[PATCH] gpio: document how to order GPIO controllers

[Linus Walleij]

On Wed, Jul 13, 2016 at 3:14 PM, Johan Hovold <johan at kernel.org> wrote:
> On Wed, Jul 06, 2016 at 02:34:39PM +0200, Linus Walleij wrote:

>> For the purpose of picking a certain named GPIO line (see below)
>> knowing this number is unnecessary. To that mechanism all gpio
>> chips are equal and the instance number does not matter.
>
> My point was that with gpio line names (and possibly topological
> information to resolve duplicates), the N number should not matter
> anymore.

Agreed. It doesn't, with the new ABI. Not for technical reasons, at all.

> If one needs to look up a particular gpiochip based on some hardware
> naming convention, why not associate a name with the chip instead of
> trying to shoehorn the dynamic gpiochip range in there?

With the new ABI it is possible to set a "label" for the gpiochip, currently
described as "a functional name for this GPIO chip, such as a product
number" and it's certainly possible to manage this name carefully for
a system to look up a certain chip.

[About obtaining topological  information from sysfs]
> And this may even be unavoidable with dynamic buses such as USB or
> greybus, where you can have multiple devices associating the same name
> with a pin. That's fine as long as the user-space interface allows for a
> way to distinguish them (e.g. through topological information).

This should work fine AFAICT. All the ABI and information is in place
to deal with this.

Yours,
Linus Walleij