linux-kernel - Re: [PATCH] pio: add arch specific gpio_is

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Message-ID: <AANLkTi=RRSOnPVCEv5ni40D71BeMSV5bccWXnZphzGTX@mail.gmail.com>
Date:	Tue, 7 Sep 2010 12:19:23 +0800
From:	Eric Miao <eric.y.miao@...il.com>
To:	Ryan Mallon <ryan@...ewatersys.com>
Cc:	David Brownell <david-b@...bell.net>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	linux-kernel@...r.kernel.org, avictor.za@...il.com,
	bn@...sdigital.com,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] pio: add arch specific gpio_is_valid() function

On Tue, Sep 7, 2010 at 12:07 PM, Ryan Mallon <ryan@...ewatersys.com> wrote:
> On 09/07/2010 03:54 PM, Eric Miao wrote:
>> On Tue, Sep 7, 2010 at 10:44 AM, Ryan Mallon <ryan@...ewatersys.com> wrote:
>>> On 09/07/2010 02:23 PM, David Brownell wrote:
>>>> Still not liking or accepting this proposed
>>>> change to the GPIO framework.
>>>>
>>>> For the AT91 case (where integers 0..N are
>>>> IRQs, but N..max are GPIOs)
>>>>
>>>> A simpler solution is just to use a bit in
>>>> the integer to indicate IRQ vs GPIO.  Like
>>>> maybe the sign bit.. which is never set on
>>>> valid GPIO numbers, but platforms could let
>>>> be set on IRQs.
>>>>
>>> How about this approach instead?
>>>
>>
>> This doesn't solve the problem with more complicated settings, e.g.
>> some GPIOs within are not valid, not just the begining ones.
>
> Agreed, but this does solve the immediate problem for AT91 in a simple
> way. Are there boards in the kernel which have holes in the gpio layout?
>
> Another possible solution is to loop through all the gpio_chips to see
> if the number maps to a valid gpio. The obvious downside to this
> approach is that the complexity of gpio_is_valid becomes reasonably high
> for something which should be a very simple test and, as you say below,
> we probably just don't need that fine-grained information.
>
>> So the real question here is the semantics of gpio_is_valid(). I'd
>> personally incline it reads as if a GPIO _number_ is valid generally,
>> (e.g. like -1 is not a valid GPIO number), instead of that specific
>> GPIO is valid on that specific platform. The latter can be judged
>> with gpio_request().
>
> Some drivers in the kernel appear to be using this behaviour to have
> optional gpios, ie setting the foo_gpio = -1 in the platform data for
> some driver. The documentation also suggests that this is what
> gpio_is_valid is intended for. However, the documentation also says we
> may want gpio_is_valid to return invalid on some other numbers.

Right. So I'd really like David to make the semantics clear. My intention
is to keep gpio_is_valid() as simple as checking a general range to
rule out most invalid cases. And just use gpio_request() to handle the
platform specific cases.

>
> ~Ryan
>
> --
> Bluewater Systems Ltd - ARM Technology Solution Centre
>
> Ryan Mallon                     5 Amuri Park, 404 Barbadoes St
> ryan@...ewatersys.com           PO Box 13 889, Christchurch 8013
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>
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