Re: PWM backlight initial state assumptions, or how pwm_bl killed my (nyan) cat^W backlight support

[Daniel Thompson]

On 06/07/17 13:41, Paul Kocialkowski wrote:
> > > Some panels have a documented powerup sequence, which usually ends with
> > > the backlight being enabled to avoid showing garbage before the panel is
> > > initialized completely.
> > > The reason for 3698d7e7d221 was a device with the display disabled out
> > > of the bootloader, where the backlight is controlled by the simple-panel
> > > driver. Enabling the backlight from the backlight driver before the
> > > panel driver requests the backlight to be enabled (before the panel is
> > > powered) would result in a white flash during boot.
> > >
> > > I tried to be careful to only let the backlight driver set the initial
> > > state to disabled if a few conditions are met: the GPIO is already
> > > configured as output and disabled, and the backlight device tree node
> > > has a phandle pointing to it, so we can expect there to be some
> > > controlling instance that will enable it when appropriate.
> > >
> > > I wonder why in your case there is a phandle link to the backlight node
> > > but nothing actually enables the backlight during boot. I would expect
> > > that to be handled by the panel driver.
> >
> > I had completely missed the fact that the panel driver is supposed to request
> > backlight enable! With that in mind, this policy no longer concerns "the whole
> > lifetime of the driver" but only the timeframe between backlight probe and
> > panel
> > probe. Thanks for clarifying this. I suppose I agree with this policy now.
> >
> > So remains the question: why does the panel not enable backlight on my device?
> >
> > I will investigate this later today. Hopefully, the probe defer logic should
> > prevent a case where the panel is probed (to the end) before the backlight
> > driver.
>
> This is actually due to the tegra drm driver not probing. It is happening on my
> setup (with a somewhat custom, out-of-tree config) on both jetson-tk1 and
> nyan_big, but also on kernelci, running both tegra_defconfig and
> multi_v7_defconfig.
>
> Since this is no longer a concern related to the pwm backlight driver, I will
> start a new thread with the relevant addresses in CC.

Thanks for the update. Much appreciated!

BTW, I got the impression from your first mail that you are chasing a 
regression between v4.11 and v4.12. If that's the case you should know 
that the backlight changes we have been discussing landed in v4.11 (so 
if you do have a regression between these two releases then mentioning 
backlight in the other thread may just be confusing).


Daniel.