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

[Alexandre Courbot]

On Fri, Dec 12, 2014 at 9:08 PM, Arnd Bergmann <arnd@xxxxxxxx> 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.
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