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    SubjectRe: [RFC PATCH 0/3] If an IRQ is a GPIO, request and configure it
    On Fri, Aug 05, 2011 at 08:43:20AM -0700, Stephen Warren wrote:
    > Russell King - ARM Linux wrote at Friday, August 05, 2011 3:40 AM:
    > > On Thu, Aug 04, 2011 at 05:00:17PM -0600, Stephen Warren wrote:
    > > > In, Mark Brown
    > > > pointed out that it was a little silly forcing every board or driver
    > > > to gpio_request() a GPIO that is later converted to an IRQ, and passed
    > > > to request_irq. The first patch in this series instead makes the core
    > > > IRQ code perform these calls when appropriate, to avoid duplicating it
    > > > everywhere.
    > >
    > > Trying to go from IRQ to GPIO is not a good idea - most of the
    > > IRQ <-> GPIO macros we have today are just plain broken. Many of them
    > > just add or subtract a constant, which means non-GPIO IRQs have an
    > > apparant GPIO number too. Couple this with the fact that all positive
    > > GPIO numbers are valid, and this is a recipe for wrong GPIOs getting
    > > used and GPIOs being requested for non-GPIO IRQs.
    > >
    > > I think this was also discussed in the past, and the conclusion was that
    > > IRQs should be kept separate from GPIOs. Maybe views have changed since
    > > then...
    > >
    > > However, if we do want to do this, then it would be much better to provide
    > > a new API for requesting GPIO IRQs, eg:
    > >
    > > gpio_request_irq()
    > >
    > > which would wrap around request_threaded_irq(), takes a GPIO number,
    > > does the GPIO->IRQ conversion internally, and whatever GPIO setup is
    > > required. Something like this:
    > With that approach, drivers need to explicitly know whether they're
    > passed a GPIO or an IRQ, and do something different, or they need to
    > choose to only accept a GPIO or IRQ.

    You completely missed the biggest reason why your approach is broken.

    + gpio = irq_to_gpio(irq);
    + if (gpio_is_valid(gpio))

    Let's look at the code:

    #define ARCH_NR_GPIOS 256

    static inline bool gpio_is_valid(int number)
    return number >= 0 && number < ARCH_NR_GPIOS;

    Now, let's take AT91:

    #define irq_to_gpio(irq) (irq)

    This doesn't define ARCH_NR_GPIOS, so it gets the default 256. Now lets
    take a random selection of the AT91 interrupt numbers:

    #define AT91RM9200_ID_US3 9 /* USART 3 */
    #define AT91RM9200_ID_MCI 10 /* Multimedia Card Interface */
    #define AT91RM9200_ID_UDP 11 /* USB Device Port */
    #define AT91RM9200_ID_TWI 12 /* Two-Wire Interface */
    #define AT91RM9200_ID_SPI 13 /* Serial Peripheral Interface */
    #define AT91RM9200_ID_SSC0 14 /* Serial Synchronous Controller 0 */
    #define AT91RM9200_ID_SSC1 15 /* Serial Synchronous Controller 1 */

    None of these are GPIOs. Yet gpio_is_valid(irq_to_gpio(AT91RM9200_ID_TWI))
    is true.

    That's the problem - it's undefined whether gpio_is_valid(irq_to_gpio(irq))
    returns true or false for any particular interrupt. There's no multiplexing
    through gpiolib for the IRQ-to-GPIO mapping either, so it doesn't work for
    off-SoC GPIOs.

    So, you can't reliably go from interrupt numbers to GPIO numbers - it's
    just not supported. So to throw this into the IRQ layer is just going to
    end up breaking a hell of a lot of platforms.

    Now, stack on top of that a discussion at the Linaro Connect conference
    this week where we discussed getting rid of IRQ numbers entirely, and
    our desire to kill off irq_to_gpio() and I think it makes this approach
    a non-starter.

    > So it seems like, as was mentioned elsewhere in this thread, the upshot of
    > this conversation is that interrupt chip drivers should do this internally,
    > both to avoid requiring a general irq_to_gpio function, and because calling
    > gpio_direction_input for GPIOs-used-as-IRQs isn't appropriate for all
    > hardware.

    That would be more appropriate, because the IRQ chip stuff at least knows
    if there's a GPIO associated with it.

    There's still the unanswered question whether we even want the IRQ layer
    to do this kind of stuff, and the previous decision on that I believe was
    in the negative. So I think Thomas needs to respond to that point first.

     \ /
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