linux-kernel - Re: [PATCH RFC 1/2] gpio: Add a block GPIO API to gpiolib

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Date:	Sun, 30 Sep 2012 17:19:14 +0200
From:	Stijn Devriendt <highguy@...il.com>
To:	Roland Stigge <stigge@...com.de>
Cc:	grant.likely@...retlab.ca, linus.walleij@...aro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	w.sang@...gutronix.de, jbe@...gutronix.de,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	bgat@...lgatliff.com
Subject: Re: [PATCH RFC 1/2] gpio: Add a block GPIO API to gpiolib

On Sun, Sep 30, 2012 at 5:09 PM, Roland Stigge <stigge@...com.de> wrote:
> Hi Stijn,
>
> On 30/09/12 16:52, Stijn Devriendt wrote:
>>> One question: How did you solve the one-value-per-file in the sysfs
>>> interface?
>>>
>> By exporting the group as a whole:
>> /sys/.../gpiogroup248/value
>> where value contains a decimal representing the group value.
>> Again, this respects the ordering of the pins:
>>
>> Actual pins: 0x2D (b 0010 1101)
>> Selected pins: 6 3 0 1
>> Readout: 6 (b 0 1 1 0)
>>
>> The export sysfs file does, however, accept multiple gpio IDs for groups.
>> Not sure if this is a 'violation' per se...
>
> If I understand correctly, it's a violation (single-value should hold
> for read and write).
>
> To solve it, I have the following in mind: /sys/.../gpiogroupXXX/
> contains files "bit0" ... "bit31" which contain a gpio number each,
> empty if "unconnected".

Unfortunately that means you can't atomically create a group.
It also creates a mess to keep ordering intact and to either
keep the current pin state or override it at allocation-time.

Rules are rules, but why make the interface overly complex when
the multi-value file is saner, cleaner and simpler?
I know I'm on the happy/corporate side of things and I can violate
whatever rule I can if it gets our product shipping, but I'd still
propose to have the multi-value approach, even in mainline.

Regards,
Stijn
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