serial: core: Report magic multiplier extra baud rates

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 can be used 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) is a 16550A
serial8250.0: ttyS0 extra baud rates supported: 230400, 460800
printk: console [ttyS0] enabled
printk: bootconsole [uart8250] disabled
serial8250.0: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
serial8250.0: ttyS1 extra baud rates supported: 230400, 460800
serial8250.0: ttyS2 at MMIO 0x1f000900 (irq = 20, base_baud = 230400) is a 16550A

Otherwise there is no clear way to figure this out, as the feature is
only reported as an obscure TTY flag in bit 16:

$ cat /sys/class/tty/ttyS[0-2]/flags
0x10010040
0x10010040
0x90000040
$
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2106260334170.37803@angie.orcam.me.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -2314,6 +2314,14 @@ uart_report_port(struct uart_driver *drv, struct uart_port *port)
 	       port->dev ? ": " : "",
 	       port->name,
 	       address, port->irq, port->uartclk / 16, uart_type(port));
+
+	/* The magic multiplier feature is a bit obscure, so report it too.  */
+	if (port->flags & UPF_MAGIC_MULTIPLIER)
+		pr_info("%s%s%s extra baud rates supported: %d, %d",
+			port->dev ? dev_name(port->dev) : "",
+			port->dev ? ": " : "",
+			port->name,
+			port->uartclk / 8, port->uartclk / 4);
 }
 
 static void