linux-kernel - Re: [PATCH 2/2] gpio: of: Allow -gpio suffix for property names

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Date:	Thu, 24 Apr 2014 20:22:44 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] gpio: of: Allow -gpio suffix for property names

On Thu, Apr 24, 2014 at 09:06:24AM -0500, Rob Herring wrote:
> On Thu, Apr 24, 2014 at 7:47 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Wed, Apr 23, 2014 at 5:28 PM, Thierry Reding
> > <thierry.reding@...il.com> wrote:
> >
> >> From: Thierry Reding <treding@...dia.com>
> >>
> >> Many bindings use the -gpio suffix in property names. Support this in
> >> addition to the -gpios suffix when requesting GPIOs using the new
> >> descriptor-based API.
> >>
> >> Signed-off-by: Thierry Reding <treding@...dia.com>
> >
> > Are the DT bindings really full of such ambiguity between
> > singular and plural? Examples?
> >
> > What happens in affected drivers today? It just doesn't work?
> 
> They mostly seem to use of_get_named_gpio.

Indeed. That has the downside of requiring manual parsing and handling
of the GPIO polarity, though.

> > Does that mean these bindings are not actively used by any
> > drivers yet so we could augment the bindings instead, or are
> > they already deployed so we must implement this?
> >
> > Would like a word from the DT people here...
> 
> Interestingly, there is not a single occurrence of '-gpio ' in
> powerpc, but a bunch in ARM. In hindsight, we should have perhaps
> enforced using -gpios only, but that doesn't really matter now. We
> have -gpio in use, so we need to support it.

I think I also saw a proposal only recently to add support for a
gpios/gpio-names type of binding

> That doesn't necessarily mean this function has to support it. For
> example, this function could a legacy method and some other function
> should be used instead (of_get_named_gpio perhaps).

The reason why I posted this is precisely because I wanted to convert
over some drivers to use the new helpers (because they make things like
polarity handling much easier). My first attempt was to fix the binding
because I was under the impression that -gpio usage was discouraged, but
people didn't like that because, you know, DT bindings being a stable
ABI and all that.

The downside of not allowing the gpiod API to support the -gpio suffix
is that we'll never be able to convert drivers that use such a binding
and will forever have a hodgepodge of GPIO APIs that we need to support.

> >>  drivers/gpio/gpiolib.c | 18 ++++++++++++------
> >>  1 file changed, 12 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> >> index 7a0b97076374..b991462c22fb 100644
> >> --- a/drivers/gpio/gpiolib.c
> >> +++ b/drivers/gpio/gpiolib.c
> >> @@ -2594,17 +2594,23 @@ static struct gpio_desc *of_find_gpio(struct device *dev, const char *con_id,
> >>                                       unsigned int idx,
> >>                                       enum gpio_lookup_flags *flags)
> >>  {
> >> +       static const char *suffixes[] = { "gpios", "gpio" };
> >>         char prop_name[32]; /* 32 is max size of property name */
> >>         enum of_gpio_flags of_flags;
> >>         struct gpio_desc *desc;
> >> +       unsigned int i;
> >>
> >> -       if (con_id)
> >> -               snprintf(prop_name, 32, "%s-gpios", con_id);
> >> -       else
> >> -               snprintf(prop_name, 32, "gpios");
> >> +       for (i = 0; i < ARRAY_SIZE(suffixes); i++) {
> >> +               if (con_id)
> >> +                       snprintf(prop_name, 32, "%s-%s", con_id, suffixes[i]);
> >> +               else
> >> +                       snprintf(prop_name, 32, "%s", suffixes[i]);
> 
> This has the side effect of searching for "gpio" as property name
> which I don't think we want to allow.

Why don't we want to allow a "gpio" property when we already allow
"gpios"?

> It also allows a DT with either suffix to work. While I don't
> necessarily think the kernel's job should be DT validation, we don't
> have any other validation today and I don't see how this change
> simplifies the code. Between stricter DT checking (in the kernel) and
> simpler code, I'd pick the latter.

I had briefly considered adding more validation here as well, such as
refusing to hand out any GPIO with idx > 0 for the -gpio suffix, but
then opted not to do that in favour of code simplicity.

Thierry

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