Re: [PATCH v5 1/4] gpiolib: Add "unknown" direction support

Grant Likely
2011-03-07T06:52:59Z

On Sun, Mar 06, 2011 at 08:43:05PM -0600, Peter Tyser wrote:
> > The thing about gpios is that how they are used is entirely dependant
> > on what they are wired up to. There is no avoiding the fact that you
> > *absolutely* must understand the usage model before even considering
> > fiddling with a gpio (ignoring the "I wonder what this knob does"
> > use-case such as when reverse engineer a board). So, while it is nice
> > to have an 'unknown' state for sysfs to report, it is certainly not
> > required. The model still remains that the pin direction must be set
> > before (or at the same time as) reading/writing the pin.
>
> As is, even if someone knows about the GPIO wiring on their board, they
> have to know Linux has a "rule" that "before I can use a GPIO, I have to
> explicitly set its direction, even if the current reported direction is
> what I want". A number of our customers have tried to use a GPIO which
> states its an 'input' as an input after exporting it, which is
> completely logical. But it doesn't work, because its not really an
> input... Why not set the direction accurately as 'unknown' so users
> intuitively know they have to set the direction before using it? You
> also mention that it would be a nice feature above, so why not include
> it?

I don't like what it does to the implementation, and I'd rather make
drivers provide accurate data at the outset.

g.