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

[Rob Herring <robh+dt@kernel.org>]

On Wed, Oct 30, 2019 at 7:03 AM Peter Ujfalusi <peter.ujfalusi@ti.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