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

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

[Sergey Shtylyov]

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);

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

[Fedor Pchelkin]

On Sun, 12. Oct 20:56, Sergey Shtylyov wrote:
> 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 --

At least MT7987A datasheet claims to support up to 4 Mbps, so I think 4 Mbps
should be chosen for the upper limit.

+ added the authors of 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
to Cc.

--
Fedor

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

[Sergey Shtylyov]

On 10/14/25 1:07 PM, Fedor Pchelkin wrote:

>> Mediatek MT798x datasheets (that I was able to get my hands on) claim

   Thank you, Fedor, BTW! :-)

>> 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 --
> 
> At least MT7987A datasheet claims to support up to 4 Mbps, so I think 4 Mbps
> should be chosen for the upper limit.

   FTR, this SoC doesn't seem to be supported by the mainline kernel (looking
at Linus' repo, at least).
   I think, we should "parametrize" the max (and maybe even min?) baud rate
adding more specific compatibles to mtk8250_of_match[] (and probably setting
this parameter to 4 Mbps for the existing entry)...

> + added the authors of 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
> to Cc.

  Just noticed that the author of this "wonderful" patch didn't even sign off
on it. "Excellent" work! :-/

> --
> Fedor

MBR, Sergey