linux-kernel - Re: [RFC v5 4/4] gpio: dt-bindings: add gpio-mmio bindings

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Date:	Fri, 29 Apr 2016 16:29:13 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Christian Lamparter <chunkeey@...glemail.com>
Cc:	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"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>,
	Álvaro Fernández Rojas <noltari@...il.com>,
	Kumar Gala <galak@...eaurora.org>,
	Alexander Shiyan <shc_work@...l.ru>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Alexandre Courbot <gnurou@...il.com>
Subject: Re: [RFC v5 4/4] gpio: dt-bindings: add gpio-mmio bindings

On Fri, Apr 29, 2016 at 2:53 AM, Christian Lamparter
<chunkeey@...glemail.com> wrote:

> From: Álvaro Fernández Rojas <noltari@...il.com>
>
> This patch adds the device tree bindings for the gpio-mmio.
> The gpio-mmio is already part of a the GPIO generic library.
>
> Signed-off-by: Álvaro Fernández Rojas <noltari@...il.com>
> Signed-off-by: Christian Lamparter <chunkeey@...glemail.com>

I share your ambition to create something generic for this class
of hardware(s).

> +Bindings for the generic driver for memory-mapped GPIO controllers.
> +
> +Required properties:
> +       - compatible: should be "linux,gpio-mmio"

Why?
"memory-mapped-gpio" sits nicely with me.

Read another very generic binding for inspiration:
Documentation/devicetree/bindings/leds/register-bit-led.txt

> +       - reg-names: must contain
> +               "dat" - data register
> +               may contain
> +               "set" - data set register
> +               "clr" - data clear register
> +               "dirout" - direction output register
> +               "dirin" - direction input register

I would just be more verbose:

data-in-register
data-out-set-register
data-out-clear-register
direction-output-register
direction-input-register

Some should be optional so we can support input-only
and output-only GPIO controllers too.

I would take this opportunity to add bindings also for stuff that
the generic MMIO driver does not support today but
could be made to support:

open-drain-register
open-source-register
debounce-register

etc

> +Optional properties:
> +       - ngpio: specifies the number of gpio mapped in the register.

Just reference the generic docs.

> +       - big-endian: force big endian register accesses.
> +       - big-endian-byte-order: assign GPIOs in reverse order.
> +       - unreadable-reg-set: data set register is not readable.
> +       - read-output-reg-set: cache value set for reads.
> +       - unreadable-reg-dir: dirout/dirin register is not readable.
> +       - no-output: GPIOs are read-only.

I think it's better to imply that if there is no data-in-register
specified, then it is output-only etc.

> +The GPIO generic library provides support for memory-mapped GPIO
> +controllers. The configuration is detected by which resources are present.
> +The simplest form of a GPIO controller that the driver support is just a
> +single "dat" register, where GPIO state can be read and/or written.
> +However, the driver supports far more:
> +       - 8/16/32/64 bits registers. The number of GPIOs is automatically
> +         determined by the width of the registers.
> +       - GPIO controllers with clear/set registers.
> +       - GPIO controllers with a single "dat" register.
> +       - Big endian bits/GPIOs ordering.


Skip this. It is Linux-specific.

Yours,
Linus Walleij