linux-kernel - Re: [RFC v2 0/2] gpio: Support for shared GPIO lines on boards

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Date:   Wed, 30 Oct 2019 08:12:53 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mark Brown <broonie@...nel.org>, Tero Kristo <t-kristo@...com>,
        Maxime Ripard <mripard@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        devicetree@...r.kernel.org
Subject: Re: [RFC v2 0/2] gpio: Support for shared GPIO lines on boards

On Wed, Oct 30, 2019 at 7:03 AM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
>
> Hi,
>
> The shared GPIO line for external components tends to be a common issue and
> there is no 'clean' way of handling it.
>
> I'm aware of the GPIOD_FLAGS_BIT_NONEXCLUSIVE flag, which must be provided when
> a driver tries to request a GPIO which is already in use.
> However the driver must know that the component is going to be used in such a
> way, which can be said to any external components with GPIO line, so in theory
> all drivers must set this flag when requesting the GPIO...
>
> But with the GPIOD_FLAGS_BIT_NONEXCLUSIVE all clients have full control of the
> GPIO line. For example any device using the same GPIO as reset/enable line can
> reset/enable other devices, which is not something the other device might like
> or can handle.
> For example a device needs to be configured after it is enabled, but some other
> driver would reset it while handling the same GPIO -> the device is not
> operational anymmore as it lost it's configuration.
>
> With the gpio-shared gpiochip we can overcome this by giving the gpio-shared
> the role of making sure that the GPIO line only changes state when it will not
> disturb any of the clients sharing the same GPIO line.

Why can't we just add a shared flag like we have for interrupts?
Effectively, we have that for resets too, it's just hardcoded in the
the drivers.

Rob