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

[Arnd Bergmann <arnd@xxxxxxxx>]

On Friday 12 December 2014 22:05:37 Alexandre Courbot wrote:
> 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.

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.

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.

	Arnd
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