linux-kernel - Re: [Gta04-owner] [PATCH 0/3] tty slave device support

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Message-ID: <554B7D33.602@hurleysoftware.com>
Date:	Thu, 07 May 2015 10:56:51 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>,
	Mark Rutland <mark.rutland@....com>
CC:	Pavel Machek <pavel@....cz>,
	List for communicating with real GTA04 owners 
	<gta04-owner@...delico.com>, NeilBrown <neil@...wn.name>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Reichel <sre@...nel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Gta04-owner] [PATCH 0/3] tty slave device support - version
 3.

On 05/07/2015 08:46 AM, Dr. H. Nikolaus Schaller wrote:

> So I don’t think that being able to control the slave is a common property
> of all UART-slave connections.
> 
> BTW: the GPS chip we use is doing something without any instructions. It starts
> to send NMEA records right after power on. It would even be useful with a unidirectional
> connection from GPS to UART. Hence there are no commands that need to be
> sent through UART. Rather it just needs some GPIO to be activated or powered down
> when /dev/tty is opened/closed.

In this example, the GPIO is a workaround for the lack of DTR.


> So one could argue that in this real case (which we try to solve in an acceptable way
> since 2 years) the fundamental/main interface (control command interface) of this chip
> is the power on/off line. And UART isn’t involved at all. Or just as an “activity detector”.

The indirect nature of control in this case only reflects the workaround,
not the underlying concept. Control is still derived from the UART status.

Both devicetree and tty/serial can already represent independent control;
what is proposed is a way to express dependent control, and in all cases,
that control stems directly from either the UART state itself or via
commands sent over that interface.

Any target not requiring UART involvement doesn't (and probably, shouldn't)
be expressed as a slave device.

Regards,
Peter Hurley
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