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(), introduce a config option to ignore characters
before the debug uart is initialized.

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

The Kconfig option is required as reading the baudrate divisor does
not seem to work for all ns16500 compatibles (which is why the last
attempt on this has been reverted in 1a67969a99).

Tested on socfpga_cyclone5_socrates.

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

2 files changed, 30 insertions, 2 deletions

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index e67ea52338..887cd687c0 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -447,6 +447,18 @@ config DEBUG_UART_SKIP_INIT
 	  Select this if the UART you want to use for debug output is already
 	  initialized by the time U-Boot starts its execution.
 
+config DEBUG_UART_NS16550_CHECK_ENABLED
+	bool "Check if UART is enabled on output"
+	depends on DEBUG_UART
+	depends on DEBUG_UART_NS16550
+	help
+	  Select this if puts()/putc() might be called before the debug UART
+	  has been initialized. If this is disabled, putc() might sit in a
+	  tight loop if it is called before debug_uart_init() has been called.
+
+	  Note that this does not work for every ns16550-compatible UART and
+	  so has to be enabled carefully or you might notice lost characters.
+
 config ALTERA_JTAG_UART
 	bool "Altera JTAG UART support"
 	depends on DM_SERIAL
diff --git a/drivers/serial/ns16550.c b/drivers/serial/ns16550.c
index 560ca2ae34..6cf2be8f2b 100644
--- a/drivers/serial/ns16550.c
+++ b/drivers/serial/ns16550.c
@@ -272,12 +272,28 @@ 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)) {
+#ifdef CONFIG_DEBUG_UART_NS16550_CHECK_ENABLED
+		if (!NS16550_read_baud_divisor(com_port))
+			return;
+#endif
+	}
 	serial_dout(&com_port->thr, ch);
 }