linux-kernel - Re: [PATCH 1/3] Device tree binding documentation for gpio-switch

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Date:	Fri, 11 Dec 2015 08:06:22 -0600
From:	Rob Herring <robh+dt@...nel.org>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Martyn Welch <martyn.welch@...labora.co.uk>,
	Alexandre Courbot <acourbot@...dia.com>,
	Michael Welling <mwelling@...e.org>,
	Markus Pargmann <mpa@...gutronix.de>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"devicetree@...r.kernel.org" <devicetree@...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>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Olof Johansson <olof@...om.net>,
	Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 1/3] Device tree binding documentation for gpio-switch

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
> <martyn.welch@...labora.co.uk> wrote:
>
>> This patch adds documentation for the gpio-switch binding. This binding
>> provides a mechanism to bind named links to gpio, with the primary
>> purpose of enabling standardised access to switches that might be standard
>> across a group of devices but implemented differently on each device.
>>
>> Signed-off-by: Martyn Welch <martyn.welch@...labora.co.uk>
>
> As mentioned in the comment to the second patch, this solves the
> following generic problem:
>
> Expose a GPIO line to userspace using a specific name
>
> That means basically naming GPIO lines and marking them as
> "not used by the operating system".
>
> This is something that has been proposed before, and postponed
> because the kernel lacks the right infrastructure.

That doesn't necessarily mean we can't define a binding.

> Markus Pargmann also did a series that add initial values to
> hogs, which is the inverse usecase of this, where you want to
> *output* something by default, then maybe also make it available
> to userspace.
>
> So what we need to see here is a patch series that does all of these
> things:
>
> - Name lines
>
> - Sets them to initial values
>
> - Mark them as read-only
>
> - Mark them as "not used by the operating system" so that they
>   can be default-exported to userspace.

No! This should not be a DT property.

Whether I want to control a GPIO in the kernel or userspace is not
known and can change over time. It could simply depend on kernel
config. There is also the case that a GPIO has no connection or kernel
driver until some time later when a DT overlay for an expansion board
is applied.

Rob
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