linux-kernel - Re: [RFC PATCH 2/2] gpio: provide a consumer when requesting a gpio

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Message-ID: <20180126073213.GW3055@rfolt0960.corp.atmel.com>
Date:   Fri, 26 Jan 2018 08:32:13 +0100
From:   Ludovic Desroches <ludovic.desroches@...rochip.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>
Subject: Re: [RFC PATCH 2/2] gpio: provide a consumer when requesting a gpio

On Wed, Jan 24, 2018 at 05:42:15PM +0200, Andy Shevchenko wrote:
> On Wed, Jan 24, 2018 at 3:07 PM, Ludovic Desroches
> <ludovic.desroches@...rochip.com> wrote:
> > On Thu, Jan 18, 2018 at 04:22:28PM +0100, Ludovic Desroches wrote:
> >> On Thu, Jan 18, 2018 at 11:30:00AM +0100, Linus Walleij wrote:
> >> > On Mon, Jan 15, 2018 at 5:24 PM, Ludovic Desroches
> >> > <ludovic.desroches@...rochip.com> wrote:
> 
> >> > I think we need to think over what is a good way to share ownership
> >> > of a pin.
> >> >
> >> > Russell pointed me to a similar problem incidentally and I briefly looked
> >> > into it: there are cases when several devices may need to hold the
> >> > same pin.
> >> >
> >> > Can't we just look up the associated gpio_chip from the GPIO range,
> >> > and in case the pin is connected between the pin controller and
> >> > the GPIO chip, then we allow the gpiochip to also take a
> >> > reference?
> 
> How do you find my proposal about introducing ownership level (not
> requested yet; exclusive; shared)?
> 

Yes but I don't see how I can fix my issue with these levels. In my
case, I need an exclusive ownership at device level not at pin level. In
reality, it is at pin level but I am in this situation because my pin
controler was introduced as non strict and also because I need to set
the configuration of the pin which is going to be used as a GPIO.

If the ownership is exclusive, pinmuxing coming from pinctrl-default
will be accepted but the GPIO request will fail even if it comes from the
same device.

If the ownership is shared then, pinmuxing coming from pinctrl-default
will be accepted but a GPIO request from another device will be accepted
too.

Both situations are incorrect in my case.

Let me know if I have not well understood your proposal. My concern is
to get out of this situation without breaking current DTs.

Regards

Ludovic

> >> It's the probably the way to go, it was Maxime's proposal and Andy seems
> >> to agree this solution.
> 
> Confirm with caveat that this is a fix for subset of cases.
> 
> > If pin_request() is called with gpio_range not NULL, it means that the
> > requests comes from a GPIO chip and the pin controller handles this pin.
> > In this case, I would say the pin is connected between the pin
> > controller and the GPIO chip. Is my assumption right?
> >
> > I am not sure it will fit all the cases:
> 
> I think it doesn't cover cases when you have UART + UART + GPIO (I
> posted early a use case example).
> 
> But at least it doesn't move things in a wrong direction.
> 
> > - case 1: device A requests the pin (pinctrl-default state) and mux it
> >   as a GPIO. Later,it requests the pin as a GPIO (gpiolib). This 'weird'
> >   situation happens because some strict pin controllers were not declared
> >   as strict and/or pinconf is needed.
> >
> > - case 2: device A requests the pin (pinctrl-default state). Device B
> >   requests the pin as a GPIO (gpiolib).
> >
> > In case 1, pin_request must not return an error. In case 2, pin_request
> > must return an error even if the pin is connected between the pin
> > controller and the GPIO chip.
> 
> For these cases looks OK to me.
> 
> >> > I.e. in that case you just allow gpio_owner to proceed and take the
> >> > pin just like with a non-strict controller.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> --
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