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

From: Alexandru M Stan <amstan@chromium.org>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: "Heiko Stuebner" <heiko@sntech.de>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Andy Gross" <agross@kernel.org>,
	"Bjorn Andersson" <bjorn.andersson@linaro.org>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Lee Jones" <lee.jones@linaro.org>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"Douglas Anderson" <dianders@chromium.org>,
	"Matthias Kaehlcke" <mka@chromium.org>,
	"Enric Balletbo i Serra" <enric.balletbo@collabora.com>,
	dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-pwm@vger.kernel.org
Subject: Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation
Date: Mon, 2 Nov 2020 10:52:49 -0800	[thread overview]
Message-ID: <CAHNYxRy4hg4rZsc-xi3MzK+RapMq76+=hGj0_E-aGcFUPB1wMA@mail.gmail.com> (raw)
In-Reply-To: <20201028151210.7e765hbq2k7i3url@holly.lan>

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

Alexandru Stan (amstan)