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

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Date:	Tue, 2 Dec 2014 15:28:11 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Benoit Parrot <bparrot@...com>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Jiri Prchal <jiri.prchal@...ignal.cz>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	"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 3:10 PM, Alexandre Courbot <gnurou@...il.com> wrote:
> 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.

Said in previous message I think: either one-per-node or lists directly
in the gpiochip node.

>>> 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.

Clever. Go for this.

>>> 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?

Yes, this is better and leaves the door open for other users.

Yours,
Linus Walleij
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