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

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Phil Reid <preid@electromag.com.au>,
	Jonathan Corbet <corbet@lwn.net>,
	Marc Zyngier <marc.zyngier@arm.com>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	Magnus Damm <magnus.damm@gmail.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Rob Herring <robh+dt@kernel.org>,
	Harish Jenny K N <harish_kandiga@mentor.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Alexander Graf <graf@amazon.com>,
	Eugeniu Rosca <erosca@de.adit-jv.com>
Subject: Re: [PATCH v6 4/8] gpiolib: Add support for GPIO lookup by line name
Date: Mon, 11 May 2020 12:18:22 +0200	[thread overview]
Message-ID: <CAMuHMdXskE8_A4gbFVWfPVdcxBO-CfaMDEehSxGAK_aC0MQ7=A@mail.gmail.com> (raw)
In-Reply-To: <CACRpkdbN82n3B+Q-QVjB1jLpJAYS19fKukkDXQm3gZsuFFFM_w@mail.gmail.com>

Hi Linus,

On Thu, Mar 26, 2020 at 10:18 PM Linus Walleij <linus.walleij@linaro.org> wrote:
> On Tue, Mar 24, 2020 at 2:57 PM Geert Uytterhoeven
> <geert+renesas@glider.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@glider.be>
> > Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
> > Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
> > Tested-by: Eugeniu Rosca <erosca@de.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.

on a _different_ 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.

Agreed.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds