linux-kernel - Re: [PATCH v2] tty/serial: Add a serial port simulator

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Date:   Fri, 29 Mar 2019 18:24:01 -0000 (UTC)
From:   Grant Edwards <grant.b.edwards@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-serial@...r.kernel.org
Subject: Re: [PATCH v2] tty/serial: Add a serial port simulator

On 2019-03-29, Corey Minyard <minyard@....org> wrote:
> On Thu, Mar 28, 2019 at 12:39:12PM -0700, H. Peter Anvin wrote:
>
>> Dumb question: this is basically a pty on steroids. Wouldn't this be
>> better done by enhancing the pty devices?

I proposed doing that several years ago, and offered to start working
on it if there was a decent chance it would be accepted into the tree.
I got no response.

> I did look at that, but it would be pretty invasive to pty.  There's
> no modem control stuff, none of the other special serial ioctls.
> And the locking in this driver is fairly strange because you have
> two serial ports looking at each other's data for modem control.
> But that might not be a big deal.
>
> Adding the speed simulation to ptys would also be really strange.
> That's not a deal-breaker, I suppose, but it's not much of a serial
> port simulation without it.

My goal wasn't really to simulate two serial ports with a null-mode
cable in-between, so the speed simluaiton wasn't on my list. my goal
was to provide a way to implement a serial port in userspace by
attaching an application to the master end of a pty.

The pty(7) man page states "The slave end of the pseudoterminal
provides an interface that behaves exactly like a classical terminal."
But, we all know that's a pretty big lie: the pty slave end implements
only a small subset of a "classical termial" device, and there are all
sorts of applications that expect to talk to a serial port which fail
miserably when connected to a pty.

-- 
Grant Edwards               grant.b.edwards        Yow! I had a lease on an
                                  at               OEDIPUS COMPLEX back in
                              gmail.com            '81 ...