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

[Rob Herring <robh+dt@kernel.org>]

On Fri, Dec 11, 2015 at 6:39 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
> On Fri, Dec 4, 2015 at 6:31 PM, Martyn Welch
> <martyn.welch@collabora.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@collabora.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