usbserial: pl2303 tx xon/xoff flow control

[Johan Hovold <johan@kernel.org>]

On Sun, May 20, 2018 at 02:22:27AM +0200, Florian Zumbiehl wrote:

> Before I investigated how to implement this patch I just saw that s/w flow
> control "didn't work", but my assumption was that that was due to buffering
> latencies, not because the kernel just ignored the request. But then,
> chances are a software implementation indeed wouldn't work very well anyway
> for exactly that reason?!

Indeed, the deep queues might prevent a software implementation from
being very useful.

> > The line discipline implementation kicks in whenever IXON is set, and
> > can be used as a fallback for devices where automatic hardware and
> > software flow control cannot be enabled concurrently in hardware for
> > example.
> 
> Well, yeah, my guess would be that to actually make it work (well), one
> would need more than that? Like, implement rate control in software to keep
> the hardware buffers empty in order to achieve short reaction times? Which
> one would probably want to disable though when it's not needed in order to
> take advantage of the buffers?

Maybe, but chances are none of this is worth the added complexity. If
you need XON/XOFF you should get a device which supports it in hardware.

> > While non-hardware assisted usb serial XON/XOFF is currently broken in
> > that transmission would not be halted, the line discipline would still
> > swallow any escape characters.
> 
> Actually, that is required even for the pl2303, as it does not swallow the
> control characters itself.

Yeah, I noticed that too.

Thanks,
Johan
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