linux-kernel - Re: [PATCH 2/2] gpio/mxc: implement reading output gpio value

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Date:	Wed, 5 Aug 2015 12:46:03 -0700
From:	Eduardo Valentin <edubezval@...il.com>
To:	Fabio Estevam <festevam@...il.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Ulises Brindis <ubrindis56@...il.com>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH 2/2] gpio/mxc: implement reading output gpio value

On Wed, Aug 05, 2015 at 03:11:40PM -0300, Fabio Estevam wrote:
> Hi Eduardo,
> 
> On Wed, Aug 5, 2015 at 3:02 PM, Eduardo Valentin <edubezval@...il.com> wrote:
> >
> > Hello Fabio,
> >
> > On Wed, Aug 05, 2015 at 02:43:05PM -0300, Fabio Estevam wrote:
> >> Hi Eduardo,
> >>
> >> On Wed, Aug 5, 2015 at 2:23 PM, Eduardo Valentin <edubezval@...il.com> wrote:
> >> > In current implementation, reading the value of an output gpio
> >> > always return 0. The reason is because when a gpio is configured
> >>
> >> Have you tried setting the SION bit for the pad?
> >
> > No, I haven't. From the bit description, it looks like it does a
> > different thing of what we would achieve with this patch. The SION bit
> > is a overwrite to the pad configuration. That is, the pin will be INPUT
> > always.
> >
> > We don't want to force it to be input. We simply want to be able to read
> > the value of an output GPIO. That is achievable by reading a different
> > register. In case the GPIO is configured as output, the GPIO block
> > documentation states its value can be read from GPIO_PSR.
> > This approach works fine, as the GPIO will be output and we can still
> > read its value.
> 
> Setting the SION bit allows you to do exactly that. Please see:
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-July/271774.html

OK. Then, what is the recommendation? Do we set this by default in the
driver code or this is left for DTS pinmux configuration?

To me seams like a bug in the gpio driver still, as the possibility to
read the value of an output gpio is missing/inconsistent.

> 
> Regards,
> 
> Fabio Estevam

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