Re: [PATCH 1/3] gpiolib: add gpio_set_direction()

From: Mark Brown
Date: Mon Mar 01 2010 - 05:35:58 EST

On Sat, Feb 27, 2010 at 09:51:08AM -0800, David Brownell wrote:
> On Saturday 27 February 2010, Mark Brown wrote:

> > Indeed, but some devices do implement a distinct tristate state for  
> > input mode pins (disabling interrupt generation logic and so on for  
> > example).

> That's a pretty sloppy usage of the term "tristate" ... yeah, there
> are people who take glee in abusing terminology to introduce confusion,
> and some of them write technical manuals with little regard to normal
> usage of terms (or trademarks, which do exist for "tristate").

It's a bit clearer when the pins have multiple functions of which GPIO
is only one - normally the tristate is something that applies over all
the functions of the GPIO.

> IRQ generation logic should be disabled until request_irq() code paths
> report otherwise. And regardless, whether a GPIO triggers an IRQ has
> nothing at all to do with its "direction".

This is an internal Linux implementation issue rather than a hardware
one - that was just an example one of the things that tristating the pin
rather than just putting it into input mode does.
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