively printing messages.

The console printing function serial8250_console_write() modifies
the interrupt register (UART_IER) under the port->lock with the
pattern: read, clear, restore.

Since some startup() callbacks are modifying UART_IER without the
port->lock locked, it is possible that the value intended to be
written by the startup() callback will get overwritten and be
lost.

CPU0                           CPU1
serial8250_console_write       omap_8250_startup
--------------------------     -----------------
spin_lock(port->lock)
oldval = read(UART_IER)
uart_console_write()
                               write(newval, UART_IER)
write(oldval, UART_IER)
spin_unlock(port->lock)

Add port->lock synchronization to the 8250 startup() callbacks
where they need to access UART_IER. This avoids racing with
serial8250_console_write().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20230525093159.223817-2-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>