[PATCH v2] serial: 8250_mtk: correct max baud rate in set_termios() method

[Sergey Shtylyov <s.shtylyov@omp.ru>]

Mediatek MT798x datasheets (that I was able to get my hands on) claim
the maximum supported baud rate to be 3 Mbps, while commit 81bb549fdf14
("serial: 8250_mtk: support big baud rate.") claimed it to be 4 Mbps --
however, it then passed undivided port->uartclk to uart_get_baud_rate()
for the maximum baud rate, while the datasheets do mention up to 52 MHz
as the baud clock's frequency.  This means that an integer overflow will
happen (when multiplying the baud variable by 256) if a baud rate higher
than 16777215 bps is passed via termios->c_ospeed. Pass the correct max
baud rate of 3 Mbps or port->uartclk, whichever happens to be less...

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
(I'm unable to use the other repos on git.kernel.org and I have to update
Linus' repo from GitHub).

Changes in version 2:
- changed the approach to the problem (and hence rewrote the description);
- removed "the" article from the subject for brevity.

 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);
+				  min(3000000U, port->uartclk));
 
 	if (baud < 115200) {
 		serial_port_out(port, MTK_UART_HIGHS, 0x0);