linux-kernel - Re: [PATCH 0/4] gpio: introduce descriptor-based interface

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Date:	Thu, 17 Jan 2013 12:25:57 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>, Alex Courbot <acourbot@...dia.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Guenter Roeck <linux@...ck-us.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	Linus WALLEIJ <linus.walleij@...ricsson.com>,
	Roland Stigge <stigge@...com.de>
Subject: Re: [PATCH 0/4] gpio: introduce descriptor-based interface

On Thu, Jan 10, 2013 at 11:08 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thursday 10 January 2013, Alex Courbot wrote:

>> > Regarding the integration of pinctrl with gpio,
>> > I was thinking in the past that we could make pinctrl provide everything
>> > that gpiolib does, and have a generic gpiolib driver on top of pinctrl
>> > so that platforms don't need to implement both interfaces but only need
>> > to provide a pure pinctrl driver. Not sure if this makes any sense.
>>
>> That would work if all GPIOs were connected to a ball, but how about GPIO
>> expanders that are external to the chip? They have no use for pinctrl AFAICT.
>> On the other hand, maybe we can have one pinctrl-gpio driver for those chips
>> where pinctrl alone can emulate all the functionality of a GPIO controller.
>> Maybe such a driver exists already?
>
> I don't think we have that yet, but it would be another option: rather
> than putting a generic gpiolib driver on top of pinctrl, we could have
> pinctrl support for all gpios that go through gpiolib, and move device
> drivers over to use pinctrl as the way to manage gpios rather than the
> classic gpio drivers. That would be a larger change though, and require
> that we pull in the pinctrl subsystem on a lot of machines that don't
> need it today.

Not quite following but have the following loose idea:

- Add API such as <linux/pinctrl/gpio.h> with struct pin_gpio_ops {}
  or similar. As orthogonal to the mux and config interfaces we already
  have.

- Add ops like .set_input(), .set_output(), .drive_high() and .drive_low()
  (etc) to the ops struct so all functionality currently provided by
  gpiolib can be implemened by a driver.

- Make global pin numbers optional in gpiolib for the next part...

- Register a generic GPIO chip on top of the pinctrl-gpio.c, preferably
  only supporting Alex' descriptors.

- Provide a userspace interface to pinctrl with something like /dev/pinctrlN
  with an ioctl() interface, solving also Roland Stigge's issue with
  driving many pins at the same time in a smooth way.

Sounds like a plan?

I'd like to avoid the either-or-approach where you have to use
pinctrl only or only gpiolib, so a compatibility layer kindof.

I'm prepping a talk at ELC so will try to jot down something more
substantial.

Yours,
Linus Walleij
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