Re: [PATCH v6 4/8] gpiolib: Add support for GPIO lookup by line name

[Linus Walleij]

On Tue, Mar 24, 2020 at 2:57 PM Geert Uytterhoeven
<geert+renesas@xxxxxxxxx> wrote:

> Currently a GPIO lookup table can only refer to a specific GPIO by a
> tuple, consisting of a GPIO controller label and a GPIO offset inside
> the controller.
>
> However, a GPIO may also carry a line name, defined by DT or ACPI.
> If present, the line name is the most use-centric way to refer to a
> GPIO.  Hence add support for looking up GPIOs by line name.
>
> Implement this by reusing the existing gpiod_lookup infrastructure.
> Rename gpiod_lookup.chip_label to gpiod_lookup.key, to make it clear
> that this field can have two meanings, and update the kerneldoc and
> GPIO_LOOKUP*() macros.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> Reviewed-by: Ulrich Hecht <uli+renesas@xxxxxxxx>
> Reviewed-by: Eugeniu Rosca <erosca@xxxxxxxxxxxxxx>
> Tested-by: Eugeniu Rosca <erosca@xxxxxxxxxxxxxx>

I kind of like this approach, however there are things here that
need to be considered: the line name is in no way globally unique,
and I think there are already quite a few GPIO chips that
have the same line names assigned for every instance of that
chip.

gpiochip_set_desc_names() only warns if there is a line with
the same name on the same gpio_chip.

I suppose we need to document that the line name look-up
will be on a first-come-first-served basis: whatever line
we find first with this name is what you will get a reference
to, no matter what chip it is on, and it is possible albeit
not recommended that some other chip has a line with the
same name.

Yours,
Linus Walleij