serial: mvebu-uart: clear state register before IRQ request

When receiving data on RX pin before ->uart_startup() is called, some
error bits in the state register could be set up (like BRK_DET).

This is harmless when using only the standard UART (error bits are
read-only), but may procude an endless loop once in the extended UART
RX interrupt handler (error bits must be cleared).

Clear the status register in ->uart_startup() to avoid this situation.

Signed-off-by: Allen Yan <yanwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mvebu-uart.c

@@ -322,6 +322,12 @@ static int mvebu_uart_startup(struct uart_port *port)
 	writel(CTRL_TXFIFO_RST | CTRL_RXFIFO_RST,
 	       port->membase + UART_CTRL(port));
 	udelay(1);
+
+	/* Clear the error bits of state register before IRQ request */
+	ret = readl(port->membase + UART_STAT);
+	ret |= STAT_BRK_ERR;
+	writel(ret, port->membase + UART_STAT);
+
 	writel(CTRL_BRK_INT, port->membase + UART_CTRL(port));
 
 	ctl = readl(port->membase + UART_INTR(port));