[PATCH v5 1/3] gpio: Cygnus: define Broadcom Cygnus GPIO binding

[Alexandre Courbot]

On Sat, Dec 13, 2014 at 12:28 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Friday 12 December 2014 22:05:37 Alexandre Courbot wrote:
>> On Fri, Dec 12, 2014 at 9:08 PM, Arnd Bergmann <arnd at arndb.de> wrote:
>> > On Thursday 11 December 2014 16:05:04 Ray Jui wrote:
>> >> +
>> >> +- linux,gpio-base:
>> >> +    Base GPIO number of this controller
>> >> +
>> >>
>> >
>> > We've NAK'ed properties like this multiple times before, and it
>> > doesn't get any better this time. What are you trying to achieve
>> > here?
>>
>> I am to blame for suggesting using this property to Ray, and I am
>> fully aware that this has been rejected before, but look at what
>> people came with recently to palliate the lack of control over the
>> GPIO number space for DT platforms:
>>
>> http://www.spinics.net/lists/arm-kernel/msg384847.html
>> https://lkml.org/lkml/2014/12/10/133
>>
>> Right now GPIO numbering for platforms using DT is a very inconsistent
>> process, subject to change by the simple action of adjusting the value
>> of ARCH_NR_GPIOS (which we did recently, btw), adding a new GPIO
>> controller, or changing the probe order of devices. For users of the
>> integer or sysfs interfaces, this results in GPIO numbers that change,
>> and drivers and/or user-space programs that behave incorrectly.
>> Ironically, the only way to have consistent numbers is to use the old
>> platform files, where you can specify the base number of a gpio_chip.
>>
>> DT is actually probably not such a bad place to provide consistency in
>> GPIO numbering. It has a global vision of the system layout, including
>> all GPIO controllers and the number of GPIOs they include, and thus
>> can make informed decisions. It provides a consistent result
>> regardless of probe order. And allowing it to assign GPIO bases to
>> controllers will free us from the nonsensical dependency of some
>> arbitrary upper-bound for GPIO numbers that ARCH_NR_GPIOS imposes on
>> us. Also about ARCH_NR_GPIOS, the plan is to eventually remove it
>> since we don't need it anymore after the removal of the global
>> gpio_descs array. This will again interfere with the numbering of GPIO
>> chips that do not have a base number provided.
>>
>> Note that I don't really like this, either - but the problem is the
>> GPIO integer interface. Until everyone has upgraded to gpiod and we
>> have a replacement for the current sysfs interface (this will take a
>> while) we have to cope with this. This issue has been bothering users
>> for years, so this time I'd like to try and solve it the less ugly
>> way. If there is a better solution, of course I'm all for it.
>
> I think the scheme will fail if you ever get gpio controllers that are
> not part of the DT: We have hotpluggable devices (PCI, USB, ...) that
> are not represented in DT and that may also provide GPIOs for internal
> uses.
>
> The current state of affairs is definitely problematic, but defining
> the GPIO numbers in DT properties would only be a relative improvement,
> not a solution, and I fear it would make it harder to change the kernel
> to remove the gpio numbers eventually.

You are absolutely right that this would be only a partial solution.
However this is a situation where there is no absolute fix (besides
dropping the GPIO numbers completely) and the relief this property
would brings makes it up for its shortcomings IMHO.

> I wonder if we could instead come up with an approach that completely
> randomizes the gpio numbers (as a compile-time option) to find any
> places that still rely on specific numbers.

A.k.a. Linus and Alex' hate mail generator. :P

Actually we are not that far from being able to do completely without
any GPIO number, and maybe that's what we should aim for. I think the
only remaining offender is the sysfs interface. If we could reach GPIO
controllers through a fixed path and just export their GPIOs there, I
believe we would have fixed the whole issue.