Re: [PATCH v2 2/4] pwm-backlight: add support for pwm-delay-us property

[Daniel Thompson]

On 06/07/17 10:24, Thierry Reding wrote:
> On Thu, Jul 06, 2017 at 10:17:18AM +0100, Daniel Thompson wrote:
> > On 06/07/17 10:12, Pavel Machek wrote:
> > > On Thu 2017-07-06 10:01:32, Thierry Reding wrote:
> > > > On Fri, Jun 30, 2017 at 01:21:07PM +0200, Enric Balletbo i Serra wrote:
> > > > > From: huang lin <hl@xxxxxxxxxxxxxx>
> > > > >
> > > > > Some panels (i.e. N116BGE-L41), in their power sequence specifications,
> > > > > request a delay between set the PWM signal and enable the backlight and
> > > > > between clear the PWM signal and disable the backlight. Add support for
> > > > > the new pwm-delay-us property to meet the timing.
> > > > >
> > > > > Note that this patch inverts current sequence. Before this patch the
> > > > > enable signal was set before the PWM signal and vice-versa on power off.
> > > > >
> > > > > I assumed that this sequence was wrong, at least it is on different panel
> > > > > datasheets that I checked, so I inverted the sequence to follow:
> > > > >
> > > > >     On power on, set the PWM signal, wait, and set the LED_EN signal.
> > > > >     On power off, clear the LED_EN signal, wait, and stop the PWM signal.
> > > >
> > > > I think this should be two separate patches to make it easier to revert
> > > > the inverted sequence should it prove to regress on other panels.
> > >
> > > Don't make this overly complex. This is trivial. No need to split it
> > > into more patches.
> >
> > Agree. IMHO getting the code that reads the (optional) new parameter correct
> > is the best way to manage risk of regression since in most cases the delay
> > will be skipped anyway.
>
> The potential regression that I'm referring to would be caused by
> inversing the sequence (GPIO enable -> PWM enable). That's completely
> unrelated to the delays introduced by this patch. Many boards use this
> driver and they've been running with the old sequence for many years.
> Granted, it's fairly unlikely to regress, but it's still a possibility.
>
> Given that both changes are logically separate, I think separate patches
> are totally appropriate. I also don't think that this would overly
> complicate things.

... and you are right on both counts!

Thanks for the detailed reply.


Daniel.