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>

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index e67ea523388fad85fab86f0f51b0f3a5fce6c7c6..887cd687c027dbbe59820f239ddeb43d01f0c2c0 100644 (file)
--- 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 560ca2ae349b4314911d6636f2d2c3dd9d358c0b..6cf2be8f2b892eb0fd7cfea13ddf825b4380724b 100644 (file)
--- 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);
 }