linux-kernel - Re: [PATCH v1 1/1] gpio: of: provide option generic property for initial line states

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Date:   Thu, 15 Jun 2017 07:56:32 +0200
From:   Oleksij Rempel <ore@...gutronix.de>
To:     Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexandre Courbot <gnurou@...il.com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        barebox@...ts.infradead.org
Subject: Re: [PATCH v1 1/1] gpio: of: provide option generic property for
 initial line states

Hi Vladimir,

On Sun, Jun 11, 2017 at 10:57:18PM +0300, Vladimir Zapolskiy wrote:
> Hi Oleksij,
> 
> On 06/11/2017 09:19 AM, Oleksij Rempel wrote:
> > Hi Rob,
> > 
> > On Fri, Jun 09, 2017 at 10:41:30AM -0500, Rob Herring wrote:
> >> On Fri, Jun 9, 2017 at 9:16 AM, Oleksij Rempel <o.rempel@...gutronix.de> wrote:
> >>> This property is needed for barebox or may be other bootloaders
> >>> to set proper gpio configuration as early as possible.
> >>>
> >>> This example is mainly based on "lines-initial-states" property
> >>> from this binding documentation:
> >>> Documentation/devicetree/bindings/gpio/gpio-pcf857x.txt
> >>
> >> Don't we have a binding for this already based on gpio-hogs.
> > 
> > Markus Pargmann was working on this topic before and suddenly was not
> > able to finish it.
> 
> that's fine, but the proposed extension is supposedly not wanted.

:(

> > I found only some not finished discussions and no code or documentation
> > in linux kernel upstream.
> > 
> > So far, I was able to find fallowing patches and discussions:
> > patch 23.08.2015, last comment 11.05.2017:
> > https://patchwork.kernel.org/patch/7096721/
> > 
> > patch 23.02.2016, last comment 09.03.2016:
> > https://patchwork.ozlabs.org/patch/586743/
> > 
> > Since there was objections about node-like design for gpio-initval, I
> > provided an array property, which is similar to initval properties already used
> > for different other device nodes. For example:
> > 
> > Documentation/devicetree/bindings/net/wireless/qcom,ath10k.txt
> > qcom,ath10k-calibration-data
> > 
> > Documentation/devicetree/bindings/gpio/gpio-pcf857x.txt
> > lines-initial-states
> 
> It'd rather be better to fix the drivers and remove the properties
> from these found device tree binding descriptions.

no sure if it is possible. WiFi calibration data is kind of initval too.

> > 
> > Please fixme if I miss something.
> 
> Why is GPIO hogging mechanism not good enough for your purpose?

Becouse it should not permanently hog a gpio. The "Hogging" should be
released as soon as some driver will request it.

Right now, if I define gpio-hog, boot loader will configure it and start
linux. But linux will never let it free.

If you have other suggestions which fit to this use case, please tell
me. I assume, right now I'm just blind.

Other way, I can imagine is to create real device nodes which use this
gpios. Compatible for this devices will be like this:
some_gpio_holder: gpio@0 {
	compatible = "gpio-for-userspace";
	gpio = <&gpio1 5 GPIO_ACTIVE_HIGH>;
};

Boot loader will configure it as needed, an linux will do nothing with
it so it can be used over /dev/gpio interface.

> --
> With best wishes,
> Vladimir

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