[PATCH 2/3] backlight: pwm_bl: Artificially add 0% during interpolation

[Daniel Vetter]

On Wed, Sep 09, 2020 at 05:03:37PM +0200, Daniel Vetter wrote:
> On Wed, Sep 9, 2020 at 4:45 PM Daniel Thompson
> <daniel.thompson at linaro.org> wrote:
> >
> > On Mon, Sep 07, 2020 at 09:50:18AM +0200, Daniel Vetter wrote:
> > > On Fri, Sep 04, 2020 at 12:38:22PM +0100, Daniel Thompson wrote:
> > > > On Mon, Jul 20, 2020 at 09:25:21PM -0700, Alexandru Stan wrote:
> > > > > Some displays need the low end of the curve cropped in order to make
> > > > > them happy. In that case we still want to have the 0% point, even though
> > > > > anything between 0% and 5%(example) would be skipped.
> > > >
> > > > For backlights it is not defined that 0 means off and, to be honest, 0
> > > > means off is actually rather weird for anything except transflexive
> > > > or front lit reflective displays[1]. There is a problem on several
> > > > systems that when the backlight slider is reduced to zero you can't
> > > > see the screen properly to turn it back up. This patch looks like it
> > > > would make that problem worse by hurting systems with will written
> > > > device trees.
> > > >
> > > > There is some nasty legacy here: some backlight displays that are off
> > > > at zero and that sucks because userspace doesn't know whether zero is
> > > > off or lowest possible setting.
> > > >
> > > > Nevertheless perhaps a better way to handle this case is for 0 to map to
> > > > 5% power and for the userspace to turn the backlight on/off as final
> > > > step in an animated backlight fade out (and one again for a fade in).
> > >
> > > Afaik chromeos encodes "0 means off" somewhere in there stack. We've
> > > gotten similar patches for the i915 backlight driver when we started
> > > obeying the panel's lower limit in our pwm backlight driver thing that's
> > > sometimes used instead of acpi.
> >
> > Out of interest... were they accepted?
> >
> > I did took a quick look at intel_panel.c and didn't see anything
> > that appeared to be special casing zero but I thought I might double
> > check.
> 
> I don't think so. Just figured I bring this up since it might explain
> why this is coming back again from an @chromium.com address.

Thanks to Alexandru pointing at the right code it's clear this got fix,
over 5 years ago:

https://source.chromium.org/chromiumos/_/chromium/chromiumos/platform2/+/6e83a6a8bb772ed8a2f939da1c7a152147c12789

	power: Write to bl_power when backlight reaches or leaves 0.
	
	Make InternalBacklight write FB_BLANK_UNBLANK to bl_power
	just before setting the internal backlight brightness to a
	nonzero value, and FB_BLANK_POWERDOWN to bl_power just
	before setting the brightness to 0.
	
	This is needed for hardware that interprets a brightness
	level of 0 as meaning "dim" rather than "off".
	
	BUG=chromium:396218
	TEST=added unit tests
	
	Change-Id: I914e333ab41db623564d5a67beac89f1a62bce9d
	Reviewed-on: https://chromium-review.googlesource.com/311010
	Commit-Ready: Dan Erat <derat at chromium.org>
	Tested-by: Dan Erat <derat at chromium.org>
	Reviewed-by: Dan Erat <derat at chromium.org>

Cheers, Daniel

> > > There's also the problem that with fancy panels with protocol (dsi, edp,
> > > ...) shutting of the backlight completely out of the proper power sequence
> > > hangs the panel (for some panels at least), so providing a backlight off
> > > that doesn't go through the drm modeset sequence isn't always possible.
> > >
> > > It's a bit a mess indeed :-/
> > > -Daniel
> > >
> > > >
> > > >
> > > > Daniel.
> > > >
> > > > >
> > > > > Signed-off-by: Alexandru Stan <amstan at chromium.org>
> > > > > ---
> > > > >
> > > > >  drivers/video/backlight/pwm_bl.c | 8 ++++++++
> > > > >  1 file changed, 8 insertions(+)
> > > > >
> > > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > > > > index 5193a72305a2..b24711ddf504 100644
> > > > > --- a/drivers/video/backlight/pwm_bl.c
> > > > > +++ b/drivers/video/backlight/pwm_bl.c
> > > > > @@ -349,6 +349,14 @@ static int pwm_backlight_parse_dt(struct device *dev,
> > > > >                   /* Fill in the last point, since no line starts here. */
> > > > >                   table[x2] = y2;
> > > > >
> > > > > +                 /*
> > > > > +                  * If we don't start at 0 yet we're increasing, assume
> > > > > +                  * the dts wanted to crop the low end of the range, so
> > > > > +                  * insert a 0 to provide a display off mode.
> > > > > +                  */
> > > > > +                 if (table[0] > 0 && table[0] < table[num_levels - 1])
> > > > > +                         table[0] = 0;
> > > > > +
> > > > >                   /*
> > > > >                    * As we use interpolation lets remove current
> > > > >                    * brightness levels table and replace for the
> > > > > --
> > > > > 2.27.0
> > > > _______________________________________________
> > > > dri-devel mailing list
> > > > dri-devel at lists.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch