Query on direction_output fn of PL061 GPIO driver.

[Baruch Siach]

Hi Viresh,

On Tue, Apr 13, 2010 at 05:10:41PM +0530, Viresh KUMAR wrote:
> On 4/13/2010 5:02 PM, Baruch Siach wrote:
> >> > 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.
> > 
> 
> thanks for your prompt reply.
> Ok. so this may cause problem on the board. 
> Anyway if this is the issue, then writing value to val register has no effects.
> 
> > Anyway, the right thing to do here is to reverse do just that, since the 
> > current way of doing things is broken.
> 
> I didn't get it completely. If i got it correctly, maybe myself or you can just
> send a patch to reverse this, i.e. writing val and setting direction.
> Am i correct?

Correct. Please send a patch, and I'll ack it after review. Make sure to test 
the patch before sending. I don't have access to this hardware anymore.  
Estimating the length of the jitter on your platform (when switching direction 
and value) would be even better. Please explain the issue and the jitter 
problem in your commit log.

baruch

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