serial: ns16550: fix debug uart putc called before init

If _debug_uart_putc() is called before _debug_uart_init(), the
ns16550 debug uart driver hangs in a tight loop waiting for the
tx FIFO to get empty.

As this can happen via a printf sneaking in before the port calls
debug_uart_init(), let's rather ignore characters before the debug
uart is initialized.

This is done by reading the baudrate divisor and aborting if is zero.

Tested on socfpga_cyclone5_socrates.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/drivers/serial/ns16550.c b/drivers/serial/ns16550.c
index f9041aa626..04b604fa2c 100644
--- a/drivers/serial/ns16550.c
+++ b/drivers/serial/ns16550.c
@@ -267,12 +267,26 @@ static inline void _debug_uart_init(void)
 	serial_dout(&com_port->lcr, UART_LCRVAL);
 }
 
+static inline int NS16550_read_baud_divisor(struct NS16550 *com_port)
+{
+	int ret;
+
+	serial_dout(&com_port->lcr, UART_LCR_BKSE | UART_LCRVAL);
+	ret = serial_din(&com_port->dll) & 0xff;
+	ret |= (serial_din(&com_port->dlm) & 0xff) << 8;
+	serial_dout(&com_port->lcr, UART_LCRVAL);
+
+	return ret;
+}
+
 static inline void _debug_uart_putc(int ch)
 {
 	struct NS16550 *com_port = (struct NS16550 *)CONFIG_DEBUG_UART_BASE;
 
-	while (!(serial_din(&com_port->lsr) & UART_LSR_THRE))
-		;
+	while (!(serial_din(&com_port->lsr) & UART_LSR_THRE)) {
+		if (!NS16550_read_baud_divisor(com_port))
+			return;
+	}
 	serial_dout(&com_port->thr, ch);
 }