serial: 8250_early: Only set divisor if valid clk & baud

commit 0ff3ab701963a845d52337ded7eebf2d1a14fe00 upstream.

If either uartclk or baud are 0, avoid calculating and setting a divisor
based on them since the output will almost certainly be garbage.

This also allows platforms such as the MIPS generic kernel, which has no
way to know a valid BASE_BASE for the board it is actually booted on at
compile time, to set BASE_BAUD to 0 and avoid early_8250 setting a bad
divisor.

This fixes a regression caused by commit 31cb9a8575ca ("earlycon:
initialise baud field of earlycon device structure"), which changed the
behavior of of_setup_earlycon such that it sets a baud rate in the
earlycon structure where previously it was left as 0. All boards
supported by the MIPS generic kernel started outputting garbage from the
boot console due to an incorrect divisor being set.

Fixes: 31cb9a8575ca ("earlycon: initialise baud field of earlycon device structure")
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
index af72ec32e404736f965cbc3f39c6adce8b42a1df..f135c1846477e46b301ce74c59a88bcef061ff8f 100644 (file)
--- a/drivers/tty/serial/8250/8250_early.c
+++ b/drivers/tty/serial/8250/8250_early.c
@@ -125,12 +125,14 @@ static void __init init_port(struct earlycon_device *device)
        serial8250_early_out(port, UART_FCR, 0);        /* no fifo */
        serial8250_early_out(port, UART_MCR, 0x3);      /* DTR + RTS */
 
-       divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * device->baud);
-       c = serial8250_early_in(port, UART_LCR);
-       serial8250_early_out(port, UART_LCR, c | UART_LCR_DLAB);
-       serial8250_early_out(port, UART_DLL, divisor & 0xff);
-       serial8250_early_out(port, UART_DLM, (divisor >> 8) & 0xff);
-       serial8250_early_out(port, UART_LCR, c & ~UART_LCR_DLAB);
+       if (port->uartclk && device->baud) {
+               divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * device->baud);
+               c = serial8250_early_in(port, UART_LCR);
+               serial8250_early_out(port, UART_LCR, c | UART_LCR_DLAB);
+               serial8250_early_out(port, UART_DLL, divisor & 0xff);
+               serial8250_early_out(port, UART_DLM, (divisor >> 8) & 0xff);
+               serial8250_early_out(port, UART_LCR, c & ~UART_LCR_DLAB);
+       }
 }
 
 int __init early_serial8250_setup(struct earlycon_device *device,