[PATCH 3/5] gpio/omap: Add DT support to GPIO driver

[Linus Walleij]

On Wed, Apr 17, 2013 at 5:41 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 04/16/2013 05:14 PM, Jon Hunter wrote:

>>> c) I have the feeling that hooking the of_xlate function for this is a
>>> bit of an abuse of the function.
>>
>> I was wondering about that. So I was grep'ing through the various xlate
>> implementations and found this [1]. Also you may recall that in the
>> of_dma_simple_xlate() we call the dma_request_channel() to allocate the
>> channel, which is very similar. However, I don't wish to get a
>> reputation as abusing APIs so would be good to know if this really is
>> abuse or not ;-)
>>
>> Cheers
>> Jon
>>
>> [1] http://permalink.gmane.org/gmane.linux.ports.arm.kernel/195124
>
> Interesting.
>
> This is really something that the core DT and GPIO and IRQ maintainers
> should weigh in on. Hence, changing them from Cc: to To: in this message
> and/or adding them.

So I guess it is OK for a driver or DT node to use a GPIO as IRQ
*only*, and then have the GPIO requested implicitly through the
xlate function in the irqdomain.

It's the use cases where one and the same driver tries to use the
same GPIO line *also* by requesting it using gpio_request()
that madness starts. In such cases the driver deserves to have
an error thrown back at it, as it definately knows the GPIO pin
and can be refactored to use gpio_to_irq() to translate it into
an IRQ rather than having it implicitly done in the xlate function.

You will just have to give the xlate function information about which
GPIOs are used as IRQs only, and only request the GPIO on these.

And I recently suggested a mechanism to do that, and that is
what I called GPIO input-hogs, which means that you mark
(e.g. from the device tree) at probe() of the gpiochip which GPIOs
will only be used as inputs, so as to generate IRQs.

This is perfectly OS-neutral information about the how the
hardware is set up in the system.

If you only allow these hogges inputs to be translated and
requested in the xlate function, everything goes smooth.

It all comes back to this: keep all applicable knowledge in the
system, so it can make proper decisions, don't try to create
mechanisms that will half-guess things.

Yours,
Linus Walleij