linux-kernel - Re: [PATCH] pinctrl: document semantics vs GPIO

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Message-ID: <50535375.4090606@wwwdotorg.org>
Date:	Fri, 14 Sep 2012 09:55:33 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Linus Walleij <linus.walleij@...aro.org>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Stephen Warren <swarren@...dia.com>,
	Anmar Oueja <anmar.oueja@...aro.org>
Subject: Re: [PATCH] pinctrl: document semantics vs GPIO

On 09/14/2012 08:30 AM, Domenico Andreoli wrote:
> On Fri, Sep 14, 2012 at 03:48:05PM +0200, Linus Walleij wrote:
>> On Fri, Sep 14, 2012 at 12:11 AM, Domenico Andreoli <cavokz@...il.com> wrote:
>>> On Thu, Sep 13, 2012 at 10:11:29AM -0600, Stephen Warren wrote:
>>
>>>> I think it makes sense to more strongly recommend that for GPIO muxing,
>>>> the GPIO driver always call into the pinctrl subsystem (if needed by the
>>>> HW) to perform that muxing, so that standalone gpio_direction_*() always
>>>> work without any use of pinctrl; the interaction between the two should
>>>> only be required if pin configuration (not just pin muxing) is also
>>>> required.
>>>
>>> Don't know. Isn't possible to reach the same effect moving this kind
>>> of knowledge into higher level helper functions and remove this bridge
>>> across the subsystems?
>>
>> I'm not following, please elaborate on this.
>>
>> What are these higher level functions, and where will they be
>> located? In which subsystem, and using what symbols/signatures and
>> so on?
> 
> If the common case is requesting the pin and then the gpio, an helper
> like this would do the trick. So why those calls into pinctrl should be
> done by the GPIO driver itself?  Pinctrl and GPIO would be separated,
> ignoring each other.
> 
> static int request_muxed_gpio(int gpio, const char *label)

That would require the driver to know when to call gpio_request() as
opposed to request_muxed_gpio() wouldn't it. Whether that is needed or
not depends on the Soc/board the driver is running on. The whole idea of
the internal GPIO->pinctrl driver communication was to avoid that.

I suppose that if we were to mandate that ever device that uses GPIOs
also have at least some (possibly empty) pinctrl state defined, then
request_muxed_gpio() could always be used. However, that's quite a
strong requirement. An also, if we were to make that rule, then we might
as well just implement this inside the existing gpio_request(), so that
no driver changes were required.
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