linux-kernel - Re: [PATCH 8/8] gpio: stmpe: configure GPIO as output by default

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Message-ID: <CACRpkdZwUnf3qfJr7btC7Bz5VXfPGaevmc3A3MTnimSXAj06+A@mail.gmail.com>
Date:	Wed, 20 Apr 2016 16:56:50 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Patrice CHOTARD <patrice.chotard@...com>
Cc:	Lee Jones <lee.jones@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Maxime Coquelin <maxime.coquelin@...com>,
	amelie.delaunay@...com, Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	Viresh Kumar <vireshk@...nel.org>,
	Shiraz Hashim <shiraz.linux.kernel@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 8/8] gpio: stmpe: configure GPIO as output by default

On Tue, Apr 19, 2016 at 2:18 PM,  <patrice.chotard@...com> wrote:

> From: Patrice Chotard <patrice.chotard@...com>
>
> Configures all GPIOs as output, in order to minimize power
> consumption when GPIOs are unused.
>
> Signed-off-by: Amelie DELAUNAY <amelie.delaunay@...com>
> Signed-off-by: Patrice Chotard <patrice.chotard@...com>

Not only do you set them all to outout but also:

> +       /* To minimize power consumption, configure unused GPIOs as outputs */
> +       for (i = 0; i < stmpe_gpio->chip.ngpio; i++)
> +               stmpe_gpio_direction_output(&stmpe_gpio->chip, i, 0);

You are driving them all low.

Now GPIO is general purpose: what if they are connected to a
line with a pull-up resistor?

That is not saving power, instead consuming more than if you
were setting them all to 1.

I am afraid this is wrong.

What you need is to be able to define in the DT (or similar)
a set of initial values for the GPIO lines, and set them to
0 for this design.

Such bindings have been discussed but no conclusion
or merged patch has emerged. Please help out in driving
a standard for this!

Yours,
Linus Walleij