Re: question on generic gpio interface

[David Brownell <david-b@pacbell.net>]

On Tuesday 17 April 2007 12:04 am, Francis Moreau wrote:

> BTW, are there any plan to make gpio usable from userspace ? I don't
> know if it makes sense but I saw some patches on LKML that did that.

Only the usual plan:  someone who wants that feature provides a driver,
which can be merged after suitable review.

For example, the gpio_keys and leds-gpio drivers already exist to
expose some kinds of GPIOs ... though not specifically as GPIOs.
And I've seen various drivers that package GPIOs on specific chips,
mostly for _exclusive_ use by userspace (no kernel access).


In this case I'm not entirely sure how it'd work.  I've seen a few
drivers which let userspace peek and poke at GPIO signals -- like
one for Gumstix boards -- but generalizing the model isn't simple.
Sub-problems include:

 - Configuring the relevant pins.  Especially for SOC cases, GPIO
   roles are multiplexed with several others.  So there are two
   issues:  (a) the platform-specific setup of that multiplexing,
   plus (b) the board-specific knowledge of what pins are truly
   available for use as GPIOs, and not otherwise in use.

 - Enumerating those GPIOs to userspace.  One SOC might have just
   a few dozen, another might have a few hundred; and then there
   are all the board-specific ones, on FPGA or I2C chips etc.

 - Exposing those pins to userspace.  It'd be unsafe to let pins
   claimed by drivers be managed by userspace; the default should
   be that only unclaimed GPIOs can be accessed.

Those points imply part of a design that includes board-specific
hooks of various kinds (maybe delegating a lot to platform code).

But nobody's yet provided code that would generalize.

- Dave