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

From: Linus Walleij <linus.walleij@linaro.org>
To: Rob Herring <robh@kernel.org>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Mark Brown <broonie@kernel.org>,
	Maxime Ripard <mripard@kernel.org>,
	Philipp Zabel <p.zabel@pengutronix.de>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>
Subject: Re: [RFC 1/2] dt-bindings: gpio: Document shared GPIO line usage
Date: Wed, 11 Dec 2019 00:54:52 +0100	[thread overview]
Message-ID: <CACRpkdZOn8vGjxCUBd9c-SQSkmwxpS+bbRk7TiOWJcpTz9BsBQ@mail.gmail.com> (raw)
In-Reply-To: <20191203235113.GA12929@bogus>

On Wed, Dec 4, 2019 at 12:51 AM Rob Herring <robh@kernel.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