Re: [PATCH] serial: sc16is7xx: fix data corruption due to unexpected xon/xoff

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Sep 23, 2026 at 02:01:17PM +0000, Martin Fuzzey wrote:
> The hardware supports automatically sending XON/OFF based on the
> FIFO status and that is already implemented by the driver.
> However that capability was not announced with the UPF_SOFT_FLOW bit.
> 
> When the port is opened with the default of XON enabled the driver
> configures that in the hardware.
> If userspace does not want XON/OFF (which is normally the case) it will
> use TCSETS to disactivate it. However the serial core optimises this
> to only notify the driver if someting imporant has changed, and for
> XON/XOFF thit is only if the  driver declares UPF_SOFT_FLOW.
> 
> So we end up with unwanted XON/XOFF in the data stream.
> 
> The problem only occurs if nothing else needed to be set by termios,
> ie if the user space requested values are the same as the default values,
> except for XON/XOFF.
> Since the default speed is 9600 this means that the problem only occurs
> when requesting 9600, otherwise the call to the driver will be made to
> change the speed, and that will also clear the XON/XOFF.
> As 9600 is quite rare these days that probably explains why this hasn't
> already been seen.
> 
> It also only occurs on the first port open after boot since the subsequent
> opens will use the "logical state" set by TCSETS but not communicated to
> the driver the first time.
> 
> Signed-off-by: Martin Fuzzey <martin.fuzzey@arrive.com>
> Cc: stable@vger.kernel.org
> ---
>  drivers/tty/serial/sc16is7xx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
> index 9b152ead050f..f0a9a3dcd0e4 100644
> --- a/drivers/tty/serial/sc16is7xx.c
> +++ b/drivers/tty/serial/sc16is7xx.c
> @@ -1485,7 +1485,7 @@ static int sc16is7xx_setup_channel(struct sc16is7xx_one *one, int i,
>  	/* Initialize port data */
>  	port->type	= PORT_SC16IS7XX;
>  	port->fifosize	= SC16IS7XX_FIFO_SIZE;
> -	port->flags	= UPF_FIXED_TYPE | UPF_LOW_LATENCY;
> +	port->flags	= UPF_FIXED_TYPE | UPF_LOW_LATENCY | UPF_SOFT_FLOW;
>  	port->iobase	= i;
>  	/*
>  	 * Use all ones as membase to make sure uart_configure_port() in
> -- 
> 2.25.1
> 

Does not apply to the tty-linus branch :(