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

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Date:	Mon, 7 Dec 2015 11:37:02 -0600
From:	Rob Herring <robh@...nel.org>
To:	Martyn Welch <martyn.welch@...labora.co.uk>,
	Linus Walleij <linus.walleij@...aro.org>
Cc:	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, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, Olof Johansson <olof@...om.net>
Subject: Re: [PATCH 1/3] Device tree binding documentation for gpio-switch

+Linus W

On Fri, Dec 04, 2015 at 05:31:13PM +0000, Martyn Welch 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.

This is good and what I suggested, but it now makes me wonder if switch 
is generic enough. This boils down to needing to expose single gpio 
lines to userspace with a defined function/use. IIRC, there's been some 
discussion about this before along with improving the userspace 
interface for GPIO in general. So I'd like to get Linus' thoughts on 
this.


> Signed-off-by: Martyn Welch <martyn.welch@...labora.co.uk>
> ---
>  .../devicetree/bindings/misc/gpio-switch.txt       | 47 ++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/misc/gpio-switch.txt
> 
> diff --git a/Documentation/devicetree/bindings/misc/gpio-switch.txt b/Documentation/devicetree/bindings/misc/gpio-switch.txt
> new file mode 100644
> index 0000000..13528bd
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/misc/gpio-switch.txt
> @@ -0,0 +1,47 @@
> +Device-Tree bindings for gpio attached switches.
> +
> +This provides a mechanism to provide a named link to specified gpios. This can
> +be useful in instances such as when theres a need to monitor a switch, which is
> +common across a family of devices, but attached to different gpios and even
> +implemented in different ways on differnet devices.
> +
> +Required properties:
> +	- compatible = "gpio-switch";
> +
> +Each signal is represented as a sub-node of "gpio-switch". The naming of
> +sub-nodes is arbitrary.
> +
> +Required sub-node properties:
> +
> +	- label: Name to be given to gpio switch.
> +	- gpios: OF device-tree gpio specification.
> +
> +Optional sub-node properties:
> +
> +	- read-only: Boolean flag to mark the gpio as read-only, i.e. the line
> +	  should not be driven by the host.

In terms a a switch use, allowing driving it would be an override of the 
switch. Is that the idea here?

> +
> +Example nodes:
> +
> +        gpio-switch {
> +                compatible = "gpio-switch";

Both from a binding and driver perspective, there is no point in 
grouping these. Each node can simply have this compatible string.

> +
> +                write-protect {
> +                        label = "write-protect";
> +                        gpios = <&gpx3 0 GPIO_ACTIVE_LOW>;
> +                        read-only;
> +                };
> +
> +                developer-switch {
> +                        label = "developer-switch";
> +                        gpios = <&gpx1 3 GPIO_ACTIVE_HIGH>;
> +                        read-only;
> +                };
> +
> +                recovery-switch {
> +                        label = "recovery-switch";
> +                        gpios = <&gpx0 7 GPIO_ACTIVE_LOW>;
> +                        read-only;
> +                };
> +        };
> +
> -- 
> 2.1.4
> 
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