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

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Date:   Thu, 26 Mar 2020 22:18:39 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jonathan Corbet <corbet@....net>,
        Harish Jenny K N <harish_kandiga@...tor.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Alexander Graf <graf@...zon.com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Phil Reid <preid@...ctromag.com.au>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>
Subject: Re: [PATCH v6 4/8] gpiolib: Add support for GPIO lookup by line name

On Tue, Mar 24, 2020 at 2:57 PM Geert Uytterhoeven
<geert+renesas@...der.be> 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@...der.be>
> Reviewed-by: Ulrich Hecht <uli+renesas@...nd.eu>
> Reviewed-by: Eugeniu Rosca <erosca@...adit-jv.com>
> Tested-by: Eugeniu Rosca <erosca@...adit-jv.com>

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