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SubjectRe: [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:


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 Boot

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