linux-kernel - Re: [PATCH 0/5] tty: add flag to suppress ready signalling on open

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Date:   Wed, 2 Dec 2020 12:48:17 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Mychaela Falconia <mychaela.falconia@...il.com>,
        Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.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 0/5] tty: add flag to suppress ready signalling on open

On Tue, Dec 01, 2020 at 08:14:07AM +0100, Jiri Slaby wrote:
> On 30. 11. 20, 22:22, Mychaela Falconia wrote:
> > 2) For situations in which the luxury of a custom USB ID is not
> > available, e.g., a situation where the device that does not tolerate
> > automatic DTR/RTS assertion on open is a physical RS-232 device that
> > can be connected to "any" serial port, the new sysfs attribute comes
> > to the rescue.
> > 
> > Johan's patch comments say that the new flag can also be brought out
> > to termios in the future, similarly to HUPCL,
> 
> The difference to other control flags is that open raises DTR/RTS in any 
> case (i.e. including O_NONBLOCK) -- provided baud rate is set (and it is 
> for casual serials). That means you cannot open a port to configure it 
> (using e.g. setserial) without actually raising the DTR/RTS.

Right, but depending on the application this may be ok (e.g. reset and
initialise on first open after boot, which may have triggered a reset
anyway).

If control over first open is needed, the sysfs interface provides that
out-of-band.

> > but I question the
> > usefulness of doing so, as it is a chicken and egg problem: one needs
> > to open the tty device in order to do termios ioctls on it, and if
> > that initial open triggers DTR/RTS hardware actions, then the end user
> > is still screwed.  If Johan or someone else can see a potential use
> > case for manipulating this new flag via termios (as opposed to sysfs
> > or USB-ID-based driver quirks), perhaps you could elaborate on it?
> 
> We would need to (ab)use another open flag (e.g. O_DIRECT). I am not 
> biased to either of solutions.

Forgot to mention that using open-flags would prevent using standard
utilities like cat, echo and terminal programs. So for that reason a
termios and/or sysfs interface is also preferred.

Johan