xen/uart: set a default baudrate if non specified neither found

If the user has set baudrate as "auto" (use whatever was configured) but
Xen cannot find any set baudrate use a default fallback baudrate of
115200.

Otherwise the console will very likely end up being non-functional, as
further values are derived from the baudrate (like the transmit timeout),
and using -1 will result in screwed parameters being derived from the
baudrate.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
master commit: 12e50da43d
master date: 2026-04-10 14:30:50 +0200

xen/drivers/char/ns16550.c

@@ -345,9 +345,16 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
         if ( divisor )
             uart->baud = uart->clock_hz / (divisor << 4);
         else
+        {
+            uart->baud = 115200;
             printk(XENLOG_ERR
                    "Automatic baud rate determination was requested,"
-                   " but a baud rate was not set up\n");
+                   " but a baud rate was not set up\n"
+                   "Setting baudrate to %u\n", uart->baud);
+            divisor = uart->clock_hz / (uart->baud << 4);
+            ns_write_reg(uart, UART_DLL, (uint8_t)divisor);
+            ns_write_reg(uart, UART_DLM, (uint8_t)(divisor >> 8));
+        }
     }
     ns_write_reg(uart, UART_LCR, lcr);