linux-kernel - Re: [PATCH v3 3/3] backlight: pwm

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Date:   Mon, 2 Nov 2020 10:52:49 -0800
From:   Alexandru M Stan <amstan@...omium.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Heiko Stuebner <heiko@...ech.de>, Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Douglas Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

On Wed, Oct 28, 2020 at 8:12 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> 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.

Sorry! Yeah, I mistakenly duplicated the "END" line in patman.

>
>
> > Signed-off-by: Alexandru Stan <amstan@...omium.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@...aro.org>
>
>
> Daniel.


Alexandru Stan (amstan)