[PATCH] RFC: leds-pwm: don't disable pwm when setting brightness to 0

[Thierry Reding]

On Thu, Oct 25, 2012 at 04:03:49PM +0800, Shawn Guo wrote:
> On Wed, Oct 24, 2012 at 03:52:46PM +0200, Uwe Kleine-König wrote:
> > This fixes disabling the LED on i.MX28. The PWM hardware delays using
> > the newly set pwm-config until the beginning of a new period.  It's very
> > likely that pwm_disable is called before the current period ends. In
> > case the LED was on brightness=max before the LED stays on because in
> > the disabled PWM block the period never ends.
> > 
> > It's unclear if the mxs-pwm driver doesn't implement the API as expected
> > (i.e. it should block until the newly set config is effective) or if the
> > leds-pwm driver makes wrong assumptions. This patch assumes the latter.
> > 
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
> > ---
> > Hello,
> > 
> > I'm not sure this is correct, but this is the workaround I'm using until
> > I get some feed back.
> 
> I'm fine with it, since it fixes a real problem.  Let's see what
> Thierry says.

I lost track of this thread somehow, so sorry for not getting back to
you earlier. The root cause of this problem seems to be that it isn't
very well defined (actually not at all) what is supposed to happen in
the case when a PWM is disabled.

There really are only two ways forward: a) we need to write down what
the PWM subsystem expects to happen when a PWM is disabled or b) keep
the currently undefined behaviour. With the latter I expect this kind
of issue to keep popping up every once in a while with all sorts of
ad-hoc solutions being implemented to solve the problem.

I think the best option would be to have some definition about what the
PWM signal should look like after a call to pwm_disable(). However this
doesn't turn out to be as trivial as it sounds. For instance, the most
straightforward definition in my opinion would be to specify that a PWM
signal should be constantly low after the call to pwm_disable(). It is
what I think most people would assume is the natural disable state of a
PWM.

However, one case where a similar problem was encountered involved a
hardware design that used an external inverter to change the polarity of
a PWM signal that was used to drive a backlight. In such a case, if the
controller were programmed to keep the output low when disabling, the
display would in fact be fully lit. This is further complicated by the
fact that the controller allows the output level of the disabled PWM
signal to be configured. This is nice because it means that pretty much
any scenario is covered, but it also doesn't make it any easier to put
this into a generic framework.

Having said that, I'm tempted to go with a simple definition like the
above anyway and handle obscure cases with board-specific quirks. I
don't see any other alternative that would allow the PWM framework to
stay relatively simple and clean.

Now I only have access to a limited amount of hardware and use-cases, so
any comments and suggestions as well as requirements on other platforms
are very welcome.

Thierry
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