This is a note to let you know that I've just added the patch titled

serial: 8250_mtk: Fix high-speed baud rates clamping

to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 551e553f0d4ab623e2a6f424ab5834f9c7b5229c Mon Sep 17 00:00:00 2001

From: Serge Semin Sergey.Semin@baikalelectronics.ru Date: Tue, 14 Jul 2020 15:41:12 +0300 Subject: serial: 8250_mtk: Fix high-speed baud rates clamping

Commit 7b668c064ec3 ("serial: 8250: Fix max baud limit in generic 8250 port") fixed limits of a baud rate setting for a generic 8250 port. In other words since that commit the baud rate has been permitted to be within [uartclk / 16 / UART_DIV_MAX; uartclk / 16], which is absolutely normal for a standard 8250 UART port. But there are custom 8250 ports, which provide extended baud rate limits. In particular the Mediatek 8250 port can work with baud rates up to "uartclk" speed.

Normally that and any other peculiarity is supposed to be handled in a custom set_termios() callback implemented in the vendor-specific 8250-port glue-driver. Currently that is how it's done for the most of the vendor-specific 8250 ports, but for some reason for Mediatek a solution has been spread out to both the glue-driver and to the generic 8250-port code. Due to that a bug has been introduced, which permitted the extended baud rate limit for all even for standard 8250-ports. The bug has been fixed by the commit 7b668c064ec3 ("serial: 8250: Fix max baud limit in generic 8250 port") by narrowing the baud rates limit back down to the normal bounds. Unfortunately by doing so we also broke the Mediatek-specific extended bauds feature.

A fix of the problem described above is twofold. First since we can't get back the extended baud rate limits feature to the generic set_termios() function and that method supports only a standard baud rates range, the requested baud rate must be locally stored before calling it and then restored back to the new termios structure after the generic set_termios() finished its magic business. By doing so we still use the serial8250_do_set_termios() method to set the LCR/MCR/FCR/etc. registers, while the extended baud rate setting procedure will be performed later in the custom Mediatek-specific set_termios() callback. Second since a true baud rate is now fully calculated in the custom set_termios() method we need to locally update the port timeout by calling the uart_update_timeout() function. After the fixes described above are implemented in the 8250_mtk.c driver, the Mediatek 8250-port should get back to normally working with extended baud rates.

Link: https://lore.kernel.org/linux-serial/20200701211337.3027448-1-danielwinkler@...

Fixes: 7b668c064ec3 ("serial: 8250: Fix max baud limit in generic 8250 port") Reported-by: Daniel Winkler danielwinkler@google.com Signed-off-by: Serge Semin Sergey.Semin@baikalelectronics.ru Cc: stable stable@vger.kernel.org Tested-by: Claire Chang tientzu@chromium.org Link: https://lore.kernel.org/r/20200714124113.20918-1-Sergey.Semin@baikalelectron... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/8250/8250_mtk.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c index f839380c2f4c..98b8a3e30733 100644 --- a/drivers/tty/serial/8250/8250_mtk.c +++ b/drivers/tty/serial/8250/8250_mtk.c @@ -306,8 +306,21 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios, } #endif

+ /* + * Store the requested baud rate before calling the generic 8250 + * set_termios method. Standard 8250 port expects bauds to be + * no higher than (uartclk / 16) so the baud will be clamped if it + * gets out of that bound. Mediatek 8250 port supports speed + * higher than that, therefore we'll get original baud rate back + * after calling the generic set_termios method and recalculate + * the speed later in this method. + */ + baud = tty_termios_baud_rate(termios); + serial8250_do_set_termios(port, termios, old);

+ tty_termios_encode_baud_rate(termios, baud, baud); + /* * Mediatek UARTs use an extra highspeed register (MTK_UART_HIGHS) * @@ -339,6 +352,11 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios, */ spin_lock_irqsave(&port->lock, flags);

+ /* + * Update the per-port timeout. + */ + uart_update_timeout(port, termios->c_cflag, baud); + /* set DLAB we have cval saved in up->lcr from the call to the core */ serial_port_out(port, UART_LCR, up->lcr | UART_LCR_DLAB); serial_dl_write(up, quot);