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

[Daniel Thompson <daniel.thompson@linaro.org>]

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.


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