This is a note to let you know that I've just added the patch titled serial: 8250: Actually allow UPF_MAGIC_MULTIPLIER baud rates to the 5.13-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: serial-8250-actually-allow-upf_magic_multiplier-baud.patch and it can be found in the queue-5.13 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 4fd479710993ffdaf609f70b7d11919e473b3703 Author: Maciej W. Rozycki <macro@xxxxxxxxxxx> Date: Thu Jun 10 20:38:34 2021 +0200 serial: 8250: Actually allow UPF_MAGIC_MULTIPLIER baud rates [ Upstream commit 78bcae8616ac277d6cb7f38e211493948ed73e30 ] Support for magic baud rate divisors of 32770 and 32769 used with SMSC Super I/O chips for extra baud rates of 230400 and 460800 respectively where base rate is 115200[1] has been added around Linux 2.5.64, which predates our repo history, but the origin could be identified as commit 2a717aad772f ("Merge with Linux 2.5.64.") with the old MIPS/Linux repo also at: <git://git.kernel.org/pub/scm/linux/kernel/git/ralf/linux.git>. Code that is now in `serial8250_do_get_divisor' was added back then to `serial8250_get_divisor', but that code would only ever trigger if one of the higher baud rates was actually requested, and that cannot ever happen, because the earlier call to `serial8250_get_baud_rate' never returns them. This is because it calls `uart_get_baud_rate' with the maximum requested being the base rate, that is clk/16 or 115200 for SMSC chips at their nominal clock rate. Fix it then and allow UPF_MAGIC_MULTIPLIER baud rates to be selected, by requesting the maximum baud rate of clk/4 rather than clk/16 if the flag has been set. Also correct the minimum baud rate, observing that these ports only support actual (non-magic) divisors of up to 32767 only. References: [1] "FDC37M81x, PC98/99 Compliant Enhanced Super I/O Controller with Keyboard/Mouse Wake-Up", Standard Microsystems Corporation, Rev. 03/27/2000, Table 31 - "Baud Rates", p. 77 Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx> Link: https://lore.kernel.org/r/alpine.DEB.2.21.2105190412280.29169@xxxxxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index fc5ab2032282..ff3f13693def 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2629,6 +2629,21 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, struct ktermios *old) { unsigned int tolerance = port->uartclk / 100; + unsigned int min; + unsigned int max; + + /* + * Handle magic divisors for baud rates above baud_base on SMSC + * Super I/O chips. Enable custom rates of clk/4 and clk/8, but + * disable divisor values beyond 32767, which are unavailable. + */ + if (port->flags & UPF_MAGIC_MULTIPLIER) { + min = port->uartclk / 16 / UART_DIV_MAX >> 1; + max = (port->uartclk + tolerance) / 4; + } else { + min = port->uartclk / 16 / UART_DIV_MAX; + max = (port->uartclk + tolerance) / 16; + } /* * Ask the core to calculate the divisor for us. @@ -2636,9 +2651,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port, * slower than nominal still match standard baud rates without * causing transmission errors. */ - return uart_get_baud_rate(port, termios, old, - port->uartclk / 16 / UART_DIV_MAX, - (port->uartclk + tolerance) / 16); + return uart_get_baud_rate(port, termios, old, min, max); } /*