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

Alexandre Courbot
2014-12-02T14:10:33Z

On Tue, Dec 2, 2014 at 9:22 AM, Benoit Parrot <bparrot@ti.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?