linux-kernel - Re: [PATCH 1/2] tty/hvc_console: Add DTR/RTS callback to handle HUPCL control

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Date:	Fri, 11 Oct 2013 14:47:07 +0200
From:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>, jslaby@...e.cz,
	gregkh@...uxfoundation.org, brueckner@...ux.vnet.com,
	heiko.carstens@...ibm.com, schwidefsky@...ibm.com,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH 1/2] tty/hvc_console: Add DTR/RTS callback to handle
 HUPCL control

Hi Benjamin,

On Fri, Oct 11, 2013 at 06:15:11PM +1100, Benjamin Herrenschmidt wrote:
> On Tue, 2013-07-02 at 17:07 +0200, Hendrik Brueckner wrote:
> > Introduce a new callback to explicitly handle the HUPCL termios control flag.
> > This prepares for a follow-up commit for the hvc_iucv device driver to
> > improve handling when to drop an established network connection.
> > 
> > The callback naming is based on the recently added tty_port interface to
> > facilitate a potential refactoring of the hvc_console to use tty_port
> > functions.
> 
> I only just noticed that ... oops. Why add those dtr_rts() calls ? We
> already have tiocmset in there which is used to set DTR on HVSI consoles
> such as hvc_opal when using hvsi_lib...
> 
> Any reason why a separate callback was needed ?

The tiocmget/tiocmset callbacks are used to set and get modem status and
triggered through an tty ioctl.

The dtr_rts() callback is different and it is used for DTS/RTS handshaking
between the hvc_console (or any other tty_port) and the tty layer.  The tty
port layer uses this callback to signal the hvc_console whether to raise or
lower the DTR/RTS lines.  This is different than the ioctl interface to
controls the modem status.

Thanks and kind regards,
  Hendrik

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