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

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Message-ID: <X9OZklRPquV8wE+E@kroah.com>
Date:   Fri, 11 Dec 2020 17:08:50 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Mychaela Falconia <mychaela.falconia@...il.com>,
        Maarten Brock <m.brock@...mierlo.com>,
        Johan Hovold <johan@...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 Fri, Dec 11, 2020 at 09:46:54AM +0100, Jiri Slaby wrote:
> On 10. 12. 20, 19:59, Mychaela Falconia wrote:
> > > O_DIRECT is an interesting hack, has anyone seen if it violates the
> > > posix rules for us to use it on a character device like this?
> > 
> > According to open(2) Linux man page, O_DIRECT does not come from POSIX
> > at all, instead it is specific to Linux, FreeBSD and SGI IRIX.  Thus
> > it seems like there aren't any POSIX rules to be violated here.
> > 
> > If we go with O_DIRECT, what semantics are we going to implement?
> > There are 3 possibilities that come to mind most readily:
> > 
> > 1) O_DIRECT applies only to the open call in which this flag is set,
> > and suppresses DTR/RTS assertion on that open.  If someone needs to do
> > multiple opens with DTR/RTS suppression being required every time,
> > then they need to include O_DIRECT every time.
> > 
> > 2) O_DIRECT applies not only immediately, but also sets a latched flag
> > whereby all subsequent opens continue to suppress auto-assertion
> > without requiring O_DIRECT every time.  This approach by itself runs
> > counter to the generic Unix way of doing things, but it may be OK if
> > there is also some ioctl to explicitly set or clear the latched flag.
> > 
> > 3) O_DIRECT applies only to the open call in which it is set, no
> > built-in latching, but there is also some ioctl to control a flag
> > enabling or disabling DTR/RTS auto-assertion on subsequent opens.
> 
> 3) -- to allow standard tools to work on the device after the quirk is set
> up once.

I'm lost, what do you mean here?

thanks,

greg k-h