Re: [PATCH 1/2] x86, gpio: Increase ARCH_NR_GPIOs to 512

[Alexandre Courbot]

On Sat, Sep 20, 2014 at 2:48 AM, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> On Fri, Sep 19, 2014 at 12:20 AM, Alexandre Courbot <gnurou@xxxxxxxxx> wrote:
>> On Tue, Sep 9, 2014 at 4:24 PM, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
>
>>> This however makes it *impossible* to use things like desc_to_gpio()
>>> and/or gpio_to_desc() so the code has to be augmented all over the
>>> place to avoid any uses of GPIO numbers on that architecture,
>>> but I am sure it *can* be done on pure ACPI or device tree
>>> systems, and that's what we should aim for.
>>
>> desc_to_gpio()/gpio_to_desc() could still work even if we remove the
>> big array of GPIO descriptors. Actually that's what I intended to do
>> when I first submitted the gpiod patches some time ago but it was
>> rejected for performance reasons.
>
> OK. I'm ready to revisit the subject.
>
>> desc_to_gpio() actually doesn't even access this array - it does its
>> job using the chip base and the beginning address of its descriptors
>> array.
>
> Right.
>
>> gpio_to_desc() would suffer a performance hit. What I initially
>> proposed was to parse the linked list of GPIO chips and check if the
>> requested number is in their assigned range. This is obviously slower
>> than accessing an array, but if we consider that we generally don't
>> have too many GPIO chips on a given hardware I don't think the hit
>> would be that bad. It would also give some incentive for people to
>> move to the gpiod interface.
>
> I think the performance hit is acceptable, because this should
> not be on a hot path anyway. I would say go ahead with this refactoring.

Great! I will come with something once my holidays are over. It should
not be a complex change.

>
>> I also have a patch in my queue that enables multiple users on the
>> same GPIO descriptor (something requested since some time already).
>> What happens with it is that descriptors ownership is fully
>> transferred to the gpio_chip instances, and the static array becomes a
>> array of double-pointers, making it considerable smaller and reducing
>> the impact of increasing its size. Maybe I should submit that change
>> just for this case?
>
> Ummmmm I think that is an orthogonal thing, but honestly I'm
> not following the double-pointers thing, so I guess I need to see
> the patch.

Yes, this is completely orthogonal, and actually this won't be needed
if we decide to get rid of that array.
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