[PATCH 6.6.y] serial: sc16is7xx: fix data corruption due to unexpected xon/xoff

[Sasha Levin <sashal@kernel.org>]

From: Martin Fuzzey <martin.fuzzey@flowbird.group>

[ Upstream commit 8bb50e95c0abd44607bc60a43032c1dbf7470d5a ]

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@flowbird.group>
Fixes: dfeae619d781 ("serial: sc16is7xx")
Cc: stable@kernel.org
Link: https://patch.msgid.link/20261001100534.1225894-1-martin.fuzzey@flowbird.group
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@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 b208181de2455..7eaf9af61542e 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -1584,7 +1584,7 @@ static int sc16is7xx_probe(struct device *dev,
 		s->p[i].port.irq	= irq;
 		s->p[i].port.type	= PORT_SC16IS7XX;
 		s->p[i].port.fifosize	= SC16IS7XX_FIFO_SIZE;
-		s->p[i].port.flags	= UPF_FIXED_TYPE | UPF_LOW_LATENCY;
+		s->p[i].port.flags	= UPF_FIXED_TYPE | UPF_LOW_LATENCY | UPF_SOFT_FLOW;
 		s->p[i].port.iobase	= i;
 		/*
 		 * Use all ones as membase to make sure uart_configure_port() in
-- 
2.53.0