GPIO - marking individual pins (not) available in device tree

[Grant Likely]

On Tue, Oct 28, 2008 at 10:53 AM, Matt Sealey <matt at genesi-usa.com> wrote:
> Is it still possible to perhaps create a node under lcd-controller which
> describes the pin groupings? Maybe we should call it a gpio-group.
>
> That way lcd-controller looks like
>
> lcd-controller {
>        compatible = "crystalfontz,something-gpio";
>        data-bus {
>                compatible - "gpio-group";
>                gpios = <gpioc 1 gpioc 8 gpioc 42 ... >
>        }
>        rw-ctrl {
>                compatible = "gpio-group";
>                gpios = <gpioc 2>
>        }

At this level, the 'compatible = "gpio-group";' is completely
irrelevant.  The binding for "crystalfontz,something-gpio" must
specify that there are two subnodes; one named data-bus and one named
rw-ctrl.  The driver, which binds against compatible in the parent
node, would know to go looking for those child nodes and to use the
gpios property inside them.  Simple.

>> Does this mean "hard-coding"?
>
> No. If you have an array of GPIO pins (gpios property) then how do you
> determine which is for data and which is some control pin? Do you
> associate these numbers in the driver somehow? Maybe a matchlist or
> an array? Given pins A B C D E F G H I J where does the data bus
> start and the control pins live? A and B? A and J? I and J?

You know because you document it in the binding.

> It would be definitely frivolous to define a whole device tree binding
> for the *order in which you MUST specify the gpios for this particular
> device*. There is obviously an implicit ordering of the GPIOs to make
> up the data bus (you'd expect an order from MSB to LSB.. or perhaps
> LSB to MSB... that might be better defined than undefined)

Why is it frivolous?  We do this all the time for reg and irqs.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.