dm: serial: Add a REQUIRE_SERIAL_CONSOLE option for boards with no serial port

Currently the serial code assumes that there is always at least one serial
port (and panics / crashes due to null pointer dereferences when there is
none).

This makes it impossible to use u-boot on boards where there is no (debug)
serial port, because e.g. all uart pins are muxed to another function.

This commit adds a CONFIG_REQUIRE_SERIAL_CONSOLE Kconfig option, which
defaults to y (preserving existing behavior), which can be set to n on
such boards to make them work.

This commit only implements this for CONFIG_DM_SERIAL=y configs, as allowing
running without a serial port for CONFIG_DM_SERIAL=n configs is non trivial,
and is not necessary at this moment.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>

drivers/serial/Kconfig

@@ -1,3 +1,14 @@
+config REQUIRE_SERIAL_CONSOLE
+	bool "Require a serial port for console"
+	# Running without a serial console is not supported by the
+	# non-dm serial code
+	depends on DM_SERIAL
+	default y
+	help
+	  Require a serial port for the console, and panic if none is found
+	  during serial port initialization (default y). Set this to n on
+	  boards which have no debug serial port whatsoever.
+
 config DM_SERIAL
 	bool "Enable Driver Model for serial drivers"
 	depends on DM

drivers/serial/serial-uclass.c

@@ -78,7 +78,9 @@ static void serial_find_console_or_panic(void)
 #undef INDEX
 	}
 
+#ifdef CONFIG_REQUIRE_SERIAL_CONSOLE
 	panic_str("No serial driver found");
+#endif
 }
 
 /* Called prior to relocation */
@@ -140,28 +142,40 @@ static int _serial_tstc(struct udevice *dev)
 
 void serial_putc(char ch)
 {
-	_serial_putc(gd->cur_serial_dev, ch);
+	if (gd->cur_serial_dev)
+		_serial_putc(gd->cur_serial_dev, ch);
 }
 
 void serial_puts(const char *str)
 {
-	_serial_puts(gd->cur_serial_dev, str);
+	if (gd->cur_serial_dev)
+		_serial_puts(gd->cur_serial_dev, str);
 }
 
 int serial_getc(void)
 {
+	if (!gd->cur_serial_dev)
+		return 0;
+
 	return _serial_getc(gd->cur_serial_dev);
 }
 
 int serial_tstc(void)
 {
+	if (!gd->cur_serial_dev)
+		return 0;
+
 	return _serial_tstc(gd->cur_serial_dev);
 }
 
 void serial_setbrg(void)
 {
-	struct dm_serial_ops *ops = serial_get_ops(gd->cur_serial_dev);
+	struct dm_serial_ops *ops;
+
+	if (!gd->cur_serial_dev)
+		return;
 
+	ops = serial_get_ops(gd->cur_serial_dev);
 	if (ops->setbrg)
 		ops->setbrg(gd->cur_serial_dev, gd->baudrate);
 }