linux-kernel - Re: [PATCH 4/5] gpio: of: Add support to have multiple gpios in gpio-hog

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Message-ID: <2337414.ntEtcfKnX0@hermes>
Date:	Thu, 10 Mar 2016 12:16:58 +0100
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Laxman Dewangan <ldewangan@...dia.com>
Cc:	Stephen Warren <swarren@...dotorg.org>, linus.walleij@...aro.org,
	robh+dt@...nel.org, pawel.moll@....com, mark.rutland@....com,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-gpio@...r.kernel.org, treding@...dia.com,
	Benoit Parrot <bparrot@...com>,
	Alexandre Courbot <acourbot@...dia.com>
Subject: Re: [PATCH 4/5] gpio: of: Add support to have multiple gpios in gpio-hog

On Thursday 10 March 2016 12:37:32 Laxman Dewangan wrote:
> 
> On Wednesday 09 March 2016 10:47 PM, Stephen Warren wrote:
> > On 03/09/2016 06:20 AM, Laxman Dewangan wrote:
> >>
> >> On Wednesday 09 March 2016 11:58 AM, Markus Pargmann wrote:
> >>> * PGP Signed by an unknown key
> >>>
> >>> Hi,
> >>>
> >>> On Tue, Mar 08, 2016 at 05:32:07PM +0530, Laxman Dewangan wrote:
> >>>> The child node for gpio hogs under gpio controller's node
> >>>> provide the mechanism to automatic GPIO request and
> >>>> configuration as part of the gpio-controller's driver
> >>>> probe function.
> >>>>
> >>>> Currently, property "gpio" takes one gpios for such
> >>>> configuration. Add support to have multiple GPIOs in
> >>>> this property so that multiple GPIOs of gpio-controller
> >>>> can be configured by this mechanism with one child node.
> >>> So if I read this correctly you want to have multiple GPIOs with the
> >>> same line name? Why don't you use multiple child nodes with individual
> >>> line names?
> >>>
> >> There is cases on which particular functional configuration needs sets
> >> of GPIO to set. On this case, making sub node for each GPIOs creates
> >> lots of sub-nodes and  add complexity on readability, usability and
> >> maintainability.
> >> Example: for my board, I wanted to set GPIO H2 to input and H0 and H1 to
> >> be output high.
> >> Instead of three nodes, I can have two here:
> >>         gpio@0,6000d000 {
> >>                 wlan_input {
> >>                         gpio-hog;
> >>                         gpios = <TEGRA_GPIO(H, 2) 0>;
> >>                         input;
> >>                 };
> >>
> >>                 wlan_output {
> >>                         gpio-hog;
> >>                         gpios = <TEGRA_GPIO(H, 0) 0 TEGRA_GPIO(H, 1) 0>;
> >>                         output-high;
> >>                 };
> >>         };
> > >
> >> So here I am grouping the multiple output GPIO together.
> >>
> >> This looks much similar if we have many GPIOs for one type of
> >> configurations.
> >>
> >> Even it looks better if we have something:
> >>         gpio@0,6000d000 {
> >>                 wlan_control {
> >>                         gpio-hog;
> >>                         gpios-input = <TEGRA_GPIO(H, 2) 0>;
> >>                         gpios-output-high = <TEGRA_GPIO(H, 0) 0
> >> TEGRA_GPIO(H, 1) 0>;
> >>                 };
> >>         };
> >
> > The problem with that is the description used when acquiring the GPIO 
> > is just "wlan_input", "wlan_output", or "wlan_control". There's 
> > nothing to indicate what those individual pins do (perhaps one is a 
> > reset signal, one is a regulator enable, etc.?) By requiring separate 
> > nodes for each GPIO, then the node name can provide a meaningful 
> > semantic name/description for each GPIO, which provides much more 
> > information.
> >
> 
> On this case, we have already property "line-name" and passed the name 
> of the gpio via this property.
> The property names is "line-name" which is good for one string. We can 
> support other property "line-names" with multiple string per GPIO index.
> 
> line-names = "wlan-reset", "wlan-enable";

There is currently a discussion about the future bindings for subnodes in GPIO
controller nodes. Please have a look at these two mail threads:

	"Device tree binding documentation for gpio-switch"
	"gpio: of: Add support to have multiple gpios in gpio-hog"

Best Regards,

Markus

> 
> 
> > If the approach in this patch is acceptable though, I think you want 
> > to update the description of "gpios" (in the GPIO hog definition 
> > section) in Documentation/devicetree/bindings/gpio/gpio.txt to mention 
> > that multiple GPIO entries are legal. Right now it says that property 
> > much contain exactly #gpio-cells, not a multiple of #gpio-cells.
> 
> I have 5th patch for this and will rearrange series as you suggested on 
> 5th patch.
> 
> 
> 

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