linux-kernel - Re: [PATCH v1 1/1] power: Add simple gpio-restart driver

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Message-ID: <20140827014300.GA29512@earth.universe>
Date:	Wed, 27 Aug 2014 03:43:00 +0200
From:	Sebastian Reichel <sre@...nel.org>
To:	David Riley <davidriley@...omium.org>
Cc:	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Grant Likely <grant.likely@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Anton Vorontsov <anton@...msg.org>,
	Marc Carino <marc.ceeeee@...il.com>,
	Anders Berg <anders.berg@....com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Ivan Khoronzhuk <ivan.khoronzhuk@...com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Haojian Zhuang <haojian.zhuang@...aro.org>,
	Jamie Lentin <jm@...tin.co.uk>,
	Doug Anderson <dianders@...omium.org>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH v1 1/1] power: Add simple gpio-restart driver

Hi David,

On Tue, Aug 26, 2014 at 04:45:05PM -0700, David Riley wrote:
> This driver registers a restart handler to set a GPIO line high/low
> to reset a board based on devicetree bindings.

Driver looks fine to me. I have some comments about the
Documentation, though:

> [...]
> diff --git a/Documentation/devicetree/bindings/gpio/gpio-restart.txt b/Documentation/devicetree/bindings/gpio/gpio-restart.txt
> new file mode 100644
> index 0000000..7cd58788
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/gpio/gpio-restart.txt
> @@ -0,0 +1,48 @@
> +Driver a GPIO line that can be used to restart the system as a
> +restart handler.

Please fix the Typo (first word).

> [...]
> +
> +The driver supports both level triggered and edge triggered power off.
> +At driver load time, the driver will request the given gpio line and
> +install a restart handler.

The wording is too driver centric IMHO. You are supposed to document
the binding in a generic way. Maybe start with something like:

"This binding supports level and edge triggered reset."

(power off is the wrong word, since there is already gpio-poweroff).

> +If the optional properties 'input' is +not found, the GPIO line
> +will be driven in the inactive state. Otherwise its configured
> +as an input.

What is this needed for?

> +When do_kernel_restart is called the various restart handlers will be tried
> +in order.  The gpio is configured as an output, and drive active, so
> +triggering a level triggered power off condition. This will also cause an
> +inactive->active edge condition, so triggering positive edge triggered
> +power off. After a delay of 100ms, the GPIO is set to inactive, thus
> +causing an active->inactive edge, triggering negative edge triggered power
> +off. After another 100ms delay the GPIO is driver active again. If the
> +power is still on and the CPU still running after a 3000ms delay, a
> +WARN_ON(1) is emitted.

I really appreciate the description of the driver (it made it easier
to review it :)), but Documentation/devicetree should avoid
Linuxisms. In other words: this is the wrong location for the
description.

> +Required properties:
> +- compatible : should be "gpio-restart".
> +- gpios : The GPIO to set high/low, see "gpios property" in
> +  Documentation/devicetree/bindings/gpio/gpio.txt. If the pin should be
> +  low to power down the board set it to "Active Low", otherwise set
> +  gpio to "Active High".
> +
> +Optional properties:
> +- input : Initially configure the GPIO line as an input. Only reconfigure
> +  it to an output when the machine_restart function is called. If this optional
> +  property is not specified, the GPIO is initialized as an output in its
> +  inactive state.
> +- priority : A priority ranging from 0 to 255 (default 128) according to
> +  the following guidelines:
> +	0:	Restart handler of last resort, with limited restart
> +		capabilities
> +	128:	Default restart handler; use if no other restart handler is
> +		expected to be available, and/or if restart functionality is
> +		sufficient to restart the entire system
> +	255:	Highest priority restart handler, will preempt all other
> +		restart handlers

You should add a short information about the property type here
(e.g. "8 bit integer" for priority).

> +Examples:
> +
> +gpio-restart {
> +	compatible = "gpio-restart";
> +	gpios = <&gpio 4 0>;
> +	priority = /bits/ 8 <200>;
> +};
> [...]

-- Sebastian

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