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

[Daniel Thompson]

On Fri, Aug 16, 2019 at 10:53:17AM -0700, Matthias Kaehlcke wrote:
> On Fri, Aug 16, 2019 at 04:54:18PM +0100, Daniel Thompson wrote:
> > On 07/08/2019 21:15, Matthias Kaehlcke 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>
> > > 
> > > Daniel (et al): do you have any more comments on this patch/series or
> > > is it ready to land?
> > 
> > I decided to leave it for a long while for others to review since I'm still
> > a tiny bit uneasy about the linear/non-linear terminology.
> > 
> > However that's my only concern, its fairly minor and I've dragged by feet
> > for more then long enough, so:
> > Reviewed-by: Daniel Thompson <daniel.thompson at linaro.org>
> 
> Thanks!
> 
> If you or someone else has another suggestion for the terminology that
> we can all agree on I'm happy to change it.

As you will see in my reply to Uwe. The term I tend to adopt when I want
to be precise about userspace behaviour is "perceptual" (e.g. that a
backlight can be mapped directly to a slider and it will feel right).

However that raises its own concerns: mostly about what is perceptual
enough.

Clear the automatic brightness curve support in the PWM driver is
perceptual.

To be honest I suspect that in most cases a true logarithmic curve (given a
sane exponent) would be perceptual enough. In other words it will feel
comfortable with a direct mapped slider and using it for animation
won't be too bad.

However when we get right down to it *that* is the information that is
actually most useful to userspace: explicit confirmation that the scale
can be mapped directly to a slider. I think it also aligned better with
Uwe's feedback (e.g. to start working towards having a preferred scale).


Daniel.