usbserial: pl2303 tx xon/xoff flow control

[Johan Hovold <johan@kernel.org>]

On Thu, May 17, 2018 at 10:29:14AM +0200, Johan Hovold wrote:
> On Thu, May 17, 2018 at 05:39:21AM +0200, Florian Zumbiehl wrote:

> > Now, I agree that signaling lack of support would be better in general, but
> > it does change what the code does for (still) unsupported cases. After all,
> > usbserial in general does not signal that it doesn't support IXON/IXANY,
> > but just pretends that it does. So, if there is a good reason why usbserial
> > ignores POSIX on this point, I would think that would still apply for the
> > remaining unsupported cases after partial support is implemented?!
> 
> There are historical reasons for a lot of things, but that's not
> necessarily a reason to continue taking shortcuts.

But on second thought, I think your approach here makes sense. If
usbserial ever gains generic IXON support, we'd just fall back to the
line-discipline implementation whenever the control characters do no
match.

Just clean up some of the lengthy conditionals (e.g. using intermediate
bools), and mention why you implemented things the way you did in the
commit message.

Thanks,
Johan
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