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

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:

quoted

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@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.