[Intel-gfx] PROBLEM: Native backlight regressed from logarithmic to linear scale

[Hans de Goede]

Hi,

On 07/22/2014 06:32 AM, Anders Kaseorg wrote:
> [1.] One line summary of the problem:    
> 
> Native backlight regressed from logarithmic to linear scale
> 
> [2.] Full description of the problem/report:
> 
> With the new default of video.use_native_backlight=0 (commit 
> v3.16-rc1~30^2~2^3), my Thinkpad T510 backlight has become very difficult 
> to control near the low end of the scale.  The lowest setting now turns 
> the backlight completely off, but the second-lowest setting is too bright.  
> Meanwhile, the difference between the highest several settings is nearly 
> imperceptible.
> 
> This happened because /sys/class/backlight/acpi_video0/brightness (which 
> has now disappeared) used a different scale than 
> /sys/class/intel_backlight/brightness; the relationship between 
> acpi_video0 and intel_backlight was not linear.  I measured the exact 
> relationship as follows:
> 
> acpi    intel
> 0       52
> 1       87
> 2       139
> 3       174
> 4       226
> 5       261
> 6       313
> 7       435
> 8       591
> 9       800
> 10      1078
> 11      1461
> 12      1914
> 13      2557
> 14      3358
> 15      4437
> 
> The relationship is close to logarithmic; a good fit is intel = 
> 60*(4/3)^acpi, or acpi = log_{4/3} (intel/60).  It’s well known that 
> perceived brightness varies logarithmically with physical luminance 
> (Fechner’s law), so it’s no surprise that the acpi_video0 scale was more 
> useful.
> 
> Since the kernel no longer uses ACPI to do this translation, it should do 
> the translation itself.

I've been thinking a bit about this, and I believe that the right answer
here is to do the linear to logarithmic mapping in user-space. The intel
backlight interface has a type of raw, clearly signalling to userspace that
it is a raw "untranslated" interface, as such any fanciness such as
creating a logarithmic scale should be done in userspace IMHO.

Regards,

Hans