linux-kernel - Re: [RFC 1/2] dt-bindings: gpio: Document shared GPIO line usage

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Message-ID: <CACRpkdZOn8vGjxCUBd9c-SQSkmwxpS+bbRk7TiOWJcpTz9BsBQ@mail.gmail.com>
Date:   Wed, 11 Dec 2019 00:54:52 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Peter Ujfalusi <peter.ujfalusi@...com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mark Brown <broonie@...nel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [RFC 1/2] dt-bindings: gpio: Document shared GPIO line usage

On Wed, Dec 4, 2019 at 12:51 AM Rob Herring <robh@...nel.org> wrote:
> On Thu, Nov 28, 2019 at 11:06:35AM +0100, Linus Walleij wrote:

> > The ambition to use refcounted GPIOs to solve this
> > usecase is probably wrong, I would say try to go for a
> > GPIO-based reset controller instead.
>
> Yes, but I think we can have that AND use the existing binding.
>
> > The fact that some Linux drivers are already using explicit
> > GPIO's for their reset handling is maybe unfortunate,
> > they will simply have to grow code to deal with a reset
> > alternatively to GPIO, like first try to grab a reset
> > handle and if that doesn't fall back to use a GPIO.
>
> I think this could just be all handled within the reset subsystem given
> that we've been consistent in using 'reset-gpios' (GPIO regulators are
> similar, but we never had such consistency with GPIO names for
> regulators). We can implement a reset driver for the 'reset-gpios'
> property that deals with the sharing. Drivers to DT nodes doesn't have
> to be 1:1. It's convenient when they are, but that's encoding the OS's
> (current) driver structure into DT.

This seems like a good approach if it can be made to work.
reset-gpios should have the right polarity flags (else drivers
and/or device trees need to be fixed...) so the driver can simply
scan over them and try to build a consensus on how to assert
or deassert a shared reset-gpios line.

It is also a natural placeholder for the connection to device
core that will inevitably need to happen the day the device
hierarchy needs to be torn up/down for a reset on some
random device.

Peter, will you have a go at it?

Yours,
Linus Walleij