[PATCH v3 2/4] backlight: Expose brightness curve type through sysfs

[Matthias Kaehlcke]

Hi Uwe,

On Fri, Aug 16, 2019 at 06:51:48PM +0200, Uwe Kleine-König wrote:
> On Tue, Jul 09, 2019 at 12:00:05PM -0700, Matthias Kaehlcke wrote:
> > Backlight brightness curves can have different shapes. The two main
> > types are linear and non-linear curves. The human eye doesn't
> > perceive linearly increasing/decreasing brightness as linear (see
> > also 88ba95bedb79 "backlight: pwm_bl: Compute brightness of LED
> > linearly to human eye"), hence many backlights use non-linear (often
> > logarithmic) brightness curves. The type of curve currently is opaque
> > to userspace, so userspace often uses more or less reliable heuristics
> > (like the number of brightness levels) to decide whether to treat a
> > backlight device as linear or non-linear.
> > 
> > Export the type of the brightness curve via the new sysfs attribute
> > 'scale'. The value of the attribute can be 'linear', 'non-linear' or
> > 'unknown'. For devices that don't provide information about the scale
> > of their brightness curve the value of the 'scale' attribute is 'unknown'.
> > 
> > Signed-off-by: Matthias Kaehlcke <mka at chromium.org>
> 
> I wonder what kind of problem you are solving here. Can you describe
> that in a few words?

The human eye perceives brightness in a logarithmic manner. For
backlights with a linear brightness curve brightness controls like
sliders need to use a mapping to achieve a behavior that is perceived
as linear-ish (more details: http://www.pathwaylighting.com/products/downloads/brochure/technical_materials_1466797044_Linear+vs+Logarithmic+Dimming+White+Paper.pdf)

As of now userspace doesn't have information about the type of the
brightness curve, and often uses heuristics to make a guess, which may
be right most of the time, but not always. The new attribute eliminates
the need to guess.