Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

Brian Norris
2019-06-11T16:55:30Z

On Tue, Jun 11, 2019 at 3:49 AM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
> This is a long standing flaw in the backlight interfaces. AFAIK generic
> userspaces end up with a (flawed) heuristic.

Bingo! Would be nice if we could start to fix this long-standing flaw.

> Basically devices with a narrow range of choices can be assumed to be
> logarithmic

That's (almost, see below) exactly what we have.

(And this is what Matthias is fighting against, now that we're
implementing both "large number of data points" and "pre-curved" at
the same time. We will have to either adapt the heuristic, or else
adapt our device trees to fit the heuristic.)

> Systems are coming along that allow us to animate the change of
> brightness (part of the reason for interpolated tables is to
> permit smooth animation rather than because the user explicitly wants
> to set the brightness to exactly 1117).

Chrome OS has done this for a long time. So "coming along" is a bit late ;)

Also, I believe Chrome OS will do animation/smoothing for all tables
(small or large) where it can: even for the small tables.

> These systems are often
> logarithmic but with a wide range of values.

NB: Chrome OS happens to use a polynomial formula (exponent = 2 or
0.5, depending on how you look at it), not logarithmic. You can see it
in all its (non)glory here:

https://chromium.googlesource.com/chromiumos/platform2/+/ee015853b227cf265491bd80ccf096b188490529/power_manager/powerd/policy/internal_backlight_controller.cc#451

Regards,
Brian