[xen stable-4.20] xen/uart: set a default baudrate if non specified neither found

[patchbot@xxxxxxx]

commit 92d668bf89c109b0fca60c018ef01d260e01f11a
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Apr 20 12:37:12 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 20 12:37:12 2026 +0200

    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@xxxxxxxxxx>
    Reviewed-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
    master commit: 12e50da43d926cc5401c6ec1146cdcedaba23243
    master date: 2026-04-10 14:30:50 +0200
---
 xen/drivers/char/ns16550.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index b443714c10..6089558aae 100644
--- a/xen/drivers/char/ns16550.c
+++ b/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);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.20