Re: Linux Keyspan USB serial driver ignoring XOFF

[Johan Hovold <johan@kernel.org>]

On Fri, Sep 20, 2019 at 02:49:51PM -0500, John Goerzen wrote:
> Hello,
> 
> I have narrowed down the issue I'm about to describe to keyspan.c; a
> Digi Edgeport/1 with identical configuration works fine.
> 
> I am configuring a Raspberry Pi running 4.19.66 (though keyspan.c hasn't
> changed since 2017) to talk to a real-live vt420.  Configuring agetty
> with systemd worked easy enough, but I found that XON/XOFF wasn't
> working.  stty -a shows ixon and ixoff as appropriate, but sending
> Ctrl-S (tested from multiple ways of sending) had no effect on output in
> bash, or scrolling output.  (Emacs, though, recognized it as the start
> of a search, so I knew it was getting down the line.)
> 
> 
> After a great deal of head-scratching on this, I went to look at the
> kernel source and found that keyspan.c does not appear to be honoring
> XOFF.  I also have a Digi Edgeport/1 on hand (which uses io_ti.c), and
> when I swapped to that, everything worked fine - Ctrl-S caused the
> expected pause.
> 
> As far as I can tell, keyspan.c simply never implemented handling of
> XOFF, but you guys are the experts there.

That's correct, the driver does not support software flow control even
if the hardware seems to have some support for assisted XON/XOFF.

Would you mind testing the below patch which may be enough to turn
sw-flow control always on. If that works, I can probably find time to
cook up a proper patch to make this configurable later this week.

Johan


From 55b46d78fe63f182923e4674659fa18f4624d6b8 Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@kernel.org>
Date: Mon, 23 Sep 2019 12:14:56 +0200
Subject: [PATCH] USB: serial: keyspan: enable XON flow control

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/serial/keyspan.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/serial/keyspan.c b/drivers/usb/serial/keyspan.c
index d34779fe4a8d..934430cbdfc4 100644
--- a/drivers/usb/serial/keyspan.c
+++ b/drivers/usb/serial/keyspan.c
@@ -2118,7 +2118,7 @@ static int keyspan_usa26_send_setup(struct usb_serial *serial,
 	msg.setLcr = 0xff;
 
 	msg.ctsFlowControl = (p_priv->flow_control == flow_cts);
-	msg.xonFlowControl = 0;
+	msg.xonFlowControl = 1;
 	msg.setFlowControl = 0xff;
 	msg.forwardingLength = 16;
 	msg.xonChar = 17;
@@ -2234,7 +2234,7 @@ static int keyspan_usa28_send_setup(struct usb_serial *serial,
 	msg.parity = 0;		/* XXX for now */
 
 	msg.ctsFlowControl = (p_priv->flow_control == flow_cts);
-	msg.xonFlowControl = 0;
+	msg.xonFlowControl = 1;
 
 	/* Do handshaking outputs, DTR is inverted relative to RTS */
 	msg.rts = p_priv->rts_state;
@@ -2388,7 +2388,7 @@ static int keyspan_usa49_send_setup(struct usb_serial *serial,
 	msg.setLcr = 0xff;
 
 	msg.ctsFlowControl = (p_priv->flow_control == flow_cts);
-	msg.xonFlowControl = 0;
+	msg.xonFlowControl = 1;
 	msg.setFlowControl = 0xff;
 
 	msg.forwardingLength = 16;
@@ -2690,7 +2690,7 @@ static int keyspan_usa67_send_setup(struct usb_serial *serial,
 	msg.setLcr = 0xff;
 
 	msg.ctsFlowControl = (p_priv->flow_control == flow_cts);
-	msg.xonFlowControl = 0;
+	msg.xonFlowControl = 1;
 	msg.setFlowControl = 0xff;
 	msg.forwardingLength = 16;
 	msg.xonChar = 17;
-- 
2.23.0