Re: Device Tree Property for Setting of Labels

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Mon, Jan 31, 2022 at 1:21 PM Matt Walker <m@xxxxxxxxxxxx> wrote:

> I have a need to reliably identify GPIO pins in userspace,
> specifically I would like the method I use to be invariant to
> declaration order in the device tree (so not based on gpiochip#.)

Not sure I understand the goal. We are trying hard to drop any global
namespace for GPIOs so each of the pins is addressed by [chip,
relative pin #].

> There doesn't seem to be any existing method to accomplish this; and
> there's no existing device tree property in pinctrl that I can see, or
> in any driver that I looked at, that would persistently set a label,
> name, or other queryable property. (Although some devices like the
> stm32pinctrl add a sysfs node like st,bank-name.)
>
> I propose that a new optional property `label` or `names` is added to
> the base pinctrl schema that drivers would then use to set the `label`
> property in `struct gpio_chip` which can be queried from userspace via
> `GPIO_GET_CHIPINFO_IOCTL`.

Names of the lines are visible if you supply gpio-line-names DT property.

> Two things come to mind to consider in this scheme
> 1) I think I've seen other solutions like this where the device tree
> node name is used instead of some additional property, but that seems
> like it would be a breaking change if accepted because then all the
> labels would change as the drivers are updated.
>
> 2) Some pinctrl devices (I'm dealing with an mcp23sxx) register
> multiple chips under the same node. Right now the label is assigned by
> <mcp23sxx>.<address> and it could be something similar such as <label
> | mcp23sxx>.<address> but if we had a property that accepted a list of
> strings then each one could have a unique label if desired.
>
> If this is an acceptable proposal then I'll put together some patches.
> Or if someone else has a method I could use instead that already
> exists I'd love to hear about it.



-- 
With Best Regards,
Andy Shevchenko