linux-kernel - Re: [Patch v2 1/2] gpio: add GPIO hogging mechanism

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Date:	Tue, 2 Dec 2014 23:10:07 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Benoit Parrot <bparrot@...com>
Cc:	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Jiri Prchal <jiri.prchal@...ignal.cz>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [Patch v2 1/2] gpio: add GPIO hogging mechanism

On Tue, Dec 2, 2014 at 9:22 AM, Benoit Parrot <bparrot@...com> wrote:
>> > +       }
>> > +
>> > +       if (tmp > MAX_PHANDLE_ARGS) {
>> > +               desc = ERR_PTR(-EINVAL);
>> > +               goto out;
>> > +       }
>> > +
>> > +       gg_data.gpiospec.args_count = tmp;
>> > +       gg_data.gpiospec.np = chip_np;
>> > +       for (i = 0; i < tmp; i++) {
>> > +               ret = of_property_read_u32(np, "gpios",
>> > +                                          &gg_data.gpiospec.args[i]);
>> > +               if (ret) {
>> > +                       desc = ERR_PTR(ret);
>> > +                       goto out;
>> > +               }
>> > +       }
>> > +
>> > +       gpiochip_find(&gg_data, of_gpiochip_find_and_xlate);
>>
>> This seems to work but only supports one GPIO per hog node. It would
>> be nice to be able to specify several GPIOs to which the same settings
>> need to be applied.
>
> This is on purpose following Linus Walleij's comment.

Could you point me to his comment? My bad for not remembering what he
said, but I'd like to understand why.

>> Using this function means that a GPIO chip module cannot be unloaded
>> if it uses GPIO hogs. Is it the intended behavior? If not, please use
>> gpiochip_request_own_desc() instead, and make sure to call
>> gpiochip_free_own_desc() for each hog when the driver is unloaded.
>
> So I guess we could add a undo_gpio_hog() function and hook it up under of_gpiochip_remove().
> Now instead of maintaining a seperate structure just to keep track of hogged descriptor,
> would it be acceptable to add a new "gpio_desc.flags" value in gpiolib.h says:
>
>    #define FLAG_GPIO_IS_HOGGED 10
>
> And key on that at removal time instead of creating a list and having to maintain that?

Definitely, that would be even better I think.

>> I would suggest to factorize this code that is similar to the one
>> found in __gpiod_get_index(). Do all the DT parsing in a function that
>> just returns a descriptor and the
>
> I would tend to agree.
> But as Linus suggested I was trying to contain the changes to gpiolib_of.c only.

If we add a FLAG_GPIO_IS_HOGGED and undo the hogs when the chip is
unloaded, I would say that this becomes a gpiolib feature. Moving it
here would also allow non-DT GPIO providers to implement hogs (it
should be particularly easy to implement for platform data). Linus, do
you agree?
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