linux-kernel - Re: [RFC] Proposal: common kernel-wide GPIO interface

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Message-ID: <44CE2B90.5030905@bootc.net>
Date:	Mon, 31 Jul 2006 17:10:56 +0100
From:	Chris Boot <bootc@...tc.net>
To:	Ben Dooks <ben@...ff.org>
Cc:	Robert Schwebel <r.schwebel@...gutronix.de>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Proposal: common kernel-wide GPIO interface

Ben Dooks wrote:
> On Sun, Jul 30, 2006 at 03:08:11PM +0200, Robert Schwebel wrote:
>> Chris,
>>
>> On Fri, Jul 28, 2006 at 09:44:40PM +0100, Chris Boot wrote:
>>> I propose to develop a common way of registering and accessing GPIO pins on 
>>> various devices.
>> I've attached the gpio framework we have developed a while ago; it is
>> not ready for upstream, only tested on pxa and has probably several
>> other drawbacks, but may be a start for your activities. One of the
>> problems we've recently seen is that for example on PowerPCs you don't
>> have such a clear "this is gpio pin x" nomenclature, so the question
>> would be how to do the mapping here.
> 
> Right, my $0.02 worth:

[snip]

Some very interesting comments and suggestions! Thanks very much for all the
info, that's exactly the sort of stuff I needed.

> 3) The sysfs interface should be configurable, as systems
>    with lots of GPIO would end up with large numbers of
>    files and directories in sysfs.

My current idea is to divide the interfaces by GPIO device and port. I've so far
not seen a GPIO device that couldn't be divided into ports of <= 32 bits. How
wide can a 'port' actually become?

Somehow I think that a separate device/file for each pin or possibly even port 
might not be a wise idea. For example, twiddling individual pins on a GPIO when 
you connect an LCD, I2C, or SPI interface seems extremely inefficient...

> 4) you probably want to ensure pull-up resistors are off if the
>    output is being driven. 

Yes, very good idea! So far I haven't managed to fry any chips by driving a 
pulled up/down output, but it's so easy to make the mistake...

Chris

-- 
Chris Boot
bootc@...tc.net
http://www.bootc.net/

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