[PATCH] serial: 8250_mtk: correct max baud rate in the set_termios() method

[PATCH] serial: 8250_mtk: correct max baud rate in the set_termios() method

[Sergey Shtylyov]

As is obvious from the code calculating the divisor in the set_termios()
method, the Mediatek UART driver uses 256-time oversampling for the high
baud rates, so passing port->uartclk to uart_get_baud_rate() for the max
acceptable baud rate makes no sense, we should divide by 256 first (this
should also prevent overflow when some arbitrary baud rate is passed via
termios->c_ospeed)...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>

---
The patch is against the master branch of Linus Torvalds' linux.git repo.

 drivers/tty/serial/8250/8250_mtk.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux/drivers/tty/serial/8250/8250_mtk.c
===================================================================
--- linux.orig/drivers/tty/serial/8250/8250_mtk.c
+++ linux/drivers/tty/serial/8250/8250_mtk.c
@@ -358,7 +358,7 @@ mtk8250_set_termios(struct uart_port *po
 	 */
 	baud = uart_get_baud_rate(port, termios, old,
 				  port->uartclk / 16 / UART_DIV_MAX,
-				  port->uartclk);
+				  port->uartclk / 256);
 
 	if (baud < 115200) {
 		serial_port_out(port, MTK_UART_HIGHS, 0x0);

Re: [PATCH] serial: 8250_mtk: correct max baud rate in the set_termios() method

[Sergey Shtylyov]

On 9/5/25 11:07 PM, Sergey Shtylyov wrote:

> As is obvious from the code calculating the divisor in the set_termios()
> method, the Mediatek UART driver uses 256-time oversampling for the high
> baud rates, so passing port->uartclk to uart_get_baud_rate() for the max
> acceptable baud rate makes no sense, we should divide by 256 first (this

   Well, I thought again and 256 seems to be an overkill, I should probably
have reverted to division by 16 instead...

> should also prevent overflow when some arbitrary baud rate is passed via
> termios->c_ospeed)...
> 
> Found by Linux Verification Center (linuxtesting.org) with the Svace static
> analysis tool.
> 
> Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
> Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>

[...]

MBR, Sergey