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

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Message-ID: <CACRpkdbsYyHRz826K0QA=Y7Dvf_sm714o5601KBD7Zo4+1hrQA@mail.gmail.com>
Date:   Thu, 18 Jan 2018 11:30:00 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Ludovic Desroches <ludovic.desroches@...rochip.com>
Cc:     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 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?

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