Linux Keyspan USB serial driver ignoring XOFF

[John Goerzen <jgoerzen@complete.org>]

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.

I twiddled many, many settings with stty but could not find anything
that would make it honor xoff/Ctrl-S with the Keyspan.  Even as a
regular user that would use that trick to pause scrolling, it had no
effect.  Different TERM settings also had no effect.

Output of stty -a:

speed 19200 baud; rows 24; columns 80; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon ixoff
-iuclc -ixany -imaxbel -iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon -iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke -flusho -extproc

Thanks,

John