serial: mvebu-uart: fix tx lost characters

Fixes missing characters on kernel console at low baud rates (i.e.9600).
The driver should poll TX_RDY or TX_FIFO_EMP instead of TX_EMP to ensure
that the transmitter holding register (THR) is ready to receive a new byte.

TX_EMP tells us when it is possible to send a break sequence via
SND_BRK_SEQ. While this also indicates that both the THR and the TSR are
empty, it does not guarantee that a new byte can be written just yet.

Fixes: 3053079 ("serial: mvebu-uart: initial support for Armada-3700 serial port")
Reviewed-by: Miquel Raynal <[email protected]>
Acked-by: Gregory CLEMENT <[email protected]>
Signed-off-by: Gabriel Matni <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/tty/serial/mvebu-uart.c

@@ -618,7 +618,7 @@ static void wait_for_xmitr(struct uart_port *port)
 	u32 val;
 
 	readl_poll_timeout_atomic(port->membase + UART_STAT, val,
-				  (val & STAT_TX_EMP), 1, 10000);
+				  (val & STAT_TX_RDY(port)), 1, 10000);
 }
 
 static void mvebu_uart_console_putchar(struct uart_port *port, int ch)