Subject: Query on direction_output fn of PL061 GPIO driver.

From: [email-masked] (Baruch Siach)
Date: Tue, 13 Apr 2010 14:32:39 +0300

Hi Viresh,

On Tue, Apr 13, 2010 at 04:20:38PM +0530, Viresh KUMAR wrote:
> I have a query on PL061 GPIO drivers API pl061_direction_output().
> Purpose of this function is to set GPIO pin in OUT mode and set/reset
> its value.
> 
> In current implementation, firstly value of GPIO pin is modified and then its
> direction is set to OUT. In our SOCs (ST SPEAr), this implementation doesn't
> work correctly (we have pl061 in our soc). Suppose previously pin is in IN mode,
> now writing val to it doesn't change anything because it is in IN mode and we
> can write to a pin only when it is in OUT mode. Now if its direction is
> changed to OUT then also its value will remain same and will not be changed to
> what we wanted.
> 
> Am i missing something?

Probably not. This is a design bug in the PL061 hardware. Changing the 
direction before the value makes the output value of the GPIO undefined 
(apparently 0) for a short period of time. This is bad news for some 
applications.

Anyway, the right thing to do here is to reverse do just that, since the 
current way of doing things is broken.

baruch

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