linux-kernel - Re: [PATCH] gpio: Add support for GPIOF_ACTIVE_LOW to gpio_request

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Message-ID: <CAAVeFuJigwAg09p4LQ+9g_LYusYe_G-y_aBWP0rr-AG-yz5V-A@mail.gmail.com>
Date:	Tue, 22 Jul 2014 13:30:59 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH] gpio: Add support for GPIOF_ACTIVE_LOW to gpio_request_one

On Thu, Jul 17, 2014 at 11:34 PM, Guenter Roeck <linux@...ck-us.net> wrote:
> On 07/17/2014 12:26 AM, Alexandre Courbot wrote:
>>
>> On Thu, Jul 17, 2014 at 3:37 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>> On 07/16/2014 11:09 PM, Alexandre Courbot wrote:
>>>>
>>>>
>>>> On Thu, Jul 17, 2014 at 8:11 AM, Guenter Roeck <linux@...ck-us.net>
>>>> wrote:
>>>>>
>>>>>
>>>>> The gpio include file and the gpio documentation declare and document
>>>>> GPIOF_ACTIVE_LOW as one of the flags to be passed to gpio_request_one
>>>>> and related functions. However, the flag is not evaluated or used.
>>>>>
>>>>> Check the flag in gpio_request_one and set the gpio internal flag
>>>>> FLAG_ACTIVE_LOW if it is set.
>>>>
>>>>
>>>>
>>>> What is the point since the integer GPIO API has no clue of the
>>>> active-low status of a GPIO? It is only used by the gpiod and sysfs
>>>> interfaces.
>>>>
>>>
>>> One can use gpio_request_one() to export a gpio pin to user space from
>>> the kernel. That code path does use the flag, as you point out yourself
>>> above.
>>
>>
>> Ok, in that case I suppose it makes sense.
>>
>> Reviewed-by: Alexandre Courbot <acourbot@...dia.com>
>>
>>> One could also argue that the integer gpio API _should_ support this as
>>> well,
>>> but that is a different question.
>>
>>
>> Probably not going to happen. The integer GPIO interface is deprecated
>> and users who need new features should seriously consider switching to
>> gpiod.
>>
>
> The new API is unfortunately not equivalent to the old one. For example,
> if I understand correctly, gpiod_get is expected to be used instead of
> gpio_request_one.

That is correct. The reason is to have a separate authority to assigns
GPIOs and prevent drivers from arbitrarily requesting any GPIO they
want, as long as everybody sticks to the gpiod interface.

> That may work nicely in a world with full DT or ACPI
> support, but doesn't work as well otherwise unless one drops the notion
> of using platform specific drivers built as modules (gpiod_add_lookup_table
> is not exported, and there is no remove function).
>
> Specifically, I don't see an easy way to convert mdio-gpio to use the new
> model, and that driver could really use support for an API which supports
> active-low pins.

If you want to benefit from the active-low property but cannot use
gpiod_get() for some reason, you can still request a GPIO by
gpio_request_one() and then convert it to a descriptor with
gpio_to_desc(), which is what I suspect your patch will allow you to
do. But the real fix would be to work any limitations that gpiod has
that prevent you from doing what you need.

I am not familiar with mdio-gpio - could you explain what makes it
impossible to convert this driver to the new model in your view?
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