[Bug 85861] backlight control doesn't work on Lenovo t440p after upgrading to 3.17 [bisected]

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=85861

--- Comment #23 from Tommi Kyntola <kynde at ts.ray.fi> ---

What are the semantics behind the 'actual_brightness' then?
Are there cases where  it can indeed differ from the set value in 'brightness'
in some reasonable way outside of rounding errors?
(it'd be nice to know if anyone is thinking about informing the kde people
about all this)

Why is it necessary to even try do the inverse for the scaling, because with
integers that simply is not a bijection unless the ranges are of equal size?

The commit in comment #14 works only by chance here, too, because with the
minimum set the ranges aren't of equal size and indeed in many spots the
brightness and actual_brightness again show different values, which would break
kde powerdevil.

The commit in comment #11 is of course different and retains the identical
ranges and make the scaling and identity operation and thus leave no room for
rounding errors of any kind.

Why not store the user value and return that for the reverse when asked for?
I tried that with quadratic scaling and it works like a charm all along the
range quite nicely (*), in fact, I'm likely to keep that patch in my own
laptop. I can clean it up a notch and sign-off, if anyone's interested.

(*) Well, I did add the inverse quadratic scaling there, too, but I only went
for it if the previous values that passed through scale could not be used, i.e.
only if there was a cache miss in a sense. Turns out, there's only one miss and
that's during a boot which is non-ciritical and could probably be avoided, too.

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