[PATCH] gpio: document how to order GPIO controllers

[Johan Hovold]

On Wed, Jul 06, 2016 at 11:34:37AM +0200, Linus Walleij wrote:
> On Tue, Jul 5, 2016 at 8:04 PM, Uwe Kleine-König
> <u.kleine-koenig at pengutronix.de> wrote:

> > root at hostname:/sys/bus/gpio/devices ls -l
> > lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip0 -> ../../../devices/platform/soc/53f00000.aips/53f9c000.gpio/gpiochip0
> > lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip1 -> ../../../devices/platform/soc/53f00000.aips/53fa4000.gpio/gpiochip1
> > lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip2 -> ../../../devices/platform/soc/53f00000.aips/53fcc000.gpio/gpiochip2
> > lrwxrwxrwx    1 root     root             0 Jul  5 20:52 gpiochip3 -> ../../../devices/platform/soc/53f00000.aips/53fd0000.gpio/gpiochip3
> >
> > That is we have:
> >
> >         Hardware name | software gpiochip
> >             GPIO4     |      gpiochip0
> >             GPIO3     |      gpiochip1
> >             GPIO1     |      gpiochip2
> >             GPIO2     |      gpiochip3
> >
> > I bet that's the probe order because when sorted by address (and so
> > by order in the device tree) we have exactly this ordering. (Compare
> > with $(grep gpio@ arch/arm/boot/dts/imx25.dtsi).)
> 
> That is super-unintuitive for a human user, I agree 100%.
> 
> > For a new interface this is OK, still I predict users will complain if
> > the numbers used don't match naturally the hardware names. And IMHO they
> > are right.
> 
> I agree, and I think that if aliases can alleviate the situation we should
> allow them and encourage them. They are the best duct-tape we can
> find for the DT systems.
> 
> Unfortunately it is not my call, because DT bindings and alias use
> is not under my jurisdiction.
> 
> I *THINK* the view of the device core maintainers is that udev
> and sysfs hierarchies should be used to uniquely identify a certain
> device, and that relying on device numbering is too fragile.

I thought the whole idea (or at least a large part of) the new
user-space interface was to allow lookups by line names precisely in
order not to have to rely on gpio numbers, which may not only change
between boots, but also between hardware revisions, etc.

What's wrong with naming the pins in DT and use that for lookups?

I've been too busy with other stuff this spring to follow the new
user-space interface development, but I assume that this had been taken
into account in the design?

Thanks,
Johan