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

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Message-ID: <20180118152228.GX2989@rfolt0960.corp.atmel.com>
Date:   Thu, 18 Jan 2018 16:22:28 +0100
From:   Ludovic Desroches <ludovic.desroches@...rochip.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     Ludovic Desroches <ludovic.desroches@...rochip.com>,
        <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 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:
> 
> > It can be useful for the pinmuxing layer to know which device is
> > requesting a GPIO. Add a consumer variant for gpiod_request to
> > reach this goal.
> >
> > GPIO chips managed by pin controllers should provide the new
> > request_consumer operation. They can rely on
> > gpiochip_generic_request_consumer instead of
> > gpiochip_generic_request.
> >
> > Signed-off-by: Ludovic Desroches <ludovic.desroches@...rochip.com>
> 
> 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?
> 

It's the probably the way to go, it was Maxime's proposal and Andy seems
to agree this solution.

> I.e. in that case you just allow gpio_owner to proceed and take the
> pin just like with a non-strict controller.
> 
> Yours,
> Linus Walleij

Regards

Ludovic