Add a hook for `uart_report_port' to let serial ports report extra 
properties beyond `irq' and `base_baud'.  Use it with the 8250 backend 
to report extra baud rates supported above the base rate for ports with 
the UPF_MAGIC_MULTIPLIER property, so that people have a way to find out 
that they are supported with their system, e.g.:

Serial: 8250/16550 driver, 5 ports, IRQ sharing enabled
printk: console [ttyS0] disabled
serial8250.0: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200 [+230400, 460800]) is a 16550A
printk: console [ttyS0] enabled
printk: console [ttyS0] enabled
printk: bootconsole [uart8250] disabled
printk: bootconsole [uart8250] disabled
serial8250.0: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200 [+230400, 460800]) is a 16550A
serial8250.0: ttyS2 at MMIO 0x1f000900 (irq = 20, base_baud = 230400) is a 16550A

Otherwise there is no clear way to figure this out.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
---
 drivers/tty/serial/8250/8250_core.c |   10 ++++++++++
 drivers/tty/serial/serial_core.c    |   11 +++++++++--
 include/linux/serial_core.h         |    3 +++
 3 files changed, 22 insertions(+), 2 deletions(-)

linux-serial-core-baud-extra.diff
Index: linux-malta-cbus-uart/drivers/tty/serial/8250/8250_core.c
===================================================================

--- linux-malta-cbus-uart.orig/drivers/tty/serial/8250/8250_core.c
+++ linux-malta-cbus-uart/drivers/tty/serial/8250/8250_core.c

@@ -952,6 +952,13 @@ static struct uart_8250_port *serial8250
 	return NULL;
 }
 
+static void serial8250_report_magic(struct uart_port *port,
+				    char *report_buf, size_t report_size)
+{
+	snprintf(report_buf, report_size,
+		 " [+%d, %d]", port->uartclk / 8, port->uartclk / 4);
+}
+
 static void serial_8250_overrun_backoff_work(struct work_struct *work)
 {
 	struct uart_8250_port *up =

@@ -1048,6 +1055,9 @@ int serial8250_register_8250_port(struct
 			}
 		}
 
+		if (up->port.flags & UPF_MAGIC_MULTIPLIER)
+			uart->port.report_extra = serial8250_report_magic;
+
 		serial8250_set_defaults(uart);
 
 		/* Possibly override default I/O functions.  */

Index: linux-malta-cbus-uart/drivers/tty/serial/serial_core.c
===================================================================

--- linux-malta-cbus-uart.orig/drivers/tty/serial/serial_core.c
+++ linux-malta-cbus-uart/drivers/tty/serial/serial_core.c

@@ -2309,6 +2309,7 @@ int uart_resume_port(struct uart_driver
 static inline void
 uart_report_port(struct uart_driver *drv, struct uart_port *port)
 {
+	char report_extra[64];
 	char address[64];
 
 	switch (port->iotype) {

@@ -2333,11 +2334,17 @@ uart_report_port(struct uart_driver *drv
 		break;
 	}
 
-	pr_info("%s%s%s at %s (irq = %d, base_baud = %d) is a %s\n",
+	if (port->report_extra)
+		port->report_extra(port, report_extra, sizeof(report_extra));
+	else
+		report_extra[0] = '\0';
+
+	pr_info("%s%s%s at %s (irq = %d, base_baud = %d%s) is a %s\n",
 	       port->dev ? dev_name(port->dev) : "",
 	       port->dev ? ": " : "",
 	       port->name,
-	       address, port->irq, port->uartclk / 16, uart_type(port));
+	       address, port->irq, port->uartclk / 16, report_extra,
+	       uart_type(port));