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

[Thierry Reding]

On Thu, Jan 03, 2013 at 09:55:14PM +0100, Uwe Kleine-König wrote:
> Hi Thierry,
> 
> On Thu, Jan 03, 2013 at 10:01:18AM +0100, Thierry Reding wrote:
> > 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
> I don't understand what you mean with "the above" here. I guess it's
> "PWM signal should be constantly low after the call to pwm_disable".

Yes, exactly.

> To cover this we could add a function pwm_disable_blurb() that accepts a
> parameter specifying the desired signal state: "high", "low" or (maybe)
> "don't care". pwm_disable would then (probably) mean
> pwm_disable_blurb("don't care"). But maybe this already contradicts your
> idea about being simple and clean?!

I'm wondering if that's really necessary. This really seems more of a
board-specific question. If you run pwm_disable() on a PWM device, it
should be turned "off" (whatever that means in the context of a board
design) after the call terminates.

Part of the problem is that we want to keep the board-specific
complexities out of client drivers. For instance in the case you
encountered, the leds-pwm driver shouldn't have to know any of the
details pertaining to the i.MX28. That is, leds-pwm should be able to
call pwm_disable() if it wants to turn off the PWM signal.

If we add pwm_disable_blurb() as you suggest, what is leds-pwm supposed
to pass in? Usually this would be "low", but on other hardware (with
additional inverter circuitry) it would be "high". We certainly don't
want to have leds-pwm handling that kind of logic. The PWM signal
polarity is entirely defined at the board-level and therefore should be
handled by board setup code (or encoded in DT).

> Also note that I had another/alternative issue with the API, i.e. when
> the pwm routines should return.

Right. All of the above would entail that pwm_config() should either
block until the configuration is active, or alternatively that when
pwm_disable() is called without the new configuration being active yet,
it is pwm_disable() that needs to wait until the configuration becomes
active.

Another alternative would be that leds-pwm wouldn't have to call
pwm_config() with a 0% duty cycle in the first place if pwm_disable() is
guaranteed to force the PWM signal to constant low.

Thierry
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