[PATCH] serial: 8250: add CONFIG_SERIAL_8250_PROBE_BAUD option

[Sheng Yu <yushenglive@gmail.com>]

Currently, the 8250 driver defaults to 9600 baud if no console options
are provided via the command line. This can result in garbled output if
the firmware or bootloader has already initialized the UART to a
different speed.

Introduce CONFIG_SERIAL_8250_PROBE_BAUD. When enabled, the driver will
attempt to read the current baud rate from the hardware registers if
no options are specified, rather than forcing the 9600 default.

Signed-off-by: Sheng Yu <yushenglive@gmail.com>
---
 drivers/tty/serial/8250/8250_core.c |  2 +-
 drivers/tty/serial/8250/8250_port.c |  5 ++++-
 drivers/tty/serial/8250/Kconfig     | 12 ++++++++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 0e81f78c6063..c11b19921a1f 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -427,7 +427,7 @@ static int univ8250_console_setup(struct console *co, char *options)
 	/* link port to console */
 	uart_port_set_cons(port, co);
 
-	retval = serial8250_console_setup(port, options, false);
+	retval = serial8250_console_setup(port, options, IS_ENABLED(CONFIG_SERIAL_8250_PROBE_BAUD));
 	if (retval != 0)
 		uart_port_set_cons(port, NULL);
 	return retval;
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 719faf92aa8a..5309b921e7b7 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3399,8 +3399,11 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
-	else if (probe)
+	else if (probe) {
 		baud = probe_baud(port);
+		pr_info("console [%s%d] probed baud rate: %d\n",
+			port->cons->name, port->cons->index, baud);
+	}
 
 	ret = uart_set_options(port, port->cons, baud, parity, bits, flow);
 	if (ret)
diff --git a/drivers/tty/serial/8250/Kconfig b/drivers/tty/serial/8250/Kconfig
index c488ff6f2865..bee6a82023d4 100644
--- a/drivers/tty/serial/8250/Kconfig
+++ b/drivers/tty/serial/8250/Kconfig
@@ -596,3 +596,15 @@ config SERIAL_OF_PLATFORM
 	  are probed through devicetree, including Open Firmware based
 	  PowerPC systems and embedded systems on architectures using the
 	  flattened device tree format.
+
+config SERIAL_8250_PROBE_BAUD
+	bool "Probe baud rate if console options are missing"
+	depends on SERIAL_8250
+	help
+	  If the "console=" command line parameter is missing options (e.g.,
+	  "console=ttyS0" instead of "console=ttyS0,115200n8"), this option
+	  allows the kernel to probe the baud rate from hardware instead of
+	  defaulting to 9600.
+
+	  If a baud rate is explicitly provided in the options, that value
+	  is always respected.
-- 
2.51.0