[PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

[Daniel Thompson]

On Wed, Oct 21, 2020 at 10:04:45PM -0700, Alexandru Stan wrote:
> The previous behavior was a little unexpected, its properties/problems:
> 1. It was designed to generate strictly increasing values (no repeats)
> 2. It had quantization errors when calculating step size. Resulting in
> unexpected jumps near the end of some segments.
> 
> Example settings:
> 	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> 	num-interpolated-steps = <16>;
> 
> Whenever num-interpolated-steps was larger than the distance
> between 2 consecutive brightness levels the table would get really
> discontinuous. The slope of the interpolation would stick with
> integers only and if it was 0 the whole line segment would get skipped.
> 
> The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
> and only starting with 16 it would start to interpolate properly.
> 
> Property #1 is not enough. The goal here is more than just monotonically
> increasing. We should still care about the shape of the curve. Repeated
> points might be desired if we're in the part of the curve where we want
> to go slow (aka slope near 0).
> 
> Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
> the calculated slope on the 32-63 segment will be almost half as it
> should be.
> 
> The most expected and simplest algorithm for interpolation is linear
> interpolation, which would handle both problems.
> Let's just implement that!
> 
> Take pairs of points from the brightness-levels array and linearly
> interpolate between them. On the X axis (what userspace sees) we'll
> now have equally sized intervals (num-interpolated-steps sized,
> as opposed to before where we were at the mercy of quantization).
> 
> END

INTERESTING.

I guess this a copy 'n paste error from some internal log book?
Better removed... but I won't lose sleep over it.


> Signed-off-by: Alexandru Stan <amstan at chromium.org>

I've waited a bit to see how strong the feelings were w.r.t. getting rid
of the division from the table initialization. It was something I was
aware of during an earlier review but it was below my personal nitpicking
threshold (which could be badly calibrated... hence waiting). However
it's all been quiet so:

Reviewed-by: Daniel Thompson <daniel.thompson at linaro.org>


Daniel.