linux-kernel - [PATCH] serial: 8250_mtk: correct max baud rate in the set

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Message-ID: <bd8764c5-c43c-4bca-996d-65367805ad5a@omp.ru>
Date: Fri, 5 Sep 2025 23:07:12 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby
	<jirislaby@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	<linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC: <linux-arm-kernel@...ts.infradead.org>,
	<linux-mediatek@...ts.infradead.org>
Subject: [PATCH] serial: 8250_mtk: correct max baud rate in the set_termios()
 method

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@....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);