linux-kernel - Re: [RFC Patch] gpio: add GPIO hogging mechanism

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Date:	Wed, 29 Oct 2014 11:41:22 -0500
From:	Benoit Parrot <bparrot@...com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
CC:	<linus.walleij@...aro.org>, <linux-gpio@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>
Subject: Re: [RFC Patch] gpio: add GPIO hogging mechanism

Maxime Ripard <maxime.ripard@...e-electrons.com> wrote on Wed [2014-Oct-29 11:45:59 +0100]:
> Hi,
> 
> On Tue, Oct 21, 2014 at 03:09:58PM -0500, Benoit Parrot wrote:
> > Based on Boris Brezillion work this is a reworked patch
> > of his initial GPIO hogging mechanism.
> > This patch provides a way to initally configure specific GPIO
> > when the gpio controller is probe.
> > 
> > The actual DT scanning to collect the GPIO specific data is performed
> > as part of the gpiochip_add().
> > 
> > The purpose of this is to allows specific GPIOs to be configured
> > without any driver specific code.
> > This particularly usueful because board design are getting
> > increassingly complex and given SoC pins can now have upward
> > of 10 mux values a lot of connections are now dependent on
> > external IO muxes to switch various modes and combination.
> > 
> > Specific drivers should not necessarily need to be aware of
> > what accounts to a specific board implementation. This board level
> > "description" should be best kept as part of the dts file.
> > 
> > Signed-off-by: Benoit Parrot <bparrot@...com>
> 
> I've been thinking about this for quite some time, it's good to see
> some progress on that :)
> 
> However, I have a slightly different use case for it: the Allwinner
> SoCs have a vdd pin coming in for every gpio bank. Nothing out of the
> ordinary so far, except that some of the boards are using a
> GPIO-controlled regulator to feed another bank vdd. That obviously
> causes a chicken-egg issue, since for the gpio-regulator driver to
> probe, it needs to gpio driver, and for the gpio driver to probe, it
> needs the regulator driver.

Unless the gpio controlling the vdd pin is from the same bank your trying to power up
I do not see the issue here.

In this patch the gpio-hogs are setup as part of the gpio-controller probe function.

> 
> I was thinking of solving this by enforcing gpio hogs, in order to
> have the gpio driver loading, grabing its gpios setting them to the
> right value to enable the current to flow in, and then let the
> regulator driver probe later on.
> 
> I don't think it's possible with your current code, would it be
> something worth considering, or would someone have a better solution?

Unless I am missing something, this should work.

> 
> Maxime
> -- 
> Maxime Ripard, Free Electrons
> Embedded Linux, Kernel and Android engineering
> http://free-electrons.com

Regards,
Benoit


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