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From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Stephen Warren <swarren@wwwdotorg.org>
Cc: Alexandre Courbot <gnurou@gmail.com>,
	"linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: Correct meaning of the GPIO active low flag
Date: Tue, 11 Feb 2014 00:21:46 +0100	[thread overview]
Message-ID: <1848152.lIn8ApGfEn@avalon> (raw)
In-Reply-To: <52F95AFE.4070904@wwwdotorg.org>

Hi Stephen,

On Monday 10 February 2014 16:04:30 Stephen Warren wrote:
> On 02/10/2014 10:52 AM, Laurent Pinchart wrote:
> > On Monday 10 February 2014 09:57:43 Stephen Warren wrote:
> >> On 02/10/2014 09:56 AM, Stephen Warren wrote:
> ...
> 
> >>> I think the flag should represent the physical level of the signal on
> >>> the board at the device pin. I'm pretty sure that's what's most
> >>> consistent with existing DT properties.
> >> 
> >> (That would have to be the GPIO source device, in order to account for
> >> any board-induced inversion)
> > 
> > Would that be the physical level at the GPIO source device output to
> > achieve a high level at the target device input pin, or the physical
> > level at the GPIO source device output to assert the signal at the target
> > device input pin ? The first case wouldn't take the receiver device
> > internal inverter into account while the second case would. In the second
> > case, how should we handle receiver devices that have configurable signal
> > polarities (essentially enabling/disabling the internal inverter from a
> > software-controller configuration) ?
> 
> I would expect the flag to represent the physical level that achieves (or
> represents, for inputs) a logically asserted value at the device.

I assume you mean "the physical level at the GPIO controller output".

> I don't think we should make the level flag influence any kind of
> configurable level within the device; that's a separate orthogonal, but
> related, concept. It'd be best if the DT binding for the device either
> (a) provided a separate property to configure that, or (b) picked a
> single one of the configurable values, and documented that all DTs
> should assume that value.

Agreed. I've phrased my question incorrectly though.

My concern with devices that have configurable input polarities is that the 
"physical level [at the GPIO controller output] that achieves (or represents, 
for inputs) a logically asserted value at the device" depends on runtime 
configuration of the device, and is thus ill-defined.

We could consider that the flag represents the physical level at the GPIO 
controller output that achieves (or represents, for inputs) a logically 
asserted value at the device, for the default configuration of the device. The 
default configuration of the device would then need to be defined. I'm unsure 
whether the default configuration should be constant, or could depend on other 
DT properties.

-- 
Regards,

Laurent Pinchart


  reply	other threads:[~2014-02-10 23:20 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-02-10 14:33 Correct meaning of the GPIO active low flag Laurent Pinchart
2014-02-10 14:50 ` Alexandre Courbot
2014-02-10 15:13   ` Laurent Pinchart
2014-02-10 16:56     ` Stephen Warren
2014-02-10 16:57       ` Stephen Warren
2014-02-10 17:52         ` Laurent Pinchart
2014-02-10 23:04           ` Stephen Warren
2014-02-10 23:21             ` Laurent Pinchart [this message]
2014-02-12 16:50               ` Stephen Warren
2014-02-13 14:43                 ` Laurent Pinchart
2014-02-13 16:49                   ` Stephen Warren
2014-02-14 23:48                     ` Laurent Pinchart
2014-02-15  0:07                       ` Stephen Warren
2014-02-15  0:20                         ` Laurent Pinchart
2014-02-18 17:58                           ` Stephen Warren
2014-02-19  0:19                             ` Laurent Pinchart

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