linux-kernel - Re: RFC: Serial port DTR/RTS

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Message-ID: <2025111241-domestic-moonstone-f75f@gregkh>
Date: Wed, 12 Nov 2025 11:46:50 -0500
From: Greg KH <gregkh@...uxfoundation.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Theodore Ts'o <tytso@....edu>, Maarten Brock <Maarten.Brock@...ls.nl>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: RFC: Serial port DTR/RTS - O_NRESETDEV

On Wed, Nov 12, 2025 at 08:09:45AM -0800, H. Peter Anvin wrote:
> On November 12, 2025 3:22:56 AM PST, Greg KH <gregkh@...uxfoundation.org> wrote:
> >On Mon, Nov 10, 2025 at 07:57:22PM -0800, H. Peter Anvin wrote:
> >> Honestly, though, I'm far less interested in what 8250-based hardware does than e.g. USB.
> >
> >hahahahahahaha {snort}
> >
> >Hah.  that's a good one.
> >
> >Oh, you aren't kidding.
> >
> >Wow, good luck with this.  USB-serial adaptors are all over the place,
> >some have real uarts in them (and so do buffering in the device, and
> >line handling in odd ways when powered up), and some are almost just a
> >straight pipe through to the USB host with control line handling ideas
> >tacked on to the side as an afterthought, if at all.
> >
> >There is no standard here, they all work differently, and even work
> >differently across the same device type with just barely enough hints
> >for us to determine what is going on.
> >
> >So don't worry about USB, if you throw that into the mix, all bets are
> >off and you should NEVER rely on that.
> >
> >Remeber USB->serial was explicitly rejected by the USB standard group,
> >only to have it come back in the "side door" through the spec process
> >when it turned out that Microsoft hated having to write a zillion
> >different vendor-specific drivers because the vendor provided ones kept
> >crashing user's machines.  So what we ended up with was "just enough" to
> >make it through the spec process, and even then line signals are
> >probably never tested so you can't rely on them.
> >
> >good luck!
> >
> >greg "this brought up too many bad memories" k-h
> 
> Ugh.
> 
> I have made it very clear that I am very aware that there is broken hardware. 

I would posit that there is NO "non-broken" usb->serial devices out
there.  The closest I have seen was the old IO-Edgeport devices, but
they were expensive and got bought out by some other company and in the
end didn't succeed due to all of the "cheap" devices/chips out there
that just did dumb tx/rx transfers over a fake serial connection.

> What I'm trying to do is to deal with the (occasional) case of
> *non*-broken hardware. Right now Linux breaks the non-broken hardware
> for it, and I don't think the existence of broken hardware is a good
> justification for that.

No, but we have to handle both somehow.

And given that we still get brand-new UART drivers sent to use every few
months, there is just more and more "broken" hardware out there overall.

Anyway, good luck coming up with a scheme to handle your crazy
connections, I would push back and say "any device that treats a serial
control line as a power signal is broken to start with" :)

greg k-h