linux-kernel - Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open

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Message-ID: <X9NM6i5mRTUTFKr9@localhost>
Date:   Fri, 11 Dec 2020 11:41:46 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Maarten Brock <m.brock@...mierlo.com>
Cc:     Mychaela Falconia <mychaela.falconia@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Mychaela N . Falconia" <falcon@...ecalypso.org>,
        linux-serial@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/7] tty: add flag to suppress ready signalling on open

On Thu, Dec 10, 2020 at 11:41:24AM +0100, Maarten Brock wrote:
> Hello Mychaela,
> 
> On 2020-12-09 23:49, Mychaela Falconia wrote:

> > But the problem is that the current status quo is fundamentally broken
> > for those hardware devices in which DTR and/or RTS have been repurposed
> > for something other than modem and flow control.  Right now whenever a
> > "cold" (never previously opened) serial port is opened for the first
> > time, that open action immediately and unstoppably asserts both DTR
> > and RTS hardware outputs, without giving userspace any opportunity to
> > say "no, please don't do it".  Yes, this behaviour is codified in a
> > bunch of standards that ultimately trace back to 1970s Original UNIX,
> > but just because it is a standard does not make it right - this
> > Unix/POSIX/Linux "standard" serial port behaviour is a bug, not a
> > feature.
> 
> I agree. And an application not configuring the required handshakes, but
> still relying on them is an equal bug.

Ehh, don't be silly. Relying on having Linux assert DTR/RTS on open is
in no way a bug; it's the standard behaviour that we must continue to
support unless explicitly overridden. Period.

> > But if there exist some custom hw devices out there that are in the
> > same predicament as my DUART28 adapter, but are different in that they
> > are classic old-fashioned RS-232 rather than integrated USB-serial,
> > with no place to assign a custom USB ID, *then* we need a non-USB-ID-
> > dependent solution such as Johan's sysfs attribute or O_DIRECT.
> 
> Any device with a classic old-fashioned RS-232 has probably already
> solved this in another way or is accepted as not working on Linux.
> 
> And then there is also the device tree (overlay?) through which a quirk
> like this can be communicated to the kernel driver. Not sure if this
> could help for a plug-and-play device like on USB.

Not every system use devicetree and no, it doesn't help with
hotpluggable buses either.

Johan