linux-kernel - Re: [RFC PATCH] gpio: support for GPIO forwarding

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Message-ID: <20150225182502.GA7586@psi-dev26.jf.intel.com>
Date:	Wed, 25 Feb 2015 10:25:02 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linus Walleij <linus.walleij@...aro.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [RFC PATCH] gpio: support for GPIO forwarding

On Wed, Feb 25, 2015 at 10:34:45AM +0900, Alexandre Courbot wrote:
> On Wed, Feb 25, 2015 at 5:34 AM, David Cohen
> <david.a.cohen@...ux.intel.com> wrote:
> > Hi,
> >
> >> If we decide to go ahead with the solution proposed by this patch for
> >> practical reasons (which are good reasons indeed), I still have one
> >> problem with its current form.
> >>
> >> As the discussion highlighted, this is an ACPI problem, so I'd very
> >> much like it to be confined to the ACPI GPIO code, to be enabled only
> >> when ACPI is, and to use function names that start with acpi_gpio. The
> >> current implementation leverages platform lookup, making said lookup
> >> less efficient in the process and bringing confusion about its
> >> purpose. Although the two processes are indeed similar, they are
> >> separate things: one is a legitimate way to map GPIOs, the other is a
> >> fixup for broken firmware.
> >>
> >> I suppose we all agree this is a hackish fix, so let's confine it as
> >> much as we can.
> >
> > Are we considering MFD cases hackish as well?
> > i.e. if we have a driver that needs to register children devices and this
> > driver needs to pass GPIO to a child.
> 
> In that case wouldn't the GPIO be best defined in the child node
> itself, for the child device's driver to directly probe?

In my case [1] I need 2 "virtual devices" (and more in future) to be
part of an USB OTG port control. I call it virtual because they are too
simple components connected to no bus and controlled by GPIOs:
- a fixed regulator controlled by GPIO
- a generic mux controlled by GPIO

I'd need to request official ACPI HID for them in order to make them
self-sufficient.

I can go ahead with this approach, but we have many examples of drivers
on upstream that are platform driver expecting to receive gpio via
platform data (e.g. extcon-gpio). The ACPI table of some products on
market were defined following this concept and won't change anymore.

Br, David

[1] https://lkml.org/lkml/2015/2/19/411
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